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WHO Classification of Tumours
Leiomyoma, NOS
Colon and rectum


Leiomyoma is a well-circumscribed benign smooth muscle neoplasm histologically characterized by the presence of spindle cells with cigar-shaped nuclei, interlacing fascicles, and a whorled pattern.

Leiomyoma of the colon and rectum
A distinctive type of leiomyoma that usually occurs in the colon and rectum is that of the muscularis mucosae. These tumours are roundish nodules that usually measure < 1 cm and not more than 2 cm. They are usually incidental endoscopic findings in older adults and are clinically indolent. Histologically, these tumours are composed of bundles of well-differentiated smooth muscle that merge into the muscularis mucosae. Although focal nuclear atypia may be present, mitotic activity is not detectable.

Intramural leiomyomas are rare. They usually measure 13 cm, and are composed of well-differentiated smooth muscle cells with limited, if any, atypia and virtually no mitotic activity, although tumours with both atypia and mitotic activity are more appropriately designated as leiomyosarcomas

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Bosman FT, Carneiro F, Hruban RH, Theise ND (Eds.)
WHO Classification of Tumours of the Digestive System.
4th Edition
International Agency for Research on Cancer: Lyon 2010