Anaplastic carcinoma of the colon: clinicopathologic study of eight cases of a poorly recognized lesion.

Eight cases of a distinctive histological variant of bowel cancer characterized by an anaplastic morphology were identified from 2,650 colonic malignancies (0.3%). The tumors were histologically composed of sheets of anaplastic tumor cells with frequent atypical mitoses, absence of gland formation, and mucicarmine and periodic acid-Schiff (PAS) negativity. Positive immunostaining for cytokeratin and vimentin was observed in eight cases and for epithelial membrane antigen in three; whereas carcinoembryonic antigen, alpha-fetoprotein, S-100 protein, HMB-45 antimelanoma antigen, leukocyte common antigen, and neuroendocrine markers were uniformly negative. Ultrastructural examination demonstrated intercellular tight junctions, focal surface microvilli, and apical terminal webs or long rootlets of microfilaments supporting a colonic derivation. At the time of diagnosis, metastases to regional lymph nodes were found in seven cases and to the liver in six. All patients in this study died of tumor within 9 months. This report emphasizes a poorly recognized variant of colonic carcinoma, characterized by a high degree of anaplasia and malignant behavior. The differential diagnosis for these lesions is discussed.