Obstructing colonic involvement by gastric-origin adenocarcinoma during adjuvant chemotherapy after curative-intent gastrectomy: a case report with literature review.

INTRODUCTION AND IMPORTANCE: Colonic involvement by gastric adenocarcinoma is rare and may mimic a new primary colorectal malignancy. Although clinicopathological assessment can support a gastric origin, it cannot reliably distinguish hematogenous or lymphatic dissemination and peritoneal implantation with secondary mural invasion. PRESENTATION OF CASE: A 52-year-old man underwent curative-intent total gastrectomy for distal gastric adenocarcinoma (pT4a pN2) in January 2024. Adjuvant folinic acid, fluorouracil, and oxaliplatin (FOLFOX) was initiated in February 2024. Approximately five months after the gastrectomy, while treatment was ongoing, he developed complete large-bowel obstruction due to a severe left-sided colonic stricture and underwent emergency left hemicolectomy. The resection contained two infiltrative lesions, including a dominant transmural lesion. Histology showed moderately differentiated adenocarcinoma. Immunohistochemistry showed CK7+, CK20+, CEA+, focal CDX2+, and SATB2-, favoring gastric-origin adenocarcinoma involving the colon over a conventional de novo colorectal primary. The route of colonic involvement remained indeterminate. The patient subsequently received palliative systemic treatment and died approximately 22 months after the initial diagnosis. CLINICAL DISCUSSION: This early, obstructing recurrence during active adjuvant therapy underscores the need to consider disease of gastric-origin in patients with a new colonic stricture after gastrectomy. Immunohistochemistry helps assign tumor origin, but it cannot establish the route of spread; microscopic peritoneal disease cannot be excluded despite the absence of macroscopic peritoneal disease at laparotomy. CONCLUSION: Gastric-origin adenocarcinoma should be considered in patients with previous gastric cancer who develop a colonic stricture. Accurate clinicopathological classification is essential for multidisciplinary treatment planning.