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Case Reports

Abstract

Histiocytic sarcoma is a rare, poorly differentiated, round-cell malignancy that sometimes manifests in the skin. These tumors originate from a hematopoietic lineage and may coincide with lymphoma or leukemia. An 82-year-old male presented with a 1.9 cm flesh-colored nodule on the right cheek and a contiguous 1.4 cm light brown pigmented macule. The nodule extended from the dermis to the subcutis and was comprised of sheets of large, non-pigmented, round cells with anaplastic nuclei and amphophilic cytoplasm. The cells were positive for monocyte markers CD68 and CD163, with focal staining for CD45, CD4, and CD30. Additionally, they demonstrated diffuse, strong positive staining for PD-L1 and lacked BRAFV600E. Epithelial, melanocytic, and muscle differentiation markers were negative. The adjacent pigmented macule was comprised of pigmented epithelioid cells lacking BRAFV600E and was diagnosed as melanoma in situ. Molecular analysis of the nodule showed a high mutational burden with loss of function mutations in TP53, NF1, BRAF p.G466V, copy number loss of CDKN2A, and TERT promoter mutation, among others. The nodule’s histology, juxtaposition to melanoma in situ, and molecular signature are features consistent with a dedifferentiated melanoma. However, the presence of histiocytic markers is unexpected in dedifferentiated melanoma, favoring a diagnosis of histiocytic sarcoma. Furthermore, judicious consideration of mutational profiles is crucial for classifying poorly differentiated or undifferentiated skin tumors, as mutations common to dedifferentiated melanoma may also occur in histiocytic sarcoma. The presence of strong PD-L1 expression indicates the potential utility of anti-PD1/PD-L1 immunotherapies to augment treatment outcome.