INTRODUCTION — Testicular neoplasms are the most common solid malignancy among males between the ages of 15 and 35, although they represent only approximately 1 percent of all solid tumors in males . The two main categories of testicular tumors are germ cell tumors (GCTs), which account for 95 percent of cases, and sex cord-stromal tumors (SCSTs) (table 1).
Testicular pathology is complex because of the spectrum of histologic subtypes and variable clinical behavior, particularly among GCTs. Prediction of biologic behavior depends upon the type(s) of tumors present, as well as clinical features, including the age of the patient and primary site (ie, testicular versus retroperitoneal), since histologically identical tumors can behave differently depending upon these clinical parameters. Accurate histologic evaluation and staging will help determine if a patient should be treated surgically (orchiectomy, retroperitoneal lymph node dissection) and whether chemotherapy is indicated.
In 2022, the World Health Organization (WHO) updated the classification of testicular tumors in "Tumor of the Urinary System and Male Genital Organs" (table 1) [2-4]. This classification scheme is relatively simple, practical, widely accepted, and of proven clinical utility. The anatomy and pathology of testicular tumors is presented here. The management of testicular GCTs and testicular SCSTs is discussed separately. (See "Initial risk-stratified treatment for advanced testicular germ cell tumors" and "Testicular sex cord stromal tumors".)
TESTICULAR ANATOMY — The testis is composed of lobules of seminiferous tubules, interlobular septa, the mediastinum testis, rete testis, the tunica albuginea, and tunica vaginalis (figure 1). The lobules are roughly pyramid shaped, and each contains one to four U-shaped seminiferous tubules. Both ends of the seminiferous tubules empty into the rete testis (which is located posteriorly in the hilum of the testis) and drain into the epididymis. Normal seminiferous tubules have a basement membrane and a thin fibrous wall, and they contain germ cells in various stages of maturation plus Sertoli cells. Blood vessels and lymphatics, mesenchymal supporting tissue, macrophages, and Leydig cells are contained within the intertubular interstitium.
Lobules are separated from each other by fibrous interlobular septa, which contain blood vessels and lymphatics. A posterior expansion of the fibrous tunica albuginea forms the mediastinum testis, which joins with the confluence of the interlobular septa. The blood vessels of the interlobular septa, together with those of the tunica albuginea, drain into those of the mediastinum testis and ultimately into those of the spermatic cord. Overlying the external surface of the tunica albuginea and separating it from the parietal tunica vaginalis is the thin mesothelial layer of the visceral tunica vaginalis .
Although not strictly part of the testis, the anatomy of the draining lymphatics determines the pattern of spread of testicular tumors. In the absence of prior surgery or invasion by tumor of the scrotum, the lymphatics of the testis drain directly to the retroperitoneal lymph nodes in a predictable fashion . (See "Retroperitoneal lymph node dissection for early-stage testicular germ cell tumors", section on 'Lymphatics'.)
PATHOLOGIC EVALUATION — Pathologic evaluation is generally performed on the entire testis rather than on a biopsy sample. Examination of the entire testis allows identification of the histopathologic tumor type, as well as an assessment of the stage and extent of the disease, both of which have an important bearing on subsequent management and prognosis (table 2A-B). Histologic features that help to determine higher stage include vascular or lymphatic invasion and spread beyond the tunica albuginea or into the spermatic cord [7,8]. Identifying a few of the major germ cell tumor (GCT) subtypes can be somewhat subjective due to morphologic overlap [9-11]. However, in many cases, these subtypes can be readily differentiated using immunohistochemistry (table 1).
GERM CELL TUMORS — Germ cell tumors (GCTs) account for approximately 95 percent of testicular tumors. They are near evenly distributed between pure seminomas (no non-seminomatous elements present), and all others, which together are termed non-seminomatous germ cell tumors (NSGCTs) (table 1). NSGCTs may contain pure components (ie, only one subtype of malignant germ cells is present), or more commonly, they may consist of a heterogeneous mixture of two or more histologic subtypes. Distinction of these various subtypes of germ cell neoplasia is primarily a morphologic diagnosis, and the use of immunohistochemical stains should be reserved for difficult cases and not for routine diagnosis [9,10,12,13]. Additionally, careful gross examination and thorough sampling of the specimen for histologic review is important, since accurate assessment of all tumor types may be crucial in planning management. (See 'Testicular anatomy' above.)
Molecular markers — The most consistent abnormality in adolescent and adult males with testicular GCTs is the presence of an isochromosome of the short arm of chromosome 12 (i12p) . Approximately 80 percent of NSGCTs and 50 percent of pure seminomas possess at least one i12p [15,16]. Most tumors in adolescent and adult males with GCTS that do not have an i12p have other chromosomal abnormalities in this region that result in overrepresentation of chromosome 12p material [17-19]. Identification of an i12p may be useful in establishing the diagnosis of a GCT in atypical cases, particularly for tumors arising in the mediastinum. In contrast, prepubertal yolk sac tumors and teratomas are not associated with isochromosome 12p [20,21].
Specific molecular markers for each testicular subtype are described separately within the corresponding sections.
Staging system — GCTs are staged using the eighth (2017) Tumor, Node, Metastasis (TNM) staging system developed jointly by the American Joint Committee on Cancer (AJCC) and the Union for International Cancer Control (UICC) (table 2A-B) . In previous versions of the AJCC staging system, tumors were pT1 if confined to the testis and epididymis, pT2 if they demonstrated vascular invasion or extension beyond the tunica vaginalis, and pT3 if they involved the spermatic cord. In the eighth edition of the AJCC staging system, the pT2 category includes tumors that demonstrated vascular invasion, extension beyond the tunica vaginalis, invasion of hilar soft tissue, and/or involvement the epididymis.
Additionally, for pure seminomas only, the pT1 category is subdivided into pT1a and pT1b for tumors that are <3 cm and ≥3 cm, respectively. (See "Clinical manifestations, diagnosis, and staging of testicular germ cell tumors", section on 'Staging'.)
Noninvasive germ cell neoplasia
Germ cell neoplasia in situ — Germ cell neoplasia in situ (GCNIS) is a noninvasive germ cell neoplasia that is defined by the presence of clearly malignant GCT elements within seminiferous tubules; these can include intratubular seminoma or, less commonly, intratubular embryonal carcinoma. However, a clear distinction of the intratubular component subtype is not always possible, and therefore, these are simply classified as GCNIS. GCNIS was previously classified as "intratubular germ cell neoplasia," a term which is no longer used.
