Synchronous Rectal Adenocarcinoma and Sigmoid Gastrointestinal Stromal Tumor: 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 6-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. Postoperatively, 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.