Abstract

Atypical spindle cell/pleomorphic lipomatous tumors (ASPLT), recently shown to harbor loss of RB-1 expression, belong within a spectrum of lipomatous neoplasms with cytologic atypia and risk of local recurrence but without documented risk of metastasis. We report a case of a 70-year-old male without prior oncologic history presenting with a 2.5 cm enlarging subcutaneous mass of the left buttock. Histopathologically, the lesion demonstrated an infiltrative, nodular proliferation of atypical spindled-to-epithelioid cells with admixed adipocytic and cartilaginous differentiation in the dermis and subcutaneous tissue. A giant-cell rich atypical mononucleated cell proliferation was also present. The tumor, including the cartilaginous and giant-cell rich areas, demonstrated marked cytologic atypia and increased mitoses with atypical forms. No lipoblasts or atypical lipoblasts were identified. By immunohistochemistry, spindled tumor cells were positive for CD34 and p16 with absent expression of RB1 and p53 (null). MDM2, ERG, SOX10, desmin, calponin, EMA, PU.1 and pan-TRK were negative. The adipocytic component expressed S100. SATB2 highlighted cartilaginous and giant-cell rich areas. These findings are consistent with a diagnosis of RB-1 deficient ASPLT. The presence of heterologous chondroid and/or osseous differentiation is not uncommon in ASPLT, yet the accompanying sarcoma-like cytologic atypia and brisk mitotic activity in these components represent highly uncommon morphologic findings. Given that there have been no reported cases of de-differentiation or malignant transformation to date, expansion of the spectrum of morphological features that can be observed in ASPLT is necessary to avoid miscategorization of these tumors as sarcomas.

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