Morphologically, the in situ neoplasia often replaces all other germ cell elements within the seminiferous tubules (picture 1). Occasionally, tumor cells may demonstrate pagetoid spread from within the tubules into the rete testis. Intratubular neoplastic cells resemble their invasive counterparts cytomorphologically and immunophenotypically.
The clinical significance of GCNIS is not entirely clear. GCNIS has been associated with GCTs of all types in adolescents and adults, except for spermatocytic tumor. However, prepubertal-type testicular teratomas, which are not associated with GCNIS, have been reported in postpubertal males . The unanswered question is whether GCNIS is a precursor lesion. There is evidence that it is [23,24], but these data come mostly from areas (eg, Denmark) where the epidemiology of GCTs is somewhat different from that seen in the United States , where little or no comparable data exist.
GCNIS is discussed in detail separately. (See "Testicular germ cell neoplasia in situ".)
Gonadoblastoma — Gonadoblastomas is a noninvasive germ cell neoplasia that contains both neoplastic germ cells and sex cord-stromal cells. Gonadoblastoma can be a precursor to more invasive tumors such as dysgerminoma or seminoma. (See "Testicular sex cord stromal tumors", section on 'Other tumor types'.)
Gonadoblastomas consist of seminoma-like cells interspersed among cells resembling immature Sertoli cells that are arranged in a characteristic pattern. The Sertoli-like cells are arrayed in a palisading fashion around small collections of eosinophilic basement membrane material. Focal calcification is a prominent feature in most cases (picture 2). Occasionally, Leydig-like cells are also present. Each of the cell types has the expected immunohistochemical phenotype. It is believed that gonadoblastoma is a premalignant lesion with the potential for the development of GCTs, especially seminoma, and gonadoblastoma can coexist with such a malignancy.
Most gonadoblastomas arise within dysgenetic gonads. Mixed gonadal dysgenesis, cryptorchidism, and abnormalities of the external genitalia, such as ambiguous genitalia or hypospadias, are common. In some cases, female external and/or internal genitalia can be seen . Although 80 percent of cases occur in phenotypic females, most have a 46 XY karyotype with the remainder exhibiting 45,X/46,XY mosaicism. Presentation is usually in childhood or adolescence in patients with male external genitalia.
Other unclassified tumors consisting of a mixture of cells with the appearances of germ cells and sex cord-stromal cells are quite rare and appear to be confined to adults. Further characterization has been hampered by their scarcity.
Seminomatous germ cell tumors — Seminoma is subclassified under the "germinoma" family of tumors (table 1) [2,3]. Pure seminomas account for approximately 50 percent of all testicular GCTs , and a seminomatous component is present in approximately 20 percent of mixed GCTs [28,29]. The average age at presentation for pure seminoma is approximately 40 years old (approximately 10 years older than for testicular NSGCTs). A seminomatous component is uncommon in GCTs in prepubertal males. Aside from age, there are no other reliable clinical correlates. In the ovary, a tumor nearly identical to seminoma is termed dysgerminoma. (See "Ovarian germ cell tumors: Pathology, epidemiology, clinical manifestations, and diagnosis".)
Specific serum tumor markers are usually normal with seminomas, although beta-human chorionic gonadotropin (hCG) may be somewhat elevated with tumors that contain scattered syncytiotrophoblastic giant cells. Nevertheless, in such cases, the serum beta-hCG is usually only mildly elevated (<100 international units/L). (See "Serum tumor markers in testicular germ cell tumors", section on 'Seminomas'.)
Seminoma — Histologic characteristics of classic seminoma include a clonal proliferation of neoplastic germ cells that demonstrates well-defined cytoplasmic borders, central to marginally located nuclei with prominent nucleoli, angled ("squared-off") nuclear membranes, and clear cytoplasm secondary to intracytoplasmic glycogen (picture 3). This overall pattern at low power is often described as having a "fried-egg" appearance. The neoplastic cells of seminoma tend to be less cohesive than in embryonal carcinoma, yolk sac tumor, or choriocarcinoma, all of which are epithelial tumors. Additionally, clusters of lymphocytes are almost invariably seen in intimate association with the tumor cells.
The differential diagnosis for seminoma includes lymphoma (especially with retroperitoneal tumors) and other GCTs, especially embryonal carcinoma (table 1). The latter distinction is most problematic when a seminoma demonstrates cytologic atypia or is poorly preserved during processing. Of note, the term anaplastic seminoma is no longer recognized by the World Health Organization (WHO) and should not be used. Instead, cases that were previously classified as anaplastic are included with the typical/classical seminomas. This change in terminology is based upon the observation that the previously so-called "anaplastic seminomas" have similar outcomes when compared with classic seminomas of comparable stage with modern treatment [30,31]. It should be recognized, however, that some seminomas have increased cytologic atypia and a higher number of mitotic figures.
Immunohistochemistry for seminomas — Immunohistochemistry for seminomas is straightforward and helpful in difficult cases. Seminoma is immunoreactive with OCT3/4, KIT (CD117), NANOG, and D2-40; however, OCT3/4 and NANOG do not distinguish seminoma from embryonal carcinoma as both are positive for this marker. By contrast, yolk sac tumor and choriocarcinomas are negative for all four of these seminoma biomarkers. Traditionally, the use of cytokeratin and CD30 immunostains have been helpful in distinguishing seminoma from embryonal carcinoma, as only the latter should be positive for these biomarkers. However, it is not infrequent for embryonal carcinoma to show only patchy, weak, or focal staining for cytokeratins and CD30, which in theory still raises the possibility of a mixed GCT instead of one that contains a pure component. Additionally, seminoma may express weak, focal (aberrant) CD30 expression, especially near areas of necrosis.
Additional immunomarkers can aid in the distinction of seminoma from embryonal carcinoma, as well as yolk sac tumor .
●OCT3/4 and NANOG – The transcription factors OCT3/4 and NANOG are very sensitive and specific markers that stain both embryonal carcinoma and seminoma but are negative in yolk sac tumor [32-34].
●Sox-2 – Sox-2 is a transcription factor that stains embryonal carcinoma, while being negative in both seminoma and yolk sac tumor (note that focal sox-2 staining has been observed in teratomatous elements and should not cause confusion; personal observation) .
