Supplementary Material for: Synchronous Rectal Adenocarcinoma and Sigmoid GIST: A Case Report

Introduction
The synchronous presentation of a colorectal adenocarcinoma and a gastrointestinal stromal tumor (GIST) is a rare clinical event that poses significant diagnostic and therapeutic challenges. These two malignancies arise from distinct histogenetic origins and are typically managed with different treatment paradigms. The incidental discovery of a GIST during the workup for a more symptomatic adenocarcinoma complicates surgical planning and requires a comprehensive, multidisciplinary approach to ensure optimal oncologic outcomes.
Case Presentation
An 85-year-old female presented with a six-month history of debilitating fecal incontinence, tenesmus, rectal bleeding, and a 13 kg weight loss. Diagnostic evaluation, including imaging and endoscopy, identified two separate lesions: a large, ulcerated mass in the mid-rectum and a pedunculated mass in the sigmoid colon. Biopsies with immunohistochemical analysis confirmed the rectal mass as a moderately differentiated adenocarcinoma (CK20+, CDX2+) and the sigmoid mass as a spindle-cell neoplasm consistent with a GIST (CD117+, DOG1+). Due to the local invasion of the rectal cancer into the posterior vaginal wall, the patient underwent an open en bloc abdominoperineal resection, which included a hysterectomy, bilateral salpingo-oophorectomy, posterior vaginectomy, and sigmoidectomy. Final pathology confirmed a pT3N0 rectal adenocarcinoma with negative margins and a completely resected 3.5 cm low-risk sigmoid GIST.
The patient was followed for 24 months with no evidence of recurrence of either tumor. Post-operatively, her presenting symptoms of tenesmus and bleeding resolved.

Conclusion
This case demonstrates the critical role of immunohistochemistry in confirming synchronous primary tumors. For complex, locally advanced disease, radical upfront en bloc resection remains the gold standard for achieving curative-intent, negative margins, even in the elderly.