Variants of mycosis fungoides

INTRODUCTION — Mycosis fungoides (MF) is the most common cutaneous T cell lymphoma (CTCL) [1]. Patients with classic MF, as originally described by Alibert and Bazin two centuries ago, initially present with erythematous patches and/or plaques (picture 1A-B), which represent the early stage of MF. Clinically, the patches and plaques vary in size and shape, typically have a scaly surface that may show variable degrees of atrophy, and are often located on sun-protected areas of the body.

It has long been recognized that early MF can mimic common dermatoses, such as psoriasis and atopic dermatitis. However, in the last few decades, the list of differential diagnoses has considerably widened, due to the description of numerous atypical variants of MF that show the clinical features and/or histopathologic patterns of all the major inflammatory skin diseases [2-6]. In this topic, we will review the clinical features, diagnosis, and management of these three well-defined subtypes of MF as well as of a number of rare MF variants. Classic MF and Sézary syndrome are discussed separately.

●(See "Clinical manifestations, pathologic features, and diagnosis of mycosis fungoides".)

●(See "Staging and prognosis of mycosis fungoides and Sézary syndrome".)

●(See "Clinical presentation, pathologic features, and diagnosis of Sézary syndrome".)

●(See "Treatment of early stage (IA to IIA) mycosis fungoides".)

●(See "Treatment of advanced stage (IIB to IV) mycosis fungoides".)

CLASSIFICATION AND GENERAL CONSIDERATIONS — The 2005 World Health Organization-European Organisation for Research and Treatment of Cancer (WHO-EORTC) classification of primary cutaneous T cell lymphomas and its 2018 update recognize only three variants/subtypes of MF with distinctive clinicopathologic features, clinical behavior, and prognosis [1,7,8]:

●Folliculotropic mycosis fungoides

●Pagetoid reticulosis

●Granulomatous slack skin

However, in the last few decades, numerous atypical variants of MF that show the clinical and/or histopathologic features of inflammatory skin diseases have been described [2-6]. This is not unexpected, as in the early stage of MF, the number of malignant T cells is small, and the dermal infiltrate consists mainly of reactive T lymphocytes, which produce inflammatory cytokines that are partly responsible for the large variety of histologic patterns observed in MF.

All major histopathologic patterns described for inflammatory skin diseases have been found in MF [6]. As MF can be a "great imitator," a high index of suspicion, as well as clinical, histopathologic, immunohistochemical, and molecular studies, are necessary for the accurate diagnosis of unusual variants of MF.

The principles of staging for MF variants are similar to those used for classic MF [9]. (See "Staging and prognosis of mycosis fungoides and Sézary syndrome".)

FOLLICULOTROPIC MYCOSIS FUNGOIDES — Folliculotropic mycosis fungoides (FMF) is the most common subtype of MF in adults. It is categorized as a distinct clinicopathologic variant in the 2005 classification of the World Health Organization-European Organisation for Research and Treatment of Cancer (WHO-EORTC) and characterized by the presence of follicle-based lesions and folliculotropism with or without follicular mucinosis as the dominant histopathologic findings [1,10].

FMF is also a common variant among pediatric patients with MF. In four studies, FMF features were found in 26 to 43 percent of patients younger than 18 years [11-14]. FMF may be the sole manifestation of the disease or may occur in conjunction with classic or other variants of MF.

Clinical presentation — The clinical manifestations are variable and include follicle-based patches, plaques, infiltrated plaques (picture 2), tumors, and prurigo nodularis-like lesions, as well as keratosis pilaris-like lesions (picture 3) and acneiform lesions (comedones, cysts) [11,12,15-24]. Alopecia is a common accompanying feature, although it is not infrequently seen also in classic MF (picture 4). In children, the most common manifestation is the presence of hypopigmented patches with follicular accentuations and alopecia.

FMF preferentially involves the head and neck [16-18]. However, a significant portion of patients display lesions on the trunk and limbs as the sole site of involvement [11,21-23]. Patients with FMF experience significant pruritus [15-23] and occasional superimposed Staphylococcus aureus infection [11].

Diagnosis and differential diagnosis — The diagnosis of FMF is based upon clinical findings and the examination of a skin biopsy. One study reported a mean time of 2.8 (range 0.3 to 10) years between the onset of the eruption and the biopsy-proven diagnosis of FMF [23].

