Abstract

Fibrofolliculomas and trichodiscomas are hamartomatous lesions occurring as asymptomatic papules most commonly on the face and neck. These lesions exist on a morphologic spectrum with varying compositions of folliculosebaceous epithelium and perifollicular mesenchyme. Trichodiscomas often demonstrate features similar to spindle cell lipoma with admixed adipocytes, myxoid stroma, and ropey collagen. A 58-year-old man presented with a lesion on the nose clinically suspicious for basal cell carcinoma. An initial shave biopsy specimen reviewed at an outside hospital showed a cytologically atypical spindle cell proliferation that was negative for cytokeratins and melanocytic markers, resulting in a diagnosis of malignant spindle cell neoplasm. The differential diagnosis included pleomorphic dermal sarcoma (PDS) and atypical fibroxanthoma (AFX). Examination of the excised lesion at our institution revealed a pleomorphic and spindle cell tumor in the dermis which extended into the underlying skeletal muscle. The tumor was associated with a fibromyxoid stroma, scattered adipocytes, and importantly, hyperplastic folliculosebaceous epithelium at the periphery. The pleomorphic tumor cells showed hyperchromatic nuclei with smudgy chromatin. Overall, the cellularity was less than would be expected for PDS/AFX. Furthermore, the tumor cells were strongly positive for CD34 and showed diffuse loss of Rb. Together with this immunohistochemical profile and absence of appreciable mitotic activity, the nuclear changes were favored to represent degenerative atypia rather than evidence of malignancy. Consequently, a diagnosis of benign CD34-positive pleomorphic spindle cell tumor was rendered, with features overlapping between spindle cell/pleomorphic lipoma and trichodiscoma. This case highlights an important diagnostic pitfall that may be avoided by recognizing characteristic architectural features of this spectrum of lesions with selection of confirmatory immunohistochemical stains.

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