Synchronous Gastric and Colonic Adenocarcinoma: A Case Report With its Molecular Implications.

Multiple primary tumors are rare but their incidence is increasing nowadays with advancements in diagnostic methods and the extended survival of individuals previously treated for malignancies. However, synchronous occurrence of gastric cancer (GC) and colonic cancer (CC) is a rare entity. A 41-year-old male came with complaints of epigastric pain associated with anorexia, rapid weight loss, and occasional constipation. Contrast-enhanced computed tomography (CECT) of the abdomen and pelvis reported mucosal thickening in the antrum, likely GC with circumferential wall thickening of the transverse colon with pericolic fat stranding suggestive of CC. Upper gastrointestinal endoscopy and colonoscopy were also done and a biopsy was taken from representative sites, which confirmed malignancy. He completed three cycles of chemotherapy preoperatively and underwent subtotal gastrectomy, D2 lymphadenectomy, gastrojejunostomy, jejunojejunostomy, and transverse colectomy simultaneously. Histopathological examination confirmed moderately differentiated gastric adenocarcinoma penetrating into the subserosa and well-differentiated colonic adenocarcinoma invading the muscularis propria. Immunohistochemical analysis of mismatch repair (MMR) proteins was done to determine the association with hereditary nonpolyposis colorectal cancer syndrome (HNPCC) or Lynch syndrome. The patient underwent postoperative chemotherapy along with immunotherapy. To conclude, synchronous occurrence of primary GC and primary CC with similar MMR protein expression in immunohistochemistry is an uncommon entity.