●KIT (CD117) – In contrast, KIT (CD117) immunoreactivity is present in seminoma but is negative in both embryonal carcinoma and yolk sac tumor. Note that a small subset of seminomas (up to approximately 20 percent) may be KIT (CD117) negative.
●AFP – Traditionally, alpha-fetoprotein (AFP) was used to stain yolk sac tumor, but background staining is frequently high, staining may only be focal, and focal staining can occasionally be seen in the non-yolk sac tumor components. Overall, AFP immunostaining has low sensitively and variable specificity for yolk sac tumor.
●SALL4 – A stem cell marker, SALL4 has been shown to stain all subtypes of GCT (except teratoma, but includes spermatocytic tumor) with high sensitivity but slightly lower specificity [35-38]. SALL4 can be used when a GCT is in the differential diagnosis of a metastatic tumor of unknown primary and is most sensitive when combined with other immunoreactive GCT biomarkers (ie, SALL4 and OCT3/4 immunoreactivity would be either a seminoma or embryonal carcinoma; secondary markers, as described above, can be used to distinguish these two tumor types). A primary or metastatic carcinoma that is SALL4 positive and negative for other germ cell markers, such as OCT3/4, NANOG, Sox2, and AFP, is most suggestive of a yolk sac tumor, but a non-GCT would have to be considered as well. Of note, testicular sex cord-stromal tumors (SCSTs) are negative for SALL4, and a small subset of non-GCTs are focally and weakly positive for SALL4. Examples of the latter include rare SALL4 positive gastric, esophageal, and colonic carcinomas [35,36,39].
●Glypican-3 – Glypican-3 is another sensitive immunomarker for yolk sac tumor. However, studies demonstrate that it stains fewer tumor cells (ie, decreased sensitivity) when compared with SALL4 .
●HCG – Staining for hCG is usually supportive of a component of choriocarcinoma. However, it is not uncommon to find isolated, scattered hCG-positive syncytiotrophoblastic giant cells in any (non-choriocarcinomatous) GCT, and their presence tends to correlate with a modestly elevated serum hCG concentration. This finding of isolated syncytiotrophoblast should not be confused with a component of choriocarcinoma in the absence of other features characteristic of this GCT subtype (eg, cytotrophoblast cells, blood lakes and possible necrosis). The biologic significance of syncytiotrophoblastic giant cells in seminoma or any (non-choriocarcinomatous) GCT is unclear, but they do not appear to affect prognosis.
●GATA3 – The trans-acting T-cell specific transcription factor that binds to the specific DNA sequence "GATA" (known as GATA3) is involved in embryogenesis , including development of the placenta  and other human tissues (parathyroid glands, auditory system, and kidneys ). For diagnostic purposes, GATA3 is typically used as a biomarker of breast and urothelial carcinomas ; however, other studies have shown GATA3 immunoreactivity in choriocarcinoma (diffuse, strong intensity), yolk sac tumor (focal to patchy, weak to moderate intensity), and within syncytiotrophoblast (in the absence of choriocarcinoma). Seminoma and embryonal carcinoma are negative for GATA3 .
Non-seminomatous germ cell tumors
Embryonal carcinoma — Pure embryonal carcinoma accounts for approximately 2 percent of all testicular GCTs , but it is a histologic component of approximately 85 percent of all mixed GCTs . The average age at presentation is approximately 30 years, similar to other NSGCTs. Embryonal carcinoma is rare in prepubertal males.
Pure embryonal carcinomas usually do not produce AFP. A more than modest elevation (ie, >60 ng/mL) in the serum AFP concentration should prompt a suspicion that a concomitant element of yolk sac tumor is present . As with seminoma, syncytiotrophoblastic giant cells within embryonal carcinomas may cause a modest elevation in the serum beta-hCG concentration. (See 'Yolk sac tumor' below.)
Microscopically, embryonal carcinoma shows epithelial differentiation with cohesive clusters and sheets of cells that show marked cytologic atypia, and this is often helpful in distinguishing them from yolk sac tumors, which generally exhibit blander cytomorphology (picture 4). Several architectural patterns exist, including glandular, papillary, and solid. None of these distinctions is of any clinical significance. Syncytiotrophoblastic giant cells may be present.
The differential diagnosis is primarily with yolk sac tumor, which is often intimately associated with embryonal carcinoma, and with seminoma. Immunohistochemical staining can be helpful with this, as well as in the broader differential diagnosis that may arise in metastatic deposits where non-GCTs are considered in the differential diagnosis. CD30 is the traditional marker used for embryonal carcinoma; however, the use of the transcription factors/stem cell markers, OCT3/4, NANOG, Sox-2, and SALL4 are the most sensitive and specific immunomarkers for embryonal carcinoma [32,33]. However, one must be aware that both OCT3/4 and NANOG also stain seminoma, and that SALL4 stains all subtypes of GCT (table 1). Keratins are typically negative to weakly positive in well-stained embryonal carcinomas, and this can be useful to distinguish from yolk sac tumors, which are nearly always strongly and diffusely positive for keratin, such as AE1/AE3.
Yolk sac tumor — The terms "yolk sac tumor" and "endodermal sinus tumor" are synonymous; however, the latter is not used in diagnostic reports. Pure yolk sac tumor is the most common malignant testicular GCT in prepubertal children , although (benign) teratomas are actually more prevalent in this age group . Pure yolk sac tumor is rarely seen in the adult, whereas a component of yolk sac tumor occurs in approximately 40 percent of mixed GCTs in adults .
Nearly all yolk sac tumors are accompanied by increased serum AFP, usually >100 ng/mL . Furthermore, the AFP level correlates with disease extent, with concentrations >1000 ng/mL often indicating the presence of extensive tumor. Yolk sac tumor does not produce hCG.
Microscopically, yolk sac tumors are the most morphologically variable of all GCTs (picture 5). Although a wide variety of patterns are recognized (eg, microcystic, reticular, papillary, glandular, solid, hepatoid), their only significance is the degree to which they can make the recognition of yolk sac tumor difficult. Hyaline-type globules and Schiller-Duval bodies are most characteristic of yolk sac tumor, but they are only present in a subset of cases. Typically, but not invariably, yolk sac tumors have lower nuclear grade, and may be associated with edematous to myxoid stroma. Following chemotherapy, yolk sac tumors may recur with sarcomatoid and/or glandular differentiation [12,49,50].
