Balloon cell melanoma of the anal canal: a wolf in sheep's clothing?

Balloon cell melanoma (BCM) is a rare histologic variant of cutaneous malignant melanoma with exceptional reports of occurrences at non-cutaneous sites. Herein we present a case of primary amelanotic BCM of anal canal, a heretofore undescribed location. Histologically, the tumor was characterized by sheets of pale cells that bore striking resemblance to foamy macrophages. Presence of rare atypical mitoses confirmed the malignant nature of the cells. Neoplastic cells were immunoreactive for S100, Melan-A, and focally for HMB-45 while were negative for myogenic, gastrointestinal stromal tumor, epithelial and neuroendocrine markers. Resemblance to foamy macrophages, bland cytology and absence of pigment imparts this tumor a deceptively benign histological appearance making it prone to diagnostic pitfalls. Awareness of this rare entity and judicious employment of immunohistochemistry is imperative in segregating it from its diverse mimics.