S1656 Not Your Standard Adenocarcinoma, a Case of Medullary Carcinoma of the Colon

Abstract

INTRODUCTION: Adenocarcinoma of the colon is the most common type of colon cancer. Medullary carcinoma (MC) of the colon is a rare histological subtype of adenocarcinoma. MC has been described to have specific histopathological features but can still be difficult to distinguish from poorly differentiated adenocarcinoma. We aim in this case report to shed light on this rare subtype of colon cancer and describe its histopathological features. CASE DESCRIPTION/METHODS: 76-year-old male presented to our hospital reporting 3 months of back pain, fatigue, and 10lb weight loss. Past medical history significant for atrial fibrillation and history of renal cell carcinoma. CT abdomen/pelvis showed a necrotic mass within the ascending colon suggestive of colon cancer. Metastatic lymph nodes were also suspected on the scan. Subsequent colonoscopy showed a large mass at the hepatic flexure. Multiple biopsies were taken from mass. Histopathologic sections showed sheets of malignant epithelial cells extending from the submucosa into the lamina propria. Immunohistochemical staining demonstrated that the malignant cells were positive for pan-cytokeratin, but negative for CK20, CDX2, S-100, CD45, synaptophysin, chromogranin, and PSA. These initial findings were consistent with invasive poorly differentiated carcinoma. Further immunohistochemical testing was done and showed mismatch repair deficiency with loss of MLH1 and PMS2 consistent with medullary carcinoma of the colon, which typically shows aberrant loss of CK20 and CDX2 (typical markers of colorectal adenocarcinoma). Due to patients advanced disease (stage IV), decision made to start palliative chemotherapy with Xeloda + Oxaliplatin + Avastin. Patient passed away approximately three months after initial diagnosis. DISCUSSION: MC of the colon is extremely rare with limited clinical and epidemiologic data. Based on population-based analysis, medullary carcinoma makes up 5-8 cases for every 10,000 colon cancers diagnosed. Most patients are diagnosed earlier in the disease course and metastatic disease is seen in 10% of patients at time of diagnosis. Immunohistochemical analysis is essential to differentiate from poorly-differentiated and undifferentiated colon adenocarcinomas. Classic findings include loss of staining for MLH-1 and CDX-2. In the current published data, overall prognosis of MC is favorable when compared to poorly differentiated or undifferentiated adenocarcinomas. However, data is limited regarding available treatment strategies and response.Figure 1