The most common entity in the microscopic differential diagnosis is embryonal carcinoma, with which yolk sac tumor often merges. Occasionally, difficulty may also be experienced in differentiating yolk sac tumor from teratoma as both can form gland-like structures. Immunohistochemical staining for AFP can be helpful when present, and is generally positive, albeit focally, in yolk sac tumor. The absence of AFP staining does not preclude the diagnosis of a yolk sac tumor as sensitivity is low. Additionally, background staining for AFP is often high and can be positive in a few scattered cells, at most, in embryonal carcinoma. The most specific staining profile for yolk sac tumor is the presence of SALL4, strong and diffuse AE1/AE3, and AFP, and an absence of OC3/4, c-kit, NANOG, and Sox-2 [34-37]. The presence of Glypican3 and GATA3, at least focally or multifocally, is also supportive of a yolk sac tumor diagnosis.
Choriocarcinoma — The most aggressive and least common type of GCT is choriocarcinoma. Widespread hematogenous dissemination occurs early, and many patients present with metastatic disease. Choriocarcinoma is present as an element of approximately 10 percent of testicular mixed GCTs , but is rare in its pure form at this site . The average age at presentation is somewhat younger than for other NSGCTs but is rare or nonexistent in the prepubertal male.
The serum beta-hCG concentration is often greater than 1000 international units/L and may even be much higher. Choriocarcinomas do not produce AFP.
Choriocarcinomas are characterized by areas of hemorrhage and necrosis, both grossly and microscopically. As such, the treatment of choriocarcinomas with chemotherapy is sometimes complicated by hemorrhagic emergencies [52,53]. The most important diagnostic characteristic is the coexistence and intimate association of both syncytiotrophoblast and cytotrophoblast cells, which distinguishes this tumor from other GCTs with only scattered syncytiotrophoblast (picture 6). Recapitulating their normal embryonic function, the syncytiotrophoblastic giant cells and cytotrophoblastic cells often display extensive vascular invasion. This characteristic is presumed to account for the propensity for early hematogenous dissemination.
Immunohistochemical staining for hCG is of limited utility in choriocarcinomas, as the antibody is not very reliable due to significant background staining and differential staining patterns. While the syncytiotrophoblast is essentially always strongly hCG positive, this is diagnostically insufficient. Staining for hCG generally does not help to identify the crucial cytotrophoblastic elements, which are less conspicuous on H&E-stained material, and at most are only weakly beta-hCG-positive. GATA3 has been shown to be a sensitive immunomarker for choriocarcinoma [44,54]. Choriocarcinomas are negative for OCT3/4, NANOG, Sox-2, and c-kit, but SALL4 may be expressed in the neoplastic cytotrophoblast (syncytiotrophoblast are negative) . Nevertheless, extensive sampling of GCTs and careful microscopic examination is the best way to detect this most aggressive tumor.
Definition — The designation teratoma refers to a varied group of tumors that show differentiation to form somatic-type tissues typical of either adult or embryonic development .Testicular teratomas may present in both prepubertal and adult males, but the prognosis differs greatly between these two age groups.
●Teratoma, prepubertal type – In children ("teratoma, prepubertal type"), teratomas most often occur before the age of four, are generally seen in their pure form, and behave in a benign fashion . This tumor subtype is classified under GCTs unrelated to germ cell neoplasia in situ and is not associated with additional genetic material from the short arm of chromosome 12. (See 'Other tumors' below.)
●Teratoma, postpubertal type – In adults ("teratoma, postpubertal type"), teratomas are usually part of a mixed GCT, and they have the potential to be found at metastatic sites , especially following chemotherapy for nonteratomatous GCT. In this setting, the teratoma appears to have a common clonal origin with the other components of the GCTs  and likely arises from pluripotent tumor cells. Analyses of metastases from GCTs and teratomas from the same patient have demonstrated a high degree of concordance in the observed genetic abnormalities.
Elevations in the serum concentration of AFP or beta-hCG cannot be attributed to teratomatous elements . Rather, elevated tumor markers indicate the coexistence of other GCT components.
The pathology of teratomas is the most confusing of all testicular GCTs. This is because the clinical import of a given set of histopathologic findings can vary widely depending on the age and the site of the primary tumor.
Few useful guidelines exist for testicular teratomas, in contrast to ovarian teratomas, where clinically meaningful criteria for the diagnosis of immaturity exist. Only immature teratomatous elements are considered malignant in an ovarian tumor, whereas both mature and immature elements are malignant in a testicular tumor in the postpubertal patient, except for the rare case of "postpubertal benign teratoma." (See 'Postpubertal benign teratoma' below.)
Thus, no distinction between mature and immature testicular teratoma/elements for these patients is necessary. Teratomas with malignant (somatic, ie, sarcomatous or carcinomatous) differentiation should be reported, since findings are suggestive of a worse prognosis. (See "Ovarian germ cell tumors: Pathology, epidemiology, clinical manifestations, and diagnosis".)
For descriptive and recognition purposes, four "variants" of teratoma are described. Of note, pathology reports in adolescent and adult males do not generally distinguish between mature and immature teratomas. In those populations, there is no clear clinical significance to the distinction between mature and immature teratomas: both have the potential to metastasize and transform into somatic-type malignancies, and they are treated in the same manner.
Mature teratomas — Mature teratomas are composed of a heterogeneous collection of differentiated cells or organoid structures, all embedded in a fibrous or myxoid stroma . Tissue types commonly seen within a mature teratoma include respiratory tract, gut, pancreas, thyroid, cartilage, squamous epithelium, and skin adnexal structures (picture 7). The degree to which undifferentiated mesenchymal elements can be present in teratomas that are termed mature is not clearly defined.
Mature cystic teratomas (dermoid cysts), common in the ovary, are quite rare in the testis. These possess only squamous epithelium, skin adnexal structures (eg, sebaceous glands), and no immature elements. No metastases have been reported from dermoid cysts, even in adult males. Similarly, uniformly benign epidermoid cysts can be seen that contain only keratinizing squamous epithelium and no other (adnexal) elements. The lack of abnormalities in chromosome 12p may be useful in distinguishing epidermoid cysts from teratomas, the latter of which contain an isochromosome of the short arm of chromosome 12 (i12p) and have a potential for malignancy .
