S6324 Beyond the Skin: A Case of Melanoma With Gastric Metastasis Detected in Life

Introduction: Malignant melanoma is a highly aggressive cancer known for its ability to metastasize to virtually any organ. Gastrointestinal involvement, particularly in the stomach, is rare and often undiagnosed during life. Gastric metastases present with nonspecific symptoms such as anemia or bleeding and are mostly discovered only at autopsy. We report a case of metastatic cutaneous melanoma with isolated gastric involvement presenting solely with chronic anemia. Case Description/Methods: A 76-year-old man was diagnosed with superficial spreading melanoma based on a shave biopsy of a lesion noted on the left chest wall in September 2019. At that time, there was no ulceration or lymphatic/perineural invasion in the skin biopsy and the sentinel lymph node biopsy was negative. Therefore, observation was recommended. Two years later, a screening computed tomography (CT) chest showed incidental left axillary adenopathy prompting a lymph node biopsy. Flow cytometry was positive for CD56, which is often seen in melanoma. Immunohistochemical staining was negative for CD45, AE1/AE3 keratin, and positive for SOX-10, HMB45, and S100, making melanoma the most likely diagnosis; additional mutation analysis showed only wild-type BRAF mutation. He was started on pembrolizumab due to the absence of aggressive features and the presence of wild-BRAF. Despite immunotherapy, his disease progressed on pembrolizumab with subsequent metastases to the brain, lung, and bladder, so he was transitioned to nivolumab and relatlimab-rmbw. Two years into treatment, he was evaluated with a CT scan for abdominal pain and severe anemia (hemoglobin 6.9) requiring blood transfusion. The CT scan showed irregular nodular gastric wall thickening. The patient underwent an endoscopy, which revealed multiple mucosal nodules with stigmata of recent bleeding that were treated with clips and Hemospray. Biopsy showed similar immunostaining to the previous lymph node biopsy, consistent with melanoma. Because of the extensive metastatic disease burden, the oncology team recommended palliative measures due to poor prognosis. Discussion: Gastric involvement in melanoma remains underreported in living patients, with most data derived from autopsies. Our case adds to the limited literature by illustrating a diagnosis during life confirmed by endoscopy and immunohistochemical staining, reinforcing the need to suspect gastrointestinal spread in patients with metastatic melanoma and anemia. Disclosure: Artificial intelligence was used to refine language and punctuation only.