Small cell undifferentiated ('neuroendocrine') carcinoma of the colon.

Three patients are described who presented with a large colonic adenoma in which a solid, undifferentiated carcinomatous component was found on microscopic examination. Despite small size (1.0 and 1.5 cm) and submucosal location in two cases, the tumours had metastasized to regional lymph nodes and the liver and death ensued at 4, 11 and 18 weeks after surgery. Immunocytochemistry was positive for carcino-embryonic antigen, low molecular weight cytokeratins and neuron specific enolase in all three cases and scanty dense core granules of neurosecretory type were found in one of two examined by electron microscopy. These 'neuroendocrine' carcinomas are compared with 'pure' adenomas and 'ordinary' poorly differentiated adenocarcinomas of the colon from which they differ, mainly by lack of glandular differentiation and mucus secretion, although two adenocarcinomas also showed patchy reactivity for neuron specific enolase. The term 'neuroendocrine' may be disputed but is now well established to describe a tumour that runs a uniquely aggressive course and for which radical surgery alone cannot provide a cure.