Immature teratomas — Immature teratoma is defined as a tumor containing undifferentiated elements resembling tissue seen in embryonic stages of development. The degree to which the tissue must be undifferentiated is not clear. The presence of primitive neuroepithelial elements is recognized as immature elements; however, the clinical significance of their presence in this setting is not well defined, and therefore, the distinction is not typically included in a pathology report.
Somatic differentiation/malignant transformation is represented by tissue with histologically frankly malignant features such as expansive growth of immature neuroectoderm resembling that of a so-called embryonic-type neuroectodermal tumor. (See 'Teratoma with somatic-type malignancy' below.)
Postpubertal benign teratoma — A small subset of postpubertal dermoid cysts and teratomas are benign. These tumors present at a mean age of 24 (range 12 to 59 years old) [22,60]. In these cases, the lesions are nearly always located near the hilum, are relatively small, have a cystic component, and are not associated with GCNIS. Additionally, fluorescent in situ hybridization for i12p was negative in all cases tested. The hypothesis is that these lesions may have been present since prior to puberty and were only found later; however, de novo lesions cannot be entirely excluded. Regardless, all such lesions were associated with a benign clinical course.
Teratoma with somatic-type malignancy — A very small fraction of teratomas have malignant somatic-type tissue elements, such as squamous cell carcinoma, adenocarcinoma, immature neuroectodermal elements, or sarcoma such as rhabdomyosarcoma [61-63]. Somatic transformation of teratoma is most frequently seen in the metastatic setting following treatment with chemotherapy. However, it can occasionally be seen in a primary resection specimen without systemic therapy (ie, naïve patients). Teratomas with these features do not respond well to cisplatin-containing chemotherapy (as is commonly used for other GCT subtypes), leading some experts to treat the histology of the transformed lesion [64-66]. (See "Rhabdomyosarcoma in childhood, adolescence, and adulthood: Treatment".)
Teratoma with malignant transformation to a primitive small round blue cell tumor is referred to as embryonic-type neuroectodermal tumor (ENT). These tumors were previously referred to as a "primitive neuroectodermal tumor" (PNET), which is no longer recognized as a diagnostic term for genitourinary tumors . This terminology change, which was made to guide appropriate management, occurred in the 2022 WHO classification scheme and is also endorsed by the Armed Forces Institute of Pathology Fascicle of Testicular Tumors ). PNET was previously used as a diagnostic term for central nervous system (CNS) tumors and bone and soft tissue tumors such as Ewing sarcoma for the latter. However, with an improved understanding of molecular alterations, testicular tumors previously classified as PNETs have been shown to more closely resemble CNS tumors and not Ewing sarcoma. The terminology change from PNET to ENT was made so that clinicians do not misinterpret a testicular PNET for an Ewing sarcoma and provide inappropriate systemic therapy. Rarely, Ewing sarcoma can be seen in the testis, but it would be in the absence of a teratoma; it would also demonstrate an EWS rearrangement and NKX2.2/CD99 expression by immunohistochemistry. (See "Clinical presentation, staging, and prognostic factors of Ewing sarcoma", section on 'Molecular findings'.)
The diagnosis of "mature" versus "immature" teratoma is of little or no proven meaning in males with testicular GCTs as both "mature" and "immature" elements are considered malignant. In children, the treatment for either mature or immature teratoma is surgical; the completeness of resection rather than the histologic grade of immaturity are the most significant prognostic parameter . Such tumors generally behave in a benign fashion, irrespective of the degree of immaturity, as long as there are no other nonteratomatous GCT elements present . The finding of a significant component of a frankly malignant component (eg, embryonal carcinoma, rhabdomyosarcoma or adenocarcinoma) carries a distinctly poorer prognosis, especially when such a component metastasizes .
Unlike the ovary, pure testicular neoplasms of one tissue type not usually primary to the testes (ie, monodermal teratoma, such as carcinoid tumors) are rarely seen. (See "Ovarian germ cell tumors: Pathology, epidemiology, clinical manifestations, and diagnosis".)
Mixed germ cell tumors — Approximately one-third of all testicular GCTs are mixed, with two or more GCT types present within a single mass . Many possible combinations of seminoma, teratoma, embryonal carcinoma, yolk sac tumor, and choriocarcinoma can be seen. A teratomatous component is identified in approximately one-third of all mixed GCTs, and the term teratocarcinoma has, in the past, been applied to cases in which teratoma coexists with embryonal carcinoma. However, this term has largely been abandoned, and these tumors are referred to as a malignant mixed GCT, with a description of the specific GCT elements present.
In adults, the epidemiologic and clinical features of mixed GCTs are similar to those of NSGCTs. The average age at diagnosis is approximately 30 years, and they are rare in prepubertal males. Elevations in serum AFP and beta-hCG reflect some of the components that are present within the tumor.
Germ cell tumors unrelated to germ cell neoplasia in situ
Spermatocytic tumor — Spermatocytic tumor is the recognized name for the group of tumors previously known as "spermatocytic seminoma." These tumors are classified under GCTs unrelated to germ cell neoplasia in situ, in the 2022 WHO classification system for testicular tumors (table 1) [2,3].
Despite being previously classified as a variant of seminoma, it has been well recognized and accepted that spermatocytic tumor differs from classic seminoma in essentially all histologic, immunohistochemical, molecular, and clinical characteristics . This distinct clinicopathologic entity accounts for 1 percent of GCTs and can occur in males of any age; but is most frequently seen in older males [28,71-73]. As an example, in one population-based study, the age at diagnosis among 58 cases identified ranged between 19 to 92 with a median of 54 years at diagnosis .
Histologically, spermatocytic tumors have a characteristic admixture of three cytologically distinct populations of neoplastic germ cells of varying size (small, medium, and large) and nuclear features (picture 8). SALL4 has been shown to be immunoreactive in spermatocytic tumors and cannot be used to distinguish from (classic) seminoma . Immunohistochemical staining for placental-like alkaline phosphatase (PLAP) and OCT3/4 is negative, in contrast to seminoma , whereas KIT has shown to be reactive in a subset of spermatocytic tumors. It has been shown that a novel SSX antibody directed against a conserved C-terminal region of SSX1, SSX2, and SSX4 (SSX CT) can be used to distinguish spermatocytic tumor and seminoma. Spermatocytic tumor is nearly always strongly and diffusely positive for SSX, whereas seminoma is typical negative or only focally positive for SSX .