Histology shows perifollicular infiltrates of variable density around the infundibulum and/or infrainfundibulum, which usually spare the bulbar area, accompanied by folliculotropism of atypical T cells. In most cases, the epidermis is only minimally involved or spared (folliculotropism instead of epidermotropism). Immunohistochemistry shows in almost all cases a CD4⁺ phenotype [11,15-23].

The clinical differential diagnosis of FMF depends upon the clinical presentation [2,11,19,24-26]:

●Alopecic lesions on the scalp – Alopecia areata, trichotillomania, and cicatricial alopecias

●Follicular spiky papules – Keratosis pilaris, lichen spinulosus, pityriasis rubra pilaris, and lichen planopilaris

●Hairless patches/flat plaques – Alopecia mucinosa, also known as idiopathic follicular mucinosis (picture 5A-B)

●Acneiform lesions – Favre-Racouchot syndrome, chloracne, follicular-comedogenic graft-versus-host disease, adult-onset acne, Demodex-induced follicular mucinosis, and lichen planus follicularis tumidus

In children, in whom FMF most often presents with hypopigmented patches with follicular accentuation and associated alopecia, the main differential diagnosis is alopecia mucinosa, which may mimic FMF both clinically and histopathologically [11].

Staging and prognosis — FMF has been traditionally considered an aggressive variant of MF, based upon earlier series reporting a greater risk of disease progression and worse prognosis, similar to that of tumor-stage MF [16-18]. However, subsequent studies suggest that this is not always the case [11,19,20,23,24].

One study performed by the authors of this topic review (including a series of 49 consecutive patients with FMF) revealed that FMF may present with two distinct patterns of clinicopathologic features, each with different prognostic implications [23]:

●Early-stage FMF, characterized by patches and/or flat plaques, keratosis pilaris-like lesions, and/or acneiform lesions mainly located on the trunk and/or limbs; relatively sparse perifollicular infiltrates on histology; and a favorable prognosis, similar to that of early MF.

●Tumor-stage/advanced-stage FMF, characterized by infiltrated plaques and/or tumors located mainly in the head and neck region, much heavier perifollicular infiltrates than early-stage FMF on histology, and survival rates similar to those of classic tumor-stage MF.

The estimated five-year survival rate was 94 percent for early-stage and 69 percent for tumor-stage FMF [23].

These observations have been confirmed by an additional study of 203 patients with FMF [27]. Among the 186 patients with skin-limited disease at the time of the diagnosis, 67 (36 percent) presented with lesions of early-stage FMF. The 10-year, disease-specific survival rates for patients with early-stage FMF and advanced-stage FMF were 93 and 40 percent, respectively. In addition to clinical stage, other unfavorable prognostic factors included age >60 years at diagnosis and presence of extensive secondary bacterial infection. Interestingly, in almost all juvenile cases reported in two studies, FMF presented with early-stage, superficial lesions on the trunk or limbs and had an indolent course [11,13].

Similar results have been reported in another small study that included 42 patients with FMF [28]. The estimated 5-year, 10-year, and 15-year, disease-specific survival rates for early-stage cutaneous disease were all 96 percent. For advanced-stage cutaneous disease, both the 5-year and 10-year, disease-specific survival rates were 70 percent (95% CI 41-98 percent), and the 15-year rate was 53 percent (95% CI 16-89 percent).

Taken together, the results of these studies suggest that the clinical staging system used for classic MF (table 1) may not be suitable for FMF, as it does not capture the unique clinicopathologic features of FMF lesions. (See "Staging and prognosis of mycosis fungoides and Sézary syndrome".)

Management — Based upon the prevailing notion that FMF is associated with higher risk of disease progression and that the perifollicular neoplastic infiltrates are less accessible to skin-targeted therapies alone, it has been suggested that FMF be treated as tumor-stage MF. (See "Treatment of advanced stage (IIB to IV) mycosis fungoides".)

However, several studies suggest that psoralen and ultraviolet A (PUVA) phototherapy, as monotherapy or in combination with interferon or systemic retinoids, may be beneficial for patients with superficial lesions who have in fact early-stage FMF [11,19-21]:

●A large, retrospective study of 203 patients with FMF from the Dutch Cutaneous Lymphoma Registry examined the response to various types of treatments in patients with early-stage FMF (n = 84); advanced-stage, skin-limited FMF (n = 102); and extracutaneous localizations at first presentation (n = 17) [29]. Among patients with early-stage FMF treated with skin-directed therapies only, including topical corticosteroids, topical nitrogen mustard, ultraviolet B (UVB) phototherapy, and PUVA therapy, 29 and 57 percent experienced complete remission or partial remission, respectively. The 10-year, disease-specific survival was 93 percent. None of these patients had disease progression. The majority of patients with advanced, skin-limited FMF were treated with PUVA with or without systemic retinoids, interferon-alpha, or local radiotherapy; local radiotherapy alone; or total skin electron beam therapy. In this group, the complete remission and partial remission rates were 25 and 50 percent, respectively, with a 10-year, disease-specific survival of 40 percent.

