Synchronous multiple primary malignancies or transdifferentiation?—A diagnostic challenge in a case of Xeroderma pigmentosum

A diagnosis of synchronous tumors is always questionable when they show similar histology. We present a case of synchronous tumors, i.e. melanoma (MM) arising from the scalp and angiosarcoma (AS) from the cheek in a patient of Xeroderma pigmentosum (XP). The presence of melanin in AS of the cheek led to a differential diagnosis and a possibility of transdifferentiation of malignant melanoma in the first instance. Immunohistochemistry (IHC) was negative for melanoma markers (Melan A, SOX-10 & S 100) and positive for vascular markers (ERG, CD31 & CD34) ruling out transdifferentiation and concluding the cheek swelling as angiosarcoma. The presence of non-cutaneous malignancies is rare in XP and the presence of MM in a nearby site and the melanin pigment in the cutaneous malignancy mislead the diagnosis. The morphology of the tumor was the key which led to the further workup of the case.