Unlike classic seminomas, spermatocytic tumors do not occur as part of mixed GCTs, do not contain an isochromosome 12p, and are not associated with germ cell neoplasia in situ (GCNIS; formally called intratubular germ cell neoplasia of unclassified type) or a significant lymphocytic infiltrate. (See "Testicular germ cell neoplasia in situ".)
Spermatocytic tumors have been shown to have a favorable prognosis even in the presence of lymphovascular invasion. The one significant adverse prognostic indicator is sarcomatoid differentiation.
Other tumors — Other GCTs unrelated to GNCIS include yolk sac tumor prepubertal-type, testicular neuroendocrine tumor prepubertal-type, mixed teratoma and yolk sac tumor prepubertal type, and teratoma prepubertal-type (table 1). (See 'Teratoma' above and 'Yolk sac tumor' above.)
Metastatic germ cell tumors — Metastatic GCTs most frequently involve retroperitoneal lymph nodes and may present as a different subtype than that of the primary tumor. As an example, metastases from a mixed GCT with embryonal carcinoma and seminoma in the primary tumor may contain only teratoma at the metastatic site and pure teratomas of the testis may have nonteratomatous metastases .
There are at least three possible explanations for these biologic differences between the primary tumor and the metastasis. Because all of these tumors are thought to be derived from totipotential germ cells, the tumor may change its phenotype, possibly due to selection of a particular clone that gained the ability to metastasize. Another possibility is that the histologic tumor type in the metastatic focus was actually present in the primary tumor but was not sampled for histologic examination. Last, teratoma in a nontesticular site is a frequent finding following chemotherapy even if the teratoma was not present in the primary testicular tumor because totipotential germ cells that differentiated into teratoma do not respond to chemotherapy (unlike the other nonteratomatous malignant germ cell elements). As with primary GCTs, elevations in serum markers are good predictors of the types of GCT present in metastases.
Extragonadal germ cell tumors — Some GCTs present in extragonadal sites, such as the retroperitoneum, mediastinum, or central nervous system, without an evident testicular primary. At least some of these cases represent metastases from occult and/or regressed testicular primaries. However, GCTs can arise in these extragonadal sites independent of testicular involvement. (See "Extragonadal germ cell tumors involving the mediastinum and retroperitoneum".)
SEX CORD-STROMAL TUMORS — Testicular sex cord-stromal tumors (SCSTs) show differentiation towards Leydig cells, Sertoli cells, and/or other types of sex cord-stromal cells (eg, granulosa cells) (table 1). In contrast to testicular germ cell tumors (GCTs), assessing the potential for malignant behavior is often difficult for SCSTs. Nevertheless, the likelihood for a given tumor to display malignant or metastatic behavior increases in the presence of local vascular invasion, large size (ie, greater than 5 cm), large numbers of mitotic figures, cytologic atypia, and necrosis .
As with GCTs, SCSTs exhibit numerous histologic patterns, and mixed tumors may infrequently occur . Morphologically recognizable elements include Leydig, Sertoli, granulosa, and theca cell types. Some SCSTs produce steroid hormones and are associated with specific endocrine syndromes, especially in prepubertal males. A minority of SCSTs exhibit some of the features of one or more of the histologic types of SCSTs but are not clearly representative of any recognized entity. Such cases are termed SCST, unclassified type, and may be more likely to demonstrate pathologic features associated with malignant or metastatic behavior .
Further details on the diagnosis and treatment of testicular SCSTs are discussed separately. (See "Testicular sex cord stromal tumors".)
Leydig cell tumors — Leydig cell tumors are the most common type of testicular SCSTs. Up to 20 percent of Leydig cell tumors in adults are classified as malignant, based predominantly upon large size, vascular invasion, and increased mitotic activity. Malignant behavior has not been documented in children [27,77,80]. However, in some cases, Leydig cell tumors with aggressive behavior are associated with fumarate hydratase inactivation, Wnt pathway activation, and copy number changes without recurrent mutations . (See "Testicular sex cord stromal tumors", section on 'Leydig cell tumors'.)
Microscopically, Leydig cell tumors consist of monomorphic sheets or nests of large cells with round, usually regular nuclei with a prominent nucleolus, and abundant eosinophilic cytoplasm. Occasional tumors may have spindle-shaped cells, or vacuolated cytoplasm . Eosinophilic crystals of Reinke may be seen in the cytoplasm in approximately one-third of cases, although they may be quite sparse and inconspicuous (picture 9). These crystals, which also have a characteristic appearance by electron microscopy, are characteristic of Leydig cells, but not diagnostic of neoplasia.
The differential diagnosis for a Leydig cell tumor includes Leydig cell hyperplasia and other neoplasms, such as lymphoma, plasmacytoma, and, occasionally, a Sertoli cell or GCT (especially the hepatoid variant of yolk sac tumor). Immunohistochemical staining with inhibin (inhibin-A) can be helpful for the distinction from most GCTs and somatic tumors, but not from Sertoli cell tumors as these can also be immunoreactive with inhibin [82,83]. Additionally, SF1, calretinin, and WT-1 immunostaining can be helpful; however, calretinin is more frequently present in Leydig cell tumors, whereas WT-1 is more frequently positive in Sertoli cell tumors. SALL4, a stem call marker that has been used to distinguish GCTs from other carcinomas and sarcomas, is negative in SCSTs, including Leydig cell tumors .
The differential diagnosis for a Leydig cell tumor also includes adrenal testicular rest tumors that are found in males with congenital adrenal hyperplasia . (See "Genetics and clinical presentation of nonclassic (late-onset) congenital adrenal hyperplasia due to 21-hydroxylase deficiency", section on 'Men'.)
Sertoli cell tumors — The general function of Sertoli cells is to facilitate spermatogenesis, and they are normally found scattered among germ cells within the seminiferous tubules. The secretory products of Sertoli cells include a variety of proteins.
Sertoli cell neoplasms may be classified as (1) Sertoli cell tumor "not otherwise specified" (NOS), (2) large cell calcifying Sertoli cell tumor, or (3) intratubular large cell hyalinizing Sertoli cell neoplasia. The previous distinction of a sclerosing subtype is no longer recognized by the WHO, and these lesions are combined into the NOS category [2,3]. Sertoliform cystadenoma has also been placed under the category of Sertoli cell tumor category and removed from the category of adnexal tumors. (See "Testicular sex cord stromal tumors", section on 'Sertoli cell tumors'.)
