EP810 - A Diagnostic Pitfall: Small-Bowel Gastrointestinal Stromal Tumor Masquerading as a Pelvic Mass

Purpose/Background: Gastrointestinal stromal tumors (GISTs) are rare mesenchymal neoplasms of the gastrointestinal tract, most commonly arising from the stomach or small intestine and accounting for approximately 1 to 3% of all gastrointestinal malignancies. Their variable growth patterns and potential for exophytic extension can result in unusual clinical presentations, including mimicking gynecologic disease by being mistaken for apparent pelvic or adnexal masses. Such atypical presentations can lead to significant changes to the surgical plan at the time of operation and often necessitate a multidisciplinary approach involving multiple surgical specialty teams.

Methods/Interventions: We present a case of a small bowel GIST that was initially suspected to be a pelvic mass of gynecologic origin, with the true diagnosis only established intraoperatively.

Results/Outcomes: A 67 year old post-menopausal female who presented with new onset pelvic pain to the gynecologic oncology service was found to have a 8.3 x 3.6 centimeters complex bilobed mass of the posterior pelvis described as inseparable from the ovaries on MRI. Tumor markers CA-125, CEA and Inhibin were found to be within normal limits on work up. She was recommended by her gynecologist to undergo a robotic assisted resection of pelvic mass, hysterectomy and bilateral salpingo-oophorectomy. Intraoperatively it was discovered that the mass was separate from the ovaries and appeared to originating from the anti-mesenteric border of the mid-jejunum and tethered to the left pelvic sidewall and posterior cul-de-sac. The ovaries, and uterus appeared grossly normal. Colorectal surgery was consulted intraoperatively and a frozen section was with results consistent with a possible GIST tumor. The mesentery was divided 5 centimeters proximal and distal to the tumor and the midline excision was then extended and the bowel was eviscerated in order to perform a side to side, functional end to end small bowel anastomosis. The patient had an uncomplicated postoperative course and was discharge on post operative day 2. Final pathology revealed a gastrointestinal stromal tumor of the small bowel that measured 11 cm with 5 mitoses per 5mm2. Given the high risk of recurrence for this tumor, the patient is recommended for Imatinib therapy

Conclusion/Discussion: This case highlights the challenge of diagnosing small bowel GISTs preoperatively because they can be difficult to distinguish from gynecologic pelvic masses. Exophytic small bowel GISTs can extend into the pelvis and closely resemble adnexal or uterine masses on imaging, especially when the organ of origin is not clearly visualized. Lesions presenting in this manner are discovered intraoperatively as was the case for this patient. This highlights the need for collaboration between gynecologic and colorectal surgeons. Definitive diagnosis requires histopathologic evaluation with immunohistochemical staining for c-KIT (CD117) and DOG1, which remain the gold standards for confirming GIST. High-risk features such as tumor size greater than 10 cm, mitotic index exceeding 10 per 50 high-power fields, or a combination of size > 5 cm with >5 mitoses per 50 HPF require adjuvant therapy with imatinib which has been shown to significantly reduce recurrence and improve overall survival. Risk stratification should also consider tumor site, as small-bowel GISTs carry a higher recurrence risk than gastric lesions. Accurate postoperative classification and early oncology referral are therefore essential to guide adjuvant treatment and surveillance.