Stage IV HER2-Positive Gastric Cancer With Immune Thrombocytopenia During Treatment

What happened, in summary

An older patient presented in June 2023 with unexplained abdominal swelling and markedly elevated tumor markers. CT showed a mass in the gastric antrum with omental involvement, metastatic lymph nodes, and liver metastases. Biopsy confirmed moderately differentiated intestinal-type gastric adenocarcinoma. Testing showed HER2 amplification, PD-L1 CPS 10, proficient mismatch repair, and Claudin18.2 overexpression. The cancer was Stage IV. In August 2023, the patient entered a clinical trial and began serplulimab, trastuzumab, and combination chemotherapy. After the second cycle, the platelet count fell to 13 × 10^9/L. Bone-marrow and immune testing supported an immune-related thrombocytopenia rather than tumor infiltration or routine chemotherapy suppression. Anticancer therapy was held while steroids, intravenous immunoglobulin, and platelet-stimulating treatments were given. A November 2023 CT showed a partial response. Because platelet recovery remained incomplete, cancer treatment was simplified to trastuzumab with S-1 and then S-1 alone. Imaging in January 2024 showed progression, so the original combination was restarted in February. Severe thrombocytopenia quickly returned, prompting permanent discontinuation of serplulimab. By March 27, 2024, CT showed further progression with enlarging gastric and omental lymphadenopathy and new liver metastases. The case highlights the challenge of balancing control of advanced gastric cancer against a serious immune-related treatment complication.