●Sertoli cell tumors, NOS – Sertoli cell tumors, NOS, which are the most common, have a greater range of histologic appearances, making recognition of this group of neoplasms challenging. The cells exhibit a variety of cytomorphologic appearances, including small polygonal cells with scant cytoplasm, fusiform cells, and cells with abundant eosinophilic cytoplasm, which may mimic those of Leydig cell tumors. Furthermore, the architecture may also vary widely, with cells arranged in sheets, pseudorosettes, tubules, slit-like spaces, nests, or cords. Sertoli cell tumors with dense fibrous stroma surrounding scattered small tubules, cords of cells, and individual cells with small nuclei and scant cytoplasm (previously referred to as "sclerosing Sertoli cell tumor") are included in this diagnostic category; mention of the sclerosed stroma can be mentioned in the pathology report if so desired.
Sertoli cell tumors, NOS, are known to harbor the CTNNB1 gene (which encodes beta catenin) mutations.
●Large cell calcifying Sertoli cell tumors – Large cell calcifying Sertoli cell tumor has very distinctive histologic features. The cells can be arranged in a variety of patterns, sometimes with tubule formation, and generally have abundant, eosinophilic cytoplasm, as the name implies (picture 10). Varying degrees of myxoid change or fibrosis can be seen in the surrounding stroma, and areas of calcification are usually a prominent feature. Neoplastic Sertoli cells are often present in the surrounding seminiferous tubules.
Large cell calcifying Sertoli cell tumors can be sporadic or associated with the Carney complex; the latter are associated with a PRKAR1A gene mutation in approximately two-thirds of cases. PRKAR1A immunohistochemistry can be used to support the diagnosis of a large cell calcifying Sertoli cell tumor; loss of staining is both sensitive and specific .
●Intratubular large cell hyalinizing Sertoli cell neoplasia – Intratubular large cell hyalinizing Sertoli cell neoplasms are frequently seen in patients with Peutz-Jeghers syndrome and are associated with gynecomastia. The WHO also classifies this neoplasm as a genetic tumor syndrome because of its association with Peutz-Jeghers syndrome . (See "Testicular sex cord stromal tumors", section on 'Additional diagnostic evaluations' and "Peutz-Jeghers syndrome: Clinical manifestations, diagnosis, and management".)
The lesion consists of an intratubular Sertoli cell proliferation that is accompanied by prominent basement membrane deposits. Lesions may be multifocal and microscopic. Secretion of aromatase by the tumor, which is then converted to androgens and estrogens, is the cause of the gynecomastia.
Histologically, the differential diagnosis of testicular Sertoli cell tumors includes juvenile granulosa cell tumor, Leydig cell tumor or mixed Sertoli-Leydig cell tumor, and pure GCT or mixed germ cell-SCST (ie, gonadoblastoma) [9,78]. It has been suggested that many tumors originally classified as Sertoli cell tumors, NOS, could be more accurately categorized as juvenile granulosa cell tumors, especially those found in preteen males.
Immunohistochemistry can be helpful with difficult cases, as Sertoli cells are typically positive for inhibin, and are negative for the broad-spectrum GCT biomarkers, SALL4, and placental alkaline phosphatase (PLAP). Inhibin positively should be interpreted with caution if the differential diagnosis includes Leydig or granulosa cell tumors. In such circumstances, WT-1 positivity can be helpful as Leydig cell should not react with this marker; however, granulosa cell tumors may express patchy WT1. SALL4 is negative in Sertoli cell tumors and therefore can be used to differentiate Sertoli cell tumors from GCTs, which are immunoreactive to SALL4 with high sensitivity and specificity . As with Leydig cell tumors, the morphologic criteria for malignancy are not well established but increased size, vascular invasion, and increased numbers of mitotic figures are worrisome features.
Granulosa cell tumors — Granulosa cell tumors are rare neoplasms that morphologically resemble their ovarian counterparts, although studies suggest they may be molecularly distinct . Granulosa cell tumors of the testis are divided into adult and juvenile types, like in the ovary. In the testis, the juvenile type is almost exclusively seen in children under the age of two years (see below). These tumors can be difficult to classify, since their histologic and immunophenotypic (eg, inhibin positive) features can overlap with other SCSTs. Granulosa cell tumors may be hormonally active or inactive. (See "Testicular sex cord stromal tumors", section on 'Granulosa cell tumors'.)
●Adult type – The histologic features of the adult type of testicular granulosa cell tumor are similar to those seen in ovarian granulosa cell tumors. The cells have elongated, grooved nuclei, scant cytoplasm, and are arranged to form solid sheets and microfollicular structures (Call-Exner bodies) (picture 11). A reticulin stain should highlight groups of cells in a granulosa cell tumor and single cells in other SCSTs.
●Juvenile type – The juvenile type of granulosa cell tumor is the most common (albeit unusual) testicular tumor of infancy , and is only rarely seen in adults. They appear to be uniformly benign. The cells of the juvenile type granulosa cell tumor lack the prominent grooving of the adult type, they tend to have somewhat more cytoplasm, and they do not show typical Call-Exner body formation. Rather, the cells are arranged in solid sheets, nests, or nodules and often form ectatic spaces filled with eosinophilic or basophilic material reminiscent of large follicles.
Fibroma thecoma family of tumors — Examples of other types of mixed SCSTs in the testis resembling those seen in the ovary, such as fibroma-thecomas, have been reported but are very uncommon. (See "Sex cord-stromal tumors of the ovary: Epidemiology, clinical features, and diagnosis in adults" and "Testicular sex cord stromal tumors", section on 'Other tumor types'.)
Mixed and other sex cord-stromal tumors
Mixed sex cord-stromal tumor — As with GCTs, mixed types of SCSTs can contain multiple components, each of which has features characteristic of the various subtypes of SCSTs (eg, Sertoli-Leydig cell tumor). (See 'Sex cord-stromal tumors' above.)
