Olfactory neuroblastoma

(A) Well-differentiated olfactory neuroblastoma (Hyams Grade I). Beneath the respiratory epithelium is a slightly lobulated diffuse growth of tumor. The cells are small with uniform nuclei and indistinct cytoplasmic borders. The background is diffusely fibrillary with pseudorosette formations.
(B) High power photomicrograph of olfactory neuroblastoma, also called esthesioneuroblastoma. When present, rosette formation (arrows) and a neurofibrillary background are helpful features in separating these lesions from other small blue cell tumors.