●The authors of this topic reported on the treatment outcomes in a retrospective cohort of 27 adults with early-stage FMF treated with PUVA monotherapy [30]. Seventy percent achieved complete remission, and 26 percent achieved partial remission. The complete remission rate was similar to that achieved in a series of 18 adult patients with classic plaque-stage MF, although the early-stage FMF group required more treatments and a higher cumulative dose of ultraviolet A (UVA).

It should be emphasized that narrowband ultraviolet B (NBUVB) phototherapy is inadequate for the treatment of FMF, as the UVB radiation penetrates the epidermis and superficial dermis but cannot reach the deep adnexal neoplastic component of FMF, thus leaving residual disease despite the improvement of the epidermal component [11,29,30].

PAGETOID RETICULOSIS — Pagetoid reticulosis (PR), also called Woringer-Kolopp disease, is a variant of MF presenting with slowly growing, localized patches or plaques with a psoriasiform or hyperkeratotic appearance, usually located on the distal extremities [1]. Histopathology reveals a psoriasiform and sometimes verrucous hyperplasia with marked pagetoid (intraepidermal) spread of highly epidermotropic, large atypical lymphocytes, singly or in nests, which occupy the entire thickness of the epidermis. The neoplastic cells are CD3⁺ expressing a CD4⁺CD8^- or CD4^-CD8⁺ or CD4^-D8^- phenotype [31-34].

The clinical differential diagnosis of PR includes papulosquamous, infectious, and neoplastic conditions. On histopathology, PR may mimic superficial spreading melanoma, pagetoid squamous cell carcinoma in situ, or extramammary Paget disease.

PR generally follows a benign course, usually with no recurrence after curative intent treatment (ie, excision, localized radiotherapy) [35]. Rare cases of local recurrence or relapses at distant cutaneous sites have been reported [36].

GRANULOMATOUS SLACK SKIN — Granulomatous slack skin (GSS) is an extremely rare clinicopathologic subtype of MF [1]. It is characterized by the slow development of bulky, infiltrated, pendulous folds of atrophic skin in the intertriginous areas (axillae and groins) reminiscent of cutis laxa [37].

GSS shares the histopathologic features of granulomatous mycosis fungoides (GMF), although the number of multifocal giant cells is much greater in GSS than in GMF, and many display 20 to 30 nuclei per cell, which is considered an almost pathognomonic sign of GSS [3]. (See 'Granulomatous mycosis fungoides' below.)

Loss of elastic fibres, elastophagocytosis, and emperipolesis (engulfment of lymphocytes) are additional characteristic findings in GSS [1,3,37]. Neoplastic lymphocytes can display a CD4⁺ or, less commonly, a CD8⁺ phenotype.

Despite the diffuse infiltrate that involves the entire dermis, GSS has a slowly progressive course with rare cases developing extracutaneous infiltration.

There is no standard of therapy for GSS. Topical nitrogen mustard, psoralen and ultraviolet A (PUVA), radiotherapy, surgical excision, methotrexate, bexarotene, and systemic therapies have all been used, depending upon the disease stage [38]. Patients with GSS require lifelong observation due to the increased risk of a second lymphoid malignancy, most commonly Hodgkin lymphoma [37,39].

OTHER CLINICOPATHOLOGIC VARIANTS

Unilesional (solitary) mycosis fungoides — MF is typically a multifocal disease. However, rare cases of solitary lesions with the clinicopathologic features of MF have been reported in both adults and children [36,40-43].

Unilesional or solitary MF usually manifests as a single erythematous, scaly patch or plaque predominantly located on non-sun-exposed areas [40]. However, hypopigmented, eczematoid, psoriasiform, poikilodermatous, and folliculotropic solitary lesions have also been reported [36,40,41,44-48]. In many patients, the lesion has been present for several years before the diagnosis.