Signet ring stromal tumor — Signet ring stroma tumors are classified as their own entity, despite their likely Sertoli cell lineage, based on the 2022 WHO classification scheme for testicular tumors [2,3,90]. Testicular signet ring stromal tumors are morphologically similar to tumors seen in the ovary (microcystic stromal tumor) and pancreas (solid pseudopapillary neoplasm). These tumors all demonstrate signet ring-like cells (ie, they lack intracytoplasmic mucin which is seen in true signet ring cells from an adenocarcinoma) associated with a thick fibrous capsule and fibrous septa. Signet ring stroma cell tumors harbor mutations of the CTNNB1 gene and therefore demonstrate nuclear staining for beta-catenin. Signet ring stromal tumors are also positive for cyclinD1, CD10, S100, and synaptophysin on immunohistochemistry; they are negative for inhibin, calretinin, desmin, and melanA. Signet ring stromal tumors are typically indolent and/or benign. However, a metastatic signet ring adenocarcinoma should always be excluded.
Myoid gonadal tumor — Although rare, myoid gonadal tumor is a distinct entity in the 2022 WHO classification of testicular tumors . Myoid gonadal stromal tumors are comprised of an intratubular, short fascicular spindle cell proliferation with myoid differentiation. Atypia, necrosis, and mitoses are infrequently seen; recurrence has not been reported (ie, benign tumors). Myoid gonadal tumors typically express SMA, S100, FOXL2, SF1, desmin and focal inhibin on immunohistochemistry. Molecular studies have also shown recurrent chromosomal copy number changes involving chromosomes 3, 6, 7, 8, 9, 11, 12, 14q, 15q, 17, 18q, 20, and 21q .
Sex cord-stromal tumor NOS — Occasionally, tumors have histologic aspects of one or more types of SCSTs, but without sufficient characteristic features to merit categorization as any recognized entity. These are termed "SCST, unclassified type" or "SCST not otherwise specified" (NOS). A significant fraction of such tumors proves to be malignant, particularly in adults.
PARATESTICULAR TUMORS — A variety of epithelial, mesothelial, and soft tissue tumors can develop in the rete testis and paratesticular tissues .
Mesothelial lesions — Mesothelial lesions involving the paratesticular region include mesothelial cysts, reactive mesothelial hyperplasia, adenomatoid tumors, benign cystic mesothelioma, well-differentiated papillary mesothelial tumor, and malignant mesothelioma . The most common is the adenomatoid tumor, which is a benign mesothelial proliferation involving the testis and/or the paratesticular tissues, often in an infiltrative fashion. This infiltrative quality can lead to the erroneous assessment of this tumor as malignant.
Epithelial tumors — Rare epithelial tumors of the paratesticular tissues include benign and malignant epithelial tumors of the rete testis, serous carcinoma, endometrioid adenocarcinoma, and mucinous cystadenocarcinoma tumors that are similar in appearance to those seen in females. Serous borderline tumors of the paratestis are histologically identical to their ovarian counterpart, and are thought to behave similarly . (See "Epithelial carcinoma of the ovary, fallopian tube, and peritoneum: Histopathology", section on 'Overview of borderline neoplasms'.)
OTHER TUMORS — Brenner tumors (transitional cell carcinomas), papillary cystadenoma, desmoplastic small round cell tumor, lymphomas, leukemias, plasmacytomas, fibromas, vascular tumors, and sarcomas (eg, rhabdomyosarcoma) have all been rarely described in the testis or paratesticular tissues . Metastatic deposits of cancers originating in a wide variety of organs, particularly the prostate and lung, have also been observed.
Testicular lymphoma — Primary testicular lymphoma is a unique and aggressive extranodal non-Hodgkin lymphoma with a high incidence of bilateral involvement, and a propensity for extranodal spread to the skin, subcutaneous tissue, bone marrow, central nervous system, and lung [95-100]. In males over 60 years old, malignant lymphoma is the most common cause of a testicular mass (image 1). Large cell diffuse B-cell types account for the majority of cases. In a series of 12 cases , seven patients presented at Ann Arbor stages I/II and five patients presented at Ann Arbor stages III/IV. All patients reached complete remission after receiving doxorubicin-based chemotherapy. The rate of relapse was 8 percent and the progression-free survival at 10 years was 88 percent. (See "Clinical presentation and initial evaluation of non-Hodgkin lymphoma", section on 'Physical examination' and "Initial treatment of limited stage diffuse large B cell lymphoma", section on 'Testicular'.)
Leukemia and plasmacytoma — In 5 percent of boys with acute lymphoblastic leukemia, the testis is involved either at presentation or as a site of relapse following initial successful induction therapy [102,103]. Less commonly, adults with acute promyelocytic leukemia may have isolated testicular relapse . Primary testicular plasmacytoma is an extremely rare tumor [105,106]. (See "Solitary extramedullary plasmacytoma".)
SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Testicular cancer".)
●Pathologic classification – Testicular neoplasms are the most common solid malignancy among males between the ages of 15 and 35. Pathologic subtypes are organized according to the World Health Organization (WHO) classification of testicular tumors (table 1). (See 'Introduction' above.)
●Germ cell tumors – Germ cell tumors (GCTs) account for most cases of testicular cancer and are mainly classified as follows (see 'Germ cell tumors' above):
•Noninvasive germ cell neoplasia – These include germ cell neoplasia in situ (GCNIS) and gonadoblastoma. (See 'Noninvasive germ cell neoplasia' above.)
•Seminomatous GCTs – These are pure seminomas without other GCT components. (See 'Seminomatous germ cell tumors' above.)
•Non-seminomatous GCTs – These include embryonal carcinoma; yolk sac tumor, postpubertal type; choriocarcinoma; teratoma, postpubertal type; and teratoma with somatic-type malignancy, and mixed GCTs, among others. (See 'Non-seminomatous germ cell tumors' above.)
Most nonseminomatous GCTs are mixed GCTs, with many possible combinations of seminoma, teratoma, embryonal carcinoma, yolk sac tumor, and choriocarcinoma. (See 'Mixed germ cell tumors' above.)
●Testicular sex cord-stromal tumors – These include Leydig cell tumors, Sertoli cell tumors, and Granulosa cell tumors, among others. (See 'Sex cord-stromal tumors' above and "Testicular sex cord stromal tumors".)
●Less common testicular tumors – A variety of other pathologic entities also exist such as GCTs unrelated to GCNIS, paratesticular tumors, metastatic deposits from other cancers, and testicular lymphoma. (See 'Germ cell tumors unrelated to germ cell neoplasia in situ' above and 'Paratesticular tumors' above and 'Other tumors' above and 'Testicular lymphoma' above.)
ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Michael Weinstein, MD, PhD, who contributed to earlier versions of this topic review.
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