Pseudoprogression of liver metastases and mixed response in primary gastric melanoma treated with nivolumab: A case report

Primary gastric melanoma (PGM) is an extremely rare malignancy with a poor prognosis, and evidence regarding the efficacy of immune checkpoint inhibitors remains limited. The present report describes the case of a 70‑year‑old male patient with metastatic PGM who was treated with nivolumab monotherapy. Histopathological examination of a gastric tumor confirmed malignant melanoma, and staging studies revealed lymph node, lung and subsequent liver metastases. After nivolumab initiation, computed tomography demonstrated shrinkage of the primary gastric lesion but enlargement of metastatic lesions with the appearance of new liver and pleural metastases, resulting in a mixed response and progressive disease according to the Response Evaluation Criteria in Solid Tumors (RECIST). However, based on Immune RECIST (iRECIST), treatment was continued because pseudoprogression was suspected. Follow‑up imaging revealed regression of liver metastases after initial enlargement, consistent with site‑specific pseudoprogression, while disease control of the primary lesion, lymph node metastases and lung metastases was maintained for ~5 months. Although the disease eventually progressed because of pleural metastases, the present case demonstrated the potential clinical benefit of nivolumab in metastatic PGM. To the best of our knowledge, the present case may represent one of the few reported instances of site‑specific pseudoprogression and mixed response in PGM, and highlights the importance of iRECIST‑guided assessment to avoid premature discontinuation of immunotherapy.