Rectal cancer with intraluminal metastasis to the duodenum

This study aims to present the diagnosis, treatment process, and clinical course of a patient with advanced rectal adenocarcinoma presenting with rare duodenal metastases. A 47-year-old male patient presented with constipation. A mass obstructing the entire lumen was detected in the rectosigmoid region, and the biopsy result was reported as rectal adenocarcinoma. Clinical staging was determined as T4aN2bM0. Due to the obstruction, a loop sigmoidostomy was performed, and Total Neoadjuvant Therapy (TNT) was planned. Within the scope of TNT, 5- Fluorouracil (5-FU), Oxaliplatin, Capecitabine, and 25 fractions of radiotherapy were administered. Surgically, Low Anterior Resection (LAR) was performed, and pathology confirmed pT4N2, with the presence of lymphovascular and perineural invasion. In the fifth postoperative month, preaortic and paraaortic lymphadenopathy was detected; a KRAS mutation was identified. Partial regression was observed in the lymph nodes following the new chemotherapy regimen (5-FU +Irinotecan+Bevacizumab), but cholestasis developed. Imaging and Endoscopic Retrograde Cholangiopancreatography (ERCP) revealed ulcerative metastatic masses in the duodenum and ampulla of Vater regions. Biopsy results and immunohistochemical analyses (CDX2, SATB2, Beta-Catenin) confirmed colon adenocarcinoma metastasis. Next-generation sequencing (NGS) analysis showed a recurrent KRAS mutation, stable microsatellite stability(MSI), and intact Mismatch Repair (MMR). Although symptomatic relief was achieved with biliary drainage and a duodenal stent, advanced metastases (liver, adrenal, bone) developed. Duodenal metastasis in patients with rectal adenocarcinoma is rare but clinically severe and has a poor prognosis. This case demonstrates the rare metastatic pathways of advanced colorectal cancer, management challenges, and palliative treatment strategies. Keywords: Rectal cancer; Duodenal metastasis; Intraluminal metastasis; KRAS mutation; Colorectal cancer.