S6295 Tumor Melanosis Following Treatment of Gastric Melanoma With Pembrolizumab

Introduction: Tumor melanosis (TM) is a rare manifestation found at the site of completely regressed melanoma. Although these lesions have a phenotypic appearance of melanoma, biopsies will demonstrate aggregates of melanin-laden macrophages in the absence of malignant melanocytes. Few cases of TM have been described in the literature to date. Here we present a rare case of a 75-year-old man with TM following treatment of gastric melanoma with pembrolizumab. Case Description/Methods: A 75-year-old man presented to the hospital for evaluation of melena and acute anemia. Esophagogastroduodenoscopy (EGD) demonstrated multiple darkly pigmented ulcerated gastric masses (largest measuring 3-4 cm) in the body and fundus. Biopsies revealed an epithelioid neoplasm in the submucosa with intracytoplasmic pigment, suspicious for melanin. Immunohistochemical staining confirmed Melan-A, SOX10, and S100, consistent with the diagnosis of gastric melanoma. After completion of pembrolizumab therapy, repeat EGD was performed and a 3 mm pigmented lesion was seen in the gastric fundus (corresponding to 1 of the sites of prior gastric melanoma), surrounded by a few pigmented satellite lesions 1-2 mm in size. Biopsies of these lesions were positive for Fontana-Masson stain, suggesting a melanin-like pigment, as well as CD68 confirming the presence of macrophages. Melan-A was negative, and this excluded the possibility of residual melanoma. Overall, the histologic findings were consistent with TM. Discussion: TM is a rare condition and most cases reported in the literature involve the skin. Our case will add to the limited number of cases of gastric TM reported to date. Histopathological examination is crucial in differentiating residual melanoma from TM.