A Rare Cutaneous Presentation of Malignant Tenosynovial Giant Cell Tumor.

Malignant tenosynovial giant cell tumor (MTGCT) is a rare and aggressive variant of tenosynovial giant cell tumors, with fewer than 60 reported cases. Although typically localized to joints and soft tissues, cutaneous presentations of MTGCT are exceedingly rare, with only a handful of documented cases involving direct dermal invasion. Here we report the case of an 88-year-old man with a history of nonmelanoma skin cancers who presented with a friable, ulcerated 2.5 cm nodule on the midchest. The lesion, present for approximately 1 month, was excised and found to be a malignant TGCT. Histopathology revealed a neoplasm with multinucleated giant cells and malignant features such as numerous and atypical mitotic figures, necrosis, and severe cellular pleomorphism. Immunohistochemistry showed positivity for CD45 and CD68, with weak partial expression of smooth muscle actin, and negative for CD34, P40, SOX10, pancytokeratin, CD163, CD1a, S100, and Melan-A, confirming the diagnosis. The patient underwent complete surgical excision. This case highlights a rare presentation of MTGCT with direct cutaneous involvement, adding to the sparse literature on this malignancy. Early recognition and accurate diagnosis of such unusual presentations are crucial because of the tumor's aggressive potential.