A Rare Case of Chemotherapy Combined with Immunotherapy for Dual Primary AFP-Positive Gastric Cancer and Synchronous Small Cell Lung Cancer.

Background: Alpha-fetoprotein-positive gastric cancer (AFPGC) is a rare subtype of gastric cancer characterized by high invasiveness and extremely poor prognosis. According to relevant studies, the median overall survival of such patients is significantly shorter than that of AFP-negative gastric cancer patients (14 months vs 40 months). Small cell lung cancer (SCLC), the most malignant type of lung cancer, has a median survival time of only 8-12 months in patients with extensive disease. To date, there have been no reported cases of dual primary cancers involving both AFPGC and SCLC, and the therapeutic role of immunotherapy in such dual primary tumors remains unclear. Case Presentation: This paper reports a case of a 72-year-old male patient who was diagnosed via imaging and pathology as having concurrent AFPGC (moderately to poorly differentiated adenocarcinoma, PD-L1 positive, Tumor mutation burden(TMB) 10.03Muts/Mb, PD-L1 Combined Positive Score (CPS)<1) combined with primary extensive-stage SCLC. The patient received CAPEOX regimen (capecitabine plus oxaliplatin) combined with tislelizumab therapy. After 4 cycles, partial response (PR) was observed in the gastric lymph nodes, and stable disease (SD) was noted in the pulmonary lesions. Following pathological confirmation of dual primary cancers, treatment continued with the original regimen, followed by maintenance therapy with tegafur gimeracil oteracil potassium capsule (teysuno) plus tislelizumab. During treatment, serum AFP levels decreased from baseline 502 μg/L to 1.56 μg/L. Both primary tumor lesions remained stably controlled for over 33 months, and the patient currently tolerates treatment well with an Eastern Cooperative Oncology Group (ECOG) performance status of 0. Conclusion: Through the long-term treatment course of this case, we validated the therapeutic efficacy of chemotherapy combined with immunotherapy (CAPEOX plus tislelizumab) for the rare aggressive dual primary tumors AFPGC and SCLC, demonstrating significant long-term maintenance benefits from the immunotherapy. Concurrently, this case confirmed the efficacy and safety of tislelizumab during the maintenance therapy phase for both tumors, offering a new treatment option for managing such complex clinical presentations.