Advanced Gastric Cancer With Cutaneous Metastases, With Effective Relief From Immunotherapy: Case Report

Cutaneous metastases from gastric cancer are rare (<1%) and typically indicate advanced disease with poor prognosis. We report a unique case of a 53-year-old male presenting with an asymptomatic left temporal erythematous nodule, later diagnosed as metastatic gastric adenocarcinoma with signet-ring cell features. Initial skin biopsy revealed adenocarcinoma (CDX2+/CK20+/CK7+), prompting gastrointestinal evaluation, which confirmed poorly differentiated gastric adenocarcinoma (30% signet-ring cell component). The patient received neoadjuvant immunochemotherapy (paclitaxel+S-1+sintilimab), followed by laparoscopic radical gastrectomy (D2 lymphadenectomy) and resection of the skin lesion. Postoperative pathology showed Becker grade 2 regression in the primary gastric tumor and Mandard grade 3 regression in the cutaneous metastasis. The surgical specimen showed metastasis in 20 of 65 lymph nodes and was staged ypT4aN3bM1 (AJCC 8th edition). Biomarker assessment demonstrated high Claudin18.2 expression (~90%) but low PD-L1 expression (CPS ~1). Postoperative surveillance showed no radiographic local recurrence or distant metastasis through April 2025; however, leptomeningeal metastasis was confirmed by cerebrospinal fluid (CSF) cytology in May 2025. This case illustrates substantial early tumor regression with perioperative chemoimmunotherapy in an unusual metastatic presentation while also demonstrating that early response does not necessarily translate into durable disease control.