In the context of a solitary lesion, the evaluation of clinical, morphologic, and histologic features are essential for the diagnosis of MF. The demonstration of T cell clonality may support the diagnosis. Clinical differential diagnosis of unilesional MF includes papulosquamous or eczematous lesions, dermatophyte infection, and Bowen's disease.

Unilesional MF generally follows a benign course, usually with no recurrence over variable follow-up periods after curative intent treatment (ie, excision or localized radiotherapy). Rare cases of local recurrence or, even more rarely, relapse at distant cutaneous sites have been reported [36,37,40,41,44,45].

In contrast with classic MF, extracutaneous spread of unilesional MF has never been reported. Whether unilesional MF is a localized manifestation of MF or a cutaneous T cell pseudolymphoma is still unclear. A study showed that compared with early MF lesions, unilesional MF is typified by high T helper (Th)1 cytokine profile and low Th2 dermal lymphocytes, and this robust, reactive T cell immune response in unilesional MF might account for the localized nature of this disease [49]. (See "Cutaneous T cell pseudolymphomas".)

Only pagetoid reticulosis (PR), but not unilesional MF, is listed as an MF subtype in the 2005 World Health Organization-European Organisation for Research and Treatment of Cancer (WHO-EORTC) classifications [1]. It remains to be seen whether the historical term "pagetoid reticulosis" will be included within unilesional MF in future classifications.

Granulomatous mycosis fungoides — Granulomatous reactions can be detected in MF cases, either at the time of the initial diagnosis or years later [50-52]. The pathogenesis of granuloma formation in MF is unknown, although it is recognized that granulomas can be induced by Th1 or Th2 cells in various diseases, including lymphoproliferative disorders [51]. Treatment of MF with interferon or bexarotene has been associated with the development of a granulomatous reaction [53,54].

Granulomatous mycosis fungoides (GMF) is mainly a histopathologic variant that may be found in histologic sections from patients with otherwise classical MF (including erythrodermic MF), as well as in unusual variants such as folliculotropic mycosis fungoides (FMF), poikilodermatous, follicular, ichthyosiform, and hyperpigmented MF [9,23,37,51].

In some cases, the clinical features may be suggestive of a granulomatous disease, as it may present as nonscaly, thick plaques or nodules, without the cutis laxa-like features typical of granulomatous slack skin. GMF may mimic benign granulomatous skin diseases, such as granuloma annulare, sarcoidosis, or granulomatous rosacea [2,55,56].

The EORTC pathologic criteria for GMF include prominent granuloma formation or numerous histiocytic giant cells or a histiocyte rich infiltrate defined by histiocytes accounting for more than 25 percent of the entire infiltrate [37]. The histopathologic patterns of GMF may vary and include epithelioid/sarcoidal, tuberculoid, periadnexal, granuloma annulare-like, palisaded, necrobiotic granuloma-like, and diffuse granulomatous infiltrate [37,51,56]. Loss of elastic fibers is a common finding, but elastophagocytosis is rare [37]. Immunohistochemically, most cases have a CD4⁺ phenotype [37,56].

The diagnosis of GMF may be difficult, particularly in cases in which the atypical lymphoid infiltrate is obscured by a predominant granulomatous component [57]. Moreover, in some cases epidermotropism, which is the major diagnostic clue for MF, may be absent, leading to a significant delay in the diagnosis and treatment [37,51,56]. The finding of a monoclonal T cell receptor gene rearrangement can support the diagnosis, although a monoclonal T cell clone can occasionally be found in non-neoplastic granulomatous disorders [58].

Scant data are available as to whether patients with GMF have a different prognosis compared with classic MF. In a retrospective case-control study of 27 patients with GMF and 54 age and stage-matched controls with classic MF, patients with GMF experienced more frequently disease progression and a poorer response to skin-directed therapies than those with classic MF [51].

GMF is associated with an increased risk of a second hematologic malignancy, most frequently a Hodgkin lymphoma, which can precede or follow the diagnosis of GMF by years or even decades [37,51].

Poikilodermatous mycosis fungoides — Poikiloderma is the combination of cutaneous atrophy, telangiectasia, and macular pigmentary changes, which result in a mottled skin appearance [59]. Poikilodermatous MF is usually a clinical variant of patch-stage MF, accounting for approximately 11 percent of all MF cases, according to one study [10]. It is predominantly located on the breast, hips, and buttocks in some, mainly affecting the flexural areas. However, it may also present with generalized involvement of diffuse poikilodermatous MF affecting over 80 percent of the skin in an "erythrodermic" pattern [60,61]. Poikilodermatous MF may be associated with patches and/or plaques of either classic MF or unusual variants [60,61]. A female predominance and lower age at diagnosis was found in poikilodermatous MF compared with classic MF [61].

