Abstract

Chondroid lipoma is an uncommon soft tissue neoplasm composed of lipoblastic cells, mature adipocytes and finely granular eosinophilic cells, within a myxochondroid matrix. The neoplastic cells are S100 and focally, CD68 positive. The differential diagnosis is typically broad and includes malignant and benign entities. We report a case of an S100 negative, TFE3 positive chondroid lipoma composed of cells with abundant coarse granular eosinophilic cytoplasm with focal myxochondroid matrix, histomorphologically and immunohistochemically masquerading as a PEComa. The tumor arose in the temple of a 55-year-old woman as two separate encapsulated masses, one superficial to the other, differing in color and composition. The superficial mahogany mass was composed of nested round cells with fine to coarse eosinophilic cytoplasmic granules and few mature adipocytes. The deeper yellow lobulated mass was composed of lipoblastic cells with scattered granular cells similar to the superficial mass. Focal myxochondroid matrix was identified in both masses. A battery of stains was performed and showed the granular tumor cells to be CD68, HMB-45, and TFE3 positive. Both the lipoblastic and granular cells were S100 negative. The immunophenotype was suggestive of PEComa; however, Next gene sequencing was performed and revealed a structural genomic alteration between the C11ORF95 and MRTFB genes [t(11;16)(11q13.1;16p13.12)]. This specific translocation has only been described in chondroid lipomas. This case highlights the varied histomorphology of chondroid lipomas, and TFE3 positivity as a potential pitfall. Careful inspection of the tumor for myxochondroid matrix is an important distinguishing element, as is the specific translocation.

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