CEA-Discordant Progression in Metastatic Rectal Cancer: A Case Report

Background

Metastatic rectal carcinoma requires coordinated multimodal management, with treatment decisions informed by disease extent, clinical status, pathology, imaging, and molecular findings. Case Presentation: A 61-year-old woman presented in November 2022 with progressive constipation, painful defecation, and mucoid rectal discharge. Loop sigmoid colostomy was performed for symptomatic metastatic rectal cancer, and intraoperative assessment demonstrated a mid-rectal growth, multiple omental and peritoneal metastases, ascites, and colonic dilatation. Histopathological examination of a peritoneal deposit confirmed metastatic poorly differentiated carcinoma. The patient received four documented cycles of palliative CapOx plus bevacizumab, followed by chemoradiotherapy and maintenance oral capecitabine. Subsequent records documented microsatellite-stable disease, a PD-L1 tumour proportion score of 17%, a combined positive score of 2, and KRAS/NRAS/PIK3CA profiling, although a complete formal molecular report was unavailable. Serial PET-CT examinations demonstrated persistent and subsequently progressive nodal metastatic disease. In July 2026, progressive para-aortic, aortocaval, left external iliac, and bilateral inguinal nodal disease was reported, with para-aortic nodal encasement of the proximal left ureter and upstream hydroureteronephrosis. Despite radiological progression, serum CEA remained low at 2.2 ng/mL.