On histopathologic examination, poikilodermatous MF shows findings of classic MF combined with changes of poikiloderma, such as epidermal atrophy, basal hydropic degeneration, pigment incontinence, and telangiectatic vessels [62,63]. Immunophenotypic analysis may show either predominance of CD8⁺ [64,65] or of CD4⁺ T cells [61].

Most reported patients had early-stage disease with nonaggressive clinical behavior and good response to phototherapy [64,65], although a few cases of tumor stage and erythroderma have been described [59,64,65]. In a large cohort study examining the outcome of 1502 patients with MF, poikilodermatous MF was associated with a reduced risk for disease progression and increased disease-specific survival [10].

Hypopigmented mycosis fungoides — Hypopigmented mycosis fungoides (HMF) accounts for approximately 3 percent of all MF cases in adults according to one large, cohort study [10]. It is the most common variant in childhood/adolescent MF and predominantly affects people with darkly pigmented skin [11,66-68]. HMF was the most common subtype (33 percent) in a Southeast Asian cohort of 239 patients with MF [69]. The loss of pigmentation has been attributed to a decrease in the number of melanocytes in lesional skin, due to a cytotoxic effect of CD8⁺ T cells, as in vitiligo [70,71]. It has been suggested that hypopigmentation may serve as a surrogate marker of an active immune response, specifically an antitumor immune response against malignant T cells in MF, regardless of the T cell phenotype [72]. (See "Vitiligo: Pathogenesis, clinical features, and diagnosis".)

HMF presents as round or irregular hypopigmented patches or flat plaques covered by fine scale, usually without atrophy. Lesions are usually asymptomatic or slightly pruritic and are mainly located on the trunk, buttocks, and limbs. On the upper limbs, in contrast with classic MF, lesions show a predilection for the dorsal, rather than the volar sun-protected surface [3,4,11,66,67,73-80].

Hypopigmented lesions may be the sole manifestation of MF or may occur in association with lesions of classic MF or other variants [11,78]. In the pediatric age group, hypopigmented lesions may be seen in combination with early-folliculotropic lesions [11].

Histopathologically, HMF lesions are indistinguishable from classic MF [11,66,67,74-77]. Striking epidermotropism of atypical lymphocytes as well as decreased melanin in the basal layer of the epidermis and melanin incontinence with melanophages in the dermis are observed in some cases [67]. In contrast with classic MF, which usually has a CD4⁺,CD8^- phenotype, HMF is often CD8⁺ [11,67,74]. It has been suggested that dominant clonal CD8⁺ T cells not showing malignant alteration in their transcriptome might be reactive [81].

The diagnosis of HMF may be difficult or delayed, due to its indolent course and close clinical resemblance to a wide range of benign conditions presenting with hypopigmented macules or patches, including vitiligo, tinea corporis, tinea versicolor, pityriasis alba, postinflammatory hypopigmentation, progressive macular hypomelanosis, idiopathic guttate hypomelanosis, sarcoidosis, and leprosy [3,11,67,72,78]. (See "Vitiligo: Pathogenesis, clinical features, and diagnosis" and "Acquired hypopigmentation disorders other than vitiligo".)

Because of the lack of specific signs and symptoms, a high index of suspicion and the examination of a skin biopsy are necessary for the diagnosis of HMF. Multiple skin biopsies may occasionally be needed for a definitive diagnosis.

HMF shows a good response to skin-directed therapies and favorable prognosis [10,11,66,67,72,74-77,79]. However, progression to tumor stage has been reported in a few cases.

Hyperpigmented mycosis fungoides — Hyperpigmented mycosis fungoides (HPMF) is a rare variant of MF, reported in a few cases and case series [82-86]. In individuals with darkly pigmented skin, it may not be as rare as reflected in the literature.

HPMF is usually characterized by hyperpigmented patches and/or plaques, some with ill-defined borders, with various degrees of skin atrophy and scaling [82-86]. These hyperpigmented lesions are usually the sole manifestation of MF, although some patients may have concomitant lesions of classic, purpuric, hypopigmented, poikilodermatous, or ichthyosiform MF [82-85].

