Medullary carcinoma is an uncommon colorectal tumour which appears poorly differentiated histologically. Consequently it may be confused with poorly differentiated adenocarcinoma NOS. The principal aim of this study was to review a large series of poorly differentiated colorectal cancers resected at a large NHS Teaching Hospital to determine how often medullary carcinomas were misclassified . Secondary aims were to investigate how often neuroendocrine differentiation or metastatic tumours were considered in the differential diagnosis, and compare clinico-pathological features between medullary and poorly differentiated adenocarcinoma NOS.
Histology slides from 302 colorectal cancer resections originally reported as poorly differentiated adenocarcinoma were reviewed and cases fulfilling WHO criteria for medullary carcinoma identified. The original pathology report was examined for any mention of medullary phenotype, consideration of neuroendocrine differentiation or metastasis from another site. Clinico-pathological features were compared to poorly differentiated adenocarcinoma NOS.
Only one third of medullary carcinomas were correctly identified between 1997 and 2018. The other two thirds were reported as poorly differentiated adenocarcinoma NOS. The possibility of an extracolonic origin or neuroendocrine carcinoma was considered in 21% and 27% of reports. Most medullary tumours exhibited mismatch repair deficiency, were located in ascending colon and caecum, and had a lower rate of vascular channel invasion and lymph node metastasis compared to poorly differentiated adenocarcinoma.
Medullary carcinoma of the colon is often mistaken for poorly differentiated adenocarcinoma NOS and occasionally for neuroendocrine or metastatic carcinoma. Greater familiarity with morphological criteria and use of mismatch repair protein staining should improve diagnosis.

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