S4038 Atypical Echo: Unexpected Recurrence of Carcinoid Tumor in a Novel Location

Introduction: Carcinoid tumors, a type of neuroendocrine tumor (NET), arise from hormone-secreting cells and present unique diagnostic challenges. NETs occur in approximately 2.5 to 5 individuals per 100,000, mainly in the gastrointestinal tract and lungs, but can also develop in rare sites. Their behavior ranges from indolent to aggressive, with recurrence rates varying by primary location and disease stage at diagnosis. This case report highlights an atypical recurrence of a carcinoid tumor in an uncommon site, emerging years after the initial diagnosis and treatment. Case Description/Methods: A 67-year-old man with a history of hypertension and a prior right hemicolectomy for a carcinoid tumor (2008) presented for routine healthcare maintenance. Elevated screening prostate-specific antigen (PSA) prompted a magnetic resonance imaging of the prostate, which revealed findings suggestive of prostate cancer and incidentally identified a 4.7 cm lobulated, enhancing soft tissue lesion contiguous with the seminal vesicles and sigmoid colon. Biopsy confirmed prostate adenocarcinoma. Subsequent workup with positron emission tomography scan showed a non-tracer avid 3 × 5 cm soft tissue attenuation near the seminal vesicle, and colonoscopy was unremarkable. Further investigation with endoscopic ultrasound revealed a 5 × 3.5 cm hypo echoic, solid peri-colonic mass 15 cm from the anal verge, which was confirmed by fine-needle aspiration to be a recurrence of the patient's known carcinoid tumor. Discussion: This case presents an unusual late (17 years) recurrence of a carcinoid tumor as a peri-colonic mass near the seminal vesicles, incidentally found during prostate cancer workup. This protracted interval and atypical pelvic location deviate from typical recurrence patterns (often within 5-10 years, involving liver/lymph nodes), highlighting long-term surveillance challenges. The diagnostic complexity arose from mimicking other pelvic malignancies, necessitating endoscopic ultrasound and fine needle aspiration for confirmation. While carcinoid tumors are indolent with potential for late recurