Mesenchymal tumors of muscularis mucosae of colon and rectum are benign leiomyomas that should be separated from gastrointestinal stromal tumors - A clinicopathologic and immunohistochemical study of eighty-eight cases

Most mesenchymal tumors of the gastrointestinal tract are currently classif ied as specific gastrointestinal stromal tumors. However, true leiomyomas a re more common In the esophagus, and they have been occasionally noted In t he colon and rectum, but the small number of reported cases does not allow for clinicopathologic profiling. This study was undertaken to characterize 88 tumors of the muscularis mucosae of the colon and rectum. Seventy tumors were obtained form the files of AFIP and 18 cases from the Department of P athology of the Haartman Institute of the University of Helsinki. The lesio ns, except one, were removed by snare polypectomy as Incidental lesions at cancer or polyp surveillance; one small tumor was an incidental finding In the rectal resection specimen. The tumors had a significant male predominan ce In both institutions (overall 2.4:1). They occurred In age range of 38 t o 85 years (median 62 years). The lesions were typically small (range 1 to 22 mM, median 4 mM) and located predominantly in the rectum and sigmoid (72 %). All tumors were composed of well-differentiated, eosinophilic smooth mu scle cells that were seen immediately beneath the mucosa obliterating the m uscularis mucosae layer and merging with It. Two tumors had significant aty pia ("symplastic leiomyoma"); mitotic activity was seen in one of these tum ors, but not In others. The lesional cells were uniformly positive for smoo th muscle actin and desmin and negative for CD34, CD117 and S100-protein, b ased on immunohistochemical studies on 20 to 24 cases with each marker. No gastrointestinal stromal tumors were identified among the tumors of muscula ris mucosae, and no CD117-positive cells, except mast cells, were seen in t he muscularis mucosae layer. None of the patients had morbidity related to the tumor. Based on follow-up data on 29 patients, leiomyomas of muscularis mucosae are benign. They should be separated from gastrointestinal stromal tumors that have a clinicopathologic spectrum including frequent disease-r elated mortality. Snare polypectomy is an adequate treatment, but ensuring the complete removal and follow-up are necessary precautions for tumors wit h any atypia or mitotic activity.