Abstract

Spindle cell lipoma, mammary type myofibroblastoma, and cellular angiofibroma show varied histologic features, but are part of the rapidly expanding category of tumors with monoallelic deletion of retinoblastoma gene (Rb1). Traditionally, these lesions have been designated as separate entities based on morphology, however, the loss of Rb1 expression, which can be shown with immunostaining, supports a more general classification as Rb-1 deleted soft tissue tumors. The current case is that of 70-year-old man who presented with a freely mobile and progressively enlarging postauricular mass. During excision, the lesion resembled an encapsulated lipomatous tumor. Biopsy revealed bland spindle cells arranged in parallel arrays, forming fascicles in a background of ropy collagen fibers, and scattered mature adipocytes. The vascular component consisted of small to medium-sized vessels, with prominently hyalinized walls and a subtle staghorn pattern. Immunohistochemistry revealed diffuse positivity for CD34 and Rb-1. STAT6 immunostaining was negative, and FISH testing for PDFGB was negative. While the histologic features are more characteristic of cellular angiofibroma of the vulvovaginal and inguinoscrotal soft tissue, the clinical presentation, including location, male gender, and age, are more typical of a spindle cell lipoma. This case is an instructive example of how molecular classification can provide a more unified understanding of these neoplasms which often show overlapping morphologic and immunohistochemical features.

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