Tumoral melanosis after immunotherapy with pembrolizumab - a response sign mimicking melanoma.

Tumoral melanosis is a rare histopathological finding characterized by aggregates of melanophages, in the absence of melanocytes, usually observed in sites of regressed melanocytic lesions, including melanoma. A 72-year-old woman with a history of a completely excised melanoma on her right arm (T3bN0M0, Stage IIb) presented with clinically-evident regional lymph node metastasis. This was treated with right axillary lymphadenectomy. Subsequently, a 2-centimeter blue-colored patch over the excision scar was identified, along with a blue nodule within the posterior aspect of the same arm, consistent with in-transit metastases. Additional metastases on the right hilar region of the lungs were detected by PET/CT. Hence, the patient began immunotherapy with pembrolizumab. After three months, a second PET/CT revealed a complete response, but the patient maintained the blue-colored patch previously observed. Given the discrepancy between the clinical and metabolic response she underwent a skin biopsy; histological examination showed findings compatible with tumoral melanosis resulting from complete regression of a metastatic lesion. In cases of metastatic melanoma under immunotherapy with anti-PD1 agents, especially pembrolizumab, tumoral melanosis has been anecdotally associated with tumor regression and favorable treatment response. The patient has been maintained on pembrolizumab, accomplishing 15 cycles, and has had a complete response to date.