Synchronous gastric and appendiceal mucinous adenocarcinomas: a rare case report

Background: Gastric mucinous adenocarcinoma (GMA) and appendiceal mucinous adenocarcinoma (AMA) are rare neoplasms that frequently present at advanced stages characterized by peritoneal dissemination. The simultaneous occurrence of these two malignancies is exceedingly uncommon. Case presentation: A 66-year-old man presented with abdominal discomfort and was initially diagnosed with gastric adenocarcinoma based on endoscopic biopsy findings. Computed tomography imaging revealed the presence of ascites and peritoneal nodules. Following neoadjuvant chemoimmunotherapy comprising XELOX and sintilimab, diagnostic laparoscopy identified a concurrent AMA accompanied by pseudomyxoma peritonei (PMP). Subsequently, the patient underwent radical gastrectomy, appendectomy, and hyperthermic intraperitoneal chemotherapy (HIPEC). Histopathological examination confirmed the synchronous presence of GMA and AMA. Importantly, serial molecular analyses demonstrated a treatment-induced transition from mismatch repair-deficient (dMMR)/microsatellite instability-high (MSI-H) status to mismatch repair-proficient (pMMR) status. Conclusion: This rare case highlights the critical importance of comprehensive preoperative imaging and meticulous surgical evaluation in the diagnosis of uncommon synchronous malignancies. Furthermore, the observed alteration in molecular phenotype following chemoimmunotherapy suggests that such treatment modalities may influence tumor biology.