Histopathologically, in addition to features of typical MF, basal hydropic degeneration changes together with melanophages are observed [82]. Diffuse vacuolar degeneration of basal keratinocytes mimicking "interface dermatitis" has been found in the majority of cases [82,87]. A band-like lymphocytic infiltrate lining up directly opposite to the dermoepidermal junction may also be noted [88].

In a small case series, most HPMF cases showed a CD8⁺ phenotype [82]. In another case series, double negative CD4^-CD8^- immunophenotypes were more frequent [86]. These cytotoxic T cells may affect neighboring melanocytes or basal keratinocytes, causing interface changes and marked melanin incontinence that results in hyperpigmentation [82].

HPMF may mimic a wide range of benign conditions, including postinflammatory hyperpigmentation, fixed drug eruption, pigmented contact dermatitis, erythema dyschromicum perstans (ashy dermatosis), cutaneous amyloidosis, atrophoderma of Pasini and Pierini, and idiopathic eruptive macular hyperpigmentation [89-91]. Hyperpigmentation induced by topical mechlorethamine, an alkylating agent used for the treatment of early-stage MF, should also be excluded [92].

Based upon very limited data, the clinical course of HPMF seems to be similar to that of classic early-stage MF, with good response to topical therapies and phototherapy and favorable prognosis [82-85].

Syringotropic mycosis fungoides — Involvement of the eccrine epithelium may be seen in some biopsies of FMF [1]. The term "syringotropic mycosis fungoides" refers to cases with prominent involvement of eccrine structures. Whether it constitutes a separate entity or falls within the spectrum of FMF is still matter of debate [93-95].

Ichthyosiform mycosis fungoides — Ichthyosiform MF may represent a specific clinical variant of MF rather than a paraneoplastic eruption [3,96-101]. (See "Cutaneous manifestations of internal malignancy", section on 'Acquired ichthyosis'.)

Patients typically present with diffuse, dry, scaling skin or well-circumscribed scaly patches or flat plaques affecting the trunk and extremities. The diagnosis of MF can be clinically suspected only in patients showing concomitant lesions of classic MF or other variants, usually FMF lesions [96,97,101].

Histologically, in addition to the typical features of MF, findings suggestive of coexistent ichthyosis vulgaris, such as parakeratosis and focally compact orthokeratosis with thinning or absence of the granular layer [96,101] and diminished filaggrin expression in the thin granular layer, are noted [102]. Most cases of ichthyosiform MF have a CD3⁺,CD4⁺ phenotype, with only a few showing a predominance of CD8⁺ lymphocytes [96-101].

Ichthyosiform MF is characterized in most cases by an indolent course and good prognosis [96,101]. Progression to tumor stage and large cell transformation have been reported in a few cases [102].

Mycosis fungoides palmaris et plantaris — Specific involvement of the palms and soles can be seen in the course of MF in approximately 10 percent of cases [103]. Yet MF that is limited predominantly to or initially presents on the palms and/or soles, a condition referred to as mycosis fungoides palmaris et plantaris (MFPP), has been only rarely reported [103].

Lesions are usually bilateral and present as erythematous hyperkeratotic patches and plaques with fissures and scales, with or without pruritus. Unusual clinical variations of MFPP include annular, hyperpigmented, vesicular, dyshidrotic, pustular, verrucous, psoriasiform, and ulcerative lesions and nail dystrophy [103-111]. If these changes are not accompanied by typical MF lesions elsewhere on the body, the diagnosis may be challenging.

On histology, the usual features of MF are found in MFPP. However, in the authors' experience, spongiosis may be more pronounced than in classic MF, making it difficult to differentiate MFPP from a spongiotic dermatitis. Immunophenotyping and analysis of T cell receptor clonality should be performed to confirm the diagnosis.

The differential diagnosis of MFPP includes primarily dermatophyte infection or inflammatory skin diseases localized to the palms and soles, such as psoriasis and eczematous dermatitides [112]. The diagnosis of MFPP should be suspected in long-standing cases of hand/foot dermatitis or psoriasis that are unresponsive to standard therapies or have atypical presentations [73,106,107,111]. Other possible differential diagnoses include secondary syphilis, hyperkeratotic lichen planus, verrucae, and granuloma annulare [73,107].

