Warning of the poor effect of chemotherapy: primary gastric malignant melanoma misdiagnosed as "gastric adenocarcinoma": a case report and literature review.

Primary gastric melanoma is a rare clinical finding. Its diagnosis can be challenging due to the complex tissue structure of the stomach and the diverse morphology of melanoma cells, which contribute to the increased likelihood of misdiagnosis. Here we report a 69-year-old female patient with primary gastric melanoma, initially misdiagnosed as "gastric adenocarcinoma". The patient initially received two cycles of neoadjuvant therapy with Tegeo in combination with oxaliplatin (SOX). Unfortunately, the response was not satisfactory. In order to guide further treatment decisions, immunohistochemistry (IHC) and next-generation sequencing (NGS) were conducted. Immunohistochemistry: CK (-), CK8/18 (-), Vimentin (+), CD56 (-), CgA (-), Syn (-), ALK (-), CD30 (-), S-100 (+), INI1 (expression), CD43 (-), CD3 (-), CD20 (-), Brg-1 (expression), CD117 (-), DOG-1 (-), CD34 (-), HMB-45 (+), melan-A (+), Her-2 (-). NGS showed no significant abnormalities in HER-2 and wild type in the range of BRAF gene detection. Through a Multidisciplinary Team (MDT) consultation, she was finally diagnosed with "primary gastric malignant melanoma". Despite the disease progressing after first-line chemotherapy with temozolomide, cisplatin, and bevacizumab, a second-line treatment with albumin paclitaxel, carboplatin, and bevacizumab resulted in a sustained partial remission (PR). The patient's condition was stable during the follow-up period. As far as we know, this is a rare case of primary gastric malignant melanoma which received persistent PR after second-line treatment. Combined with the pathological manifestations of MM and recent clinical studies, the pathological features and treatment of primary gastric malignant melanoma were discussed.