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. Conclusion: This case demonstrates the importance of integrating serial imaging, pathology, clinical assessment, and verified molecular findings when monitoring metastatic rectal carcinoma, particularly when serum CEA does not reflect radiological disease progression.