MFPP is predominantly a localized stage IA disease and as such has an indolent course, although cases with rapid progression beyond acral regions have been described [106]. Treatment modalities that have been used for MFPP include topical steroids, nitrogen mustard, topical or systemic retinoids, phototherapy, laser treatments, radiotherapy, methotrexate, and combinations of multiple agents and treatment modalities [106,107,109,113]. (See "Treatment of early stage (IA to IIA) mycosis fungoides".)

Papular and pityriasis lichenoides chronica-like mycosis fungoides — The papular variant of MF was initially defined in 2005 as papules with histopathologic findings of MF, absence of spontaneous regression, and no other evidence of MF or lymphomatoid drug reaction [114]. So far, less than 40 cases have been reported in the literature, with only a few subsequently developing typical patches or plaques of classic MF [115-124]. Lesions were typically widespread, often symmetric, and without apparent predilection for sun-protected areas. All reported cases of papular MF were CD30^- [115,124].

The differential diagnosis of papular MF includes primarily other clonal T cell lymphoproliferative disorders, such as lymphomatoid papulosis (LyP) type B and pityriasis lichenoides chronica (PLC) and, less often, persistent arthropod bite reactions and lymphomatoid drug eruption [115,124-126]. Whether papular MF is actually a variant of MF or an atypical form of LyP type B is somewhat controversial [115]. (See "Lymphomatoid papulosis" and "Pityriasis lichenoides chronica".)

PLC-like MF is a unique expression of papular MF, reported in a few adults and children [126-128]. It is characterized by papules that resemble PLC and histopathologic findings consistent with MF, including a dermal infiltrate composed of lymphocytes that are CD30^-. The main differential diagnoses are LyP type B and PLC.

Because evolution into typical MF has not been documented for any of the reported PLC-like MF cases, the alternative term "lymphomatoid pityriasis lichenoides chronica" has been proposed to describe this condition [114,125].

Pigmented purpuric dermatosis-like mycosis fungoides — Pigmented purpuric dermatosis (PPD)-like MF is a rare purpuric variant of MF, reported both in adults and children. It may be the sole manifestation of the disease or may appear in conjunction with classic lesions or unusual variants of MF [129-136]. It is unclear whether PPD-like MF represents a purpuric dermatosis evolving to MF or a primary MF presenting with clinical and histopathologic features of PPD [129-136]. (See "Pigmented purpuric dermatoses (capillaritis)", section on 'Relationship to mycosis fungoides'.)

On histopathologic examination, PPD-like MF reveals features of MF (ie, lichenoid infiltrate, epidermotropism, atypical lymphocytes) as well as extravasation of erythrocytes in the papillary dermis and presence of siderophages typically seen in PPD. Immunohistochemical analysis performed in a minority of the published cases showed CD4⁺, CD8⁺, or CD4^-,CD8^- double negative phenotypes [82,135,137].

Clinical clues to the diagnosis of PPD-like MF are the extensive distribution of lesions (in PPD, lesions are usually limited to the lower limbs), presence of reticular arrangement, and/or the coexistence of other lesions suspicious of large plaque parapsoriasis or MF [138,139].

Although MF-like histopathologic patterns can be observed in classic cases of PPD [139-141], the presence of intraepidermal lymphocytes larger than those in the dermis along with atypical lymphocytes in the dermis and papillary dermal fibrosis favor the diagnosis of MF [87]. The finding of a monoclonal T cell receptor gene rearrangement can be helpful to confirm the diagnosis, although T cell clonality may also be detected in otherwise classic cases of PPD [140,141]. It has been suggested that cases of PPD with T cell clonality but histopathologic features not consistent with MF should be regarded as cases of cutaneous T cell lymphoid dyscrasia and undergo a close clinical and histopathologic follow-up [3].

Bullous mycosis fungoides — Vesiculobullous lesions are a rare manifestation of MF, usually appearing months to years after the onset of classic MF/Sézary syndrome [142-147]. In rare instances, bullous lesions may be the primary manifestation of MF.

Bullous MF presents with flaccid or tense, often multiple blisters appearing on both lesional and normal-looking skin, sometimes with a tendency to form ulcers. The trunk and limbs are the areas predominantly affected.

Histopathology reveals subcorneal, intraepithelial, or subepidermal blisters, which may contain atypical T cells. The presence of typical features of MF (ie, epidermotropism with atypical lymphocytes) is the main clue to the diagnosis [143-146,148].

The differential diagnosis includes bacterial or viral infections, autoimmune bullous disorders, and drug eruptions. The coexistence of bullous pemphigoides or pemphigus foliaceous with MF has been reported in a few cases [142,149]. Of note, bulla formation can be an adverse effect of treatments for MF, such as topical mechlorethamine, systemic interferon, and psoralen and ultraviolet A (PUVA) [145].

The limited literature on this rare phenotype of MF generally portrays a negative prognosis [143-148].

Interstitial mycosis fungoides — Interstitial MF is a rare variant that may mimic histopathologically and, less often, clinically interstitial granuloma annulare (IGA), inflammatory morphea, or interstitial granulomatous dermatitis (IGD) [150-155]. (See "Granuloma annulare: Epidemiology, clinical manifestations, and diagnosis" and "Pathogenesis, clinical manifestations, and diagnosis of morphea (localized scleroderma) in adults".)

In the largest published series of interstitial MF, which included 21 patients, this histopathologic variant was detected in biopsies of otherwise conventional patches or plaques of MF and mostly as a transient histopathologic pattern [154]. However, based upon earlier reports and the authors' experience, the interstitial pattern is found in flat or slightly elevated plaques that, in contrast with classic MF, lack scaling and atrophy.

On histopathology, interstitial MF shows a dermal interstitial infiltrate composed mainly of CD3⁺ lymphocytes with variable amounts of CD68⁺ histiocytes that do not outnumber the lymphocytes and occasional mucin deposition. Epidermotropic lymphocytes are present at least focally in many but not in all cases. In a series of 21 patients with interstitial MF, a CD8⁺ phenotype was found in 9 of 18 tested cases [154].

IGA and IGD can be differentiated from interstitial MF by the predominance of histiocytes. Inflammatory morphea is differentiated from interstitial MF by the relative predominance of B cells and plasma cells in the interstitium and presence of sclerosis. The detection of T cell clonality by polymerase chain reaction (PCR) favors the diagnosis of MF. However, clonality has been detected, albeit rarely, also in granuloma annulare [156]. A rare coexistence of MF and IGA has been reported [157].

The clinical behavior, prognosis, and treatment are not established due to the rarity of this variant.

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: Primary cutaneous lymphoma".)

SUMMARY AND RECOMMENDATIONS

●Classification – The 2005 World Health Organization-European Organisation for Research and Treatment of Cancer (WHO-EORTC) classification of primary cutaneous T cell lymphomas and its 2018 update recognize three variants of mycosis fungoides (MF) with distinctive clinicopathologic features, clinical behavior, and prognosis: folliculotropic mycosis fungoides (FMF), pagetoid reticulosis (PR), and granulomatous slack skin (GSS). However, many atypical MF variants that share the clinical features and histopathologic patterns of all of the major inflammatory skin diseases have been described. (See 'Classification and general considerations' above.)

●Folliculotropic mycosis fungoides – FMF is characterized by the presence of follicle-based patches, plaques, infiltrated plaques (picture 2), tumors, prurigo nodularis-like lesions, keratosis pilaris-like lesions (picture 3), or acneiform lesions, in many cases with alopecia. Although FMF has been traditionally considered an aggressive variant of MF, an early-stage form, characterized by patches and/or flat plaques, keratosis pilaris-like or acneiform lesions, and favorable prognosis has been recognized. (See 'Folliculotropic mycosis fungoides' above.)

●Pagetoid reticulosis – PR, also called Woringer-Kolopp disease, presents as slowly growing, localized patches or plaques with a psoriasiform or hyperkeratotic appearance, usually located on the distal extremities. PR generally follows a benign course, with no recurrence after curative intent treatment. (See 'Pagetoid reticulosis' above.)

●Granulomatous slack skin – GSS is an extremely rare clinicopathologic subtype of MF characterized by the slow development of bulky, infiltrated, pendulous folds of atrophic skin in the intertriginous areas reminiscent of cutis laxa. Treatments include topical nitrogen mustard, phototherapy, radiotherapy, surgical excision, and systemic therapies. GSS is associated with an increased risk of a second lymphoid malignancy, most commonly Hodgkin lymphoma. (See 'Granulomatous slack skin' above.)

●Other variants – Other uncommon clinicopathologic variants include unilesional, granulomatous, poikilodermatous, and hypopigmented MF. Rare variants mimicking ichthyosis vulgaris, pityriasis lichenoides chronica, pigmented purpuric dermatosis, and bullous diseases have also been described. (See 'Other clinicopathologic variants' above.)