Medullary carcinoma presenting as an obstructive colonic mass requiring emergency surgery: a case report

Introduction and importance: Medullary carcinoma (MC) is a rare, poorly differentiated colorectal carcinoma that typically arises in the right colon and is usually mismatch repair–deficient (dMMR). Presentation as an acute bowel obstruction is rarely documented, and data on emergency management are scarce. Case presentation: A 77-year-old man receiving anticoagulant therapy presented with 5 days of obstructive symptoms and acute kidney injury. Computed tomography showed a transverse colon mass causing ileal obstruction, with suspected invasion of the gastric antrum. After anti-Xa activity was confirmed to be undetectable, an emergency right hemicolectomy with complete mesocolic excision was performed, along with segmental ileal resection, an atypical (wedge) stapled resection of the invaded antral wall, and the creation of a protective ileostomy. Histology showed MC with loss of MLH1 and PMS2 expression (pT4b N1a, V1, L1, Pn1; American Joint Committee on Cancer 8th-edition stage IIIC, with no evidence of distant metastasis). Recovery was complicated by Kidney Disease: Improving Global Outcomes stage 3 acute kidney injury; the patient was discharged on postoperative day 17, received adjuvant immunotherapy, and was alive without recurrence at 6 months. Clinical discussion: Loss of MLH1 and PMS2 establishes dMMR status but, strictly, not microsatellite instability-high status, and most, but not all, MCs are dMMR. At 77 years of age, it is usually sporadic, yet Lynch syndrome cannot be excluded without BRAF V600E testing or MLH1 promoter methylation testing. Transverse colon MC is best regarded as a proximal, midgut-type disease, which supports right-sided resection with complete mesocolic excision. Adjuvant immunotherapy was selected for stage III dMMR disease, for which atezolizumab combined with chemotherapy has shown benefit. Conclusion: MC may present as acute obstruction of the transverse colon. Mismatch repair immunohistochemistry is recommended for all colorectal cancers; following emergency resection, solid or undifferentiated morphology should prompt immediate testing, because the result integrates pathological diagnosis, Lynch syndrome triage, and multidisciplinary adjuvant planning, including eligibility for immune checkpoint inhibitors.