RECTAL GIST: AN UNEXPECTED DIAGNOSIS

Objectives: Gastrointestinal stromal tumors (GIST) are connective tissue tumors commonly seen in the stomach, but can also occur in the small intestine, esophagus, colon, rectum, and mesentery. GISTs have a spindle-cell histology, similar to that of leiomyomas, and often present with vague symptoms.
A 49-year-old woman was found to have uterine fibroids during an evaluation for abnormal uterine bleeding. A large rectal mass was also palpated during the bimanual exam. Her elective hysterectomy was postponed, and she was referred to general surgery clinic. A rectal exam under anesthesia demonstrated a nonobstructive anterior mass approximately 4-5 centimeters from the anal verge that was bulging into the rectum. The rectal mucosa did not show any obvious lesions, and biopsy revealed benign colonic mucosa without malignancy. She proceeded with her hysterectomy. The mass was palpated in the rectovaginal septum and was not continuous with the uterus or cervix. General Surgery was consulted intra-operatively, and the decision was made to complete the hysterectomy and proceed with diagnostic workup afterward. A pelvic MRI showed a well-circumscribed hypoenhancing mass originating from the posterior vaginal wall; it was favored to be benign or low-grade given its characteristics. The patient returned for excision of the mass. Several solid multi-lobulated masses were enucleated from the rectovaginal septal space. A specimen was sent to pathology as a frozen section intra-operatively; the tissue architecture was reported as consistent with uterine leiomyoma. Meanwhile, a 1-mm lesion was taken as a posterior and inferior margin. It involved the rectum, and a rectorrhaphy was performed. The final pathology report identified the masses as a 10-cm GIST. The most posterior margin best demonstrated the GIST arising from the muscularis propria of the rectum. This was high grade, with high mitotic rate >15 per 5mm2 and multiple positive margins. The patient was referred to Medical Oncology and was started on Gleevec therapy. Her case has been discussed at tumor board; adjuvant radiation was not advised due to lack of clear-cut data. She was presented with the option of surgical resection, which would involve a permanent ostomy; she declined. Repeat imaging has shown no obvious distant metastases or residual rectovaginal region mass.
The simultaneous presence of uterine leiomyoma and rectal GIST – both of which can have similar gross and microscopic appearances – draws attention to this case. Recognizing that patients can have two simultaneous medical problems is critical in preventing a missed diagnosis and delay in care.

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