Histopathologic study of the rectum in 1,464 consecutive rectal specimens in a single Japanese hospital: II. malignant lesions.

The author investigated histopathology of 1,464 consecutive rectal specimens in of our pathology laboratory in Japan. A review of pathological reports was done by computer. Observation of histological slides was performed, when appropriate. The rectal specimens were composed of 1,041 benign lesions and 423 malignant lesions. The 423 malignant lesions were composed of 367 cases of primary rectal carcinoma, 41 cases of carcinoma in adenoma, 7 cases of neuroendocrine tumor, 3 cases of malignant lymphoma, 2 cases of gastrointestinal stromal tumors (GIST), and 3 cases of metastatic carcinoma. Of the 367 cases of primary rectal carcinoma, 37 cases were early carcinomas whose invasion was limited up to the submucosa (early rectal carcinoma). The remaining 330 cases were advanced carcinoma invading beyond the proper muscle layer. The histological types were well differentiated adenocarcinoma in 197 cases, moderately differentiated adenocarcinoma in 129 cases, poorly differentiated adenocarcinoma in 10 cases, mucinous adenocarcinoma in 24 cases, signet ring cell carcinoma in 6 cases, squamous cell carcinoma in 1 case In the 41 cases of carcinoma in adenoma, the carcinoma was well to moderately differentiated adenocarcinoma, and all cases were early carcinomas without invasion or with little invasions to subserosa. The size of carcinoma in adenoma was as follows: < 10 mm, 5 cases; 10-15 mm, 8 cases; 15-20 mm, 23 cases; > 20mm, 5 cases. The background adenoma was as follows: tubular adenoma (n=15), tubulo-villous adenoma (n=14), and villous adenoma (n=12). The 7 cases of neuroendocrine carcinoma consisted of 6 low grade neuroendocrine tumors (carcinoids) and 1 high grade neuroendocrine carcinoma (small cell carcinoma). All were submucosal lesions. Immunohistochemically, the tumor cells were positive for two or more of synaptophysin, chromogranin, neuron-specific enolase, CD56. In small cell carcinoma, KIT and PDGFRA were consistently positive. The 3 cases of malignant lymphoma were diffuse large B-cell lymphomas positive for CD20 and CD79a and negative for NK/T cell markers. The two cases of GIST was spindle cell type, and the risk was intermediate. Kit mutations were recognized in both GISTs. No PDGFRA mutations were seen. Of the 3 metastatic carcinomas, one was a metastasis from prostatic adenocarcinoma, and the remaining two was adenocarcinoma of unknown primary sites.