Colorectal smooth-muscle tumors. A pathobiologic study with immunohistochemistry and histomorphometry.

Smooth-muscle tumors are an interesting group of tumors that show considerable site specificity in their pathobiology. Recent work has also shown that some previously so-called smooth-muscle tumors were not, in fact, truly leiomyogenic, hence the origin of the more embracing term stromal tumors. The purpose of this retrospective study was to delineate the tumor subsets constituting colorectal stromal tumors, and to determine the histopathologic correlates for biologic aggressiveness for these tumors. The cohort was constituted of 12 patients; the mean follow-up was 6.6 years with a median of 5.0 years. Immunohistochemical evaluations showed tumoral positivity for muscle-specific actin (12 of 12), vimentin (11 of 12), desmin (two of 12), and S100 protein (zero of 12). Electron microscopic examinations corroborated this leiomyogenicity profile (five of five). Semiquantitative histomorphometric analysis showed that tumor size, cellularity, mitoses, and necrosis, in that order, correlated with biologic aggressiveness. The immunohistochemistry results for this cohort of colorectal stromal tumors vindicated the traditional histochemical evaluations in that all tumors showed features of leiomyogenicity. For colorectal smooth-muscle tumors, tumor size appears to be the best predictor for biologic aggressiveness. This study reinforces the concept of site-specificity for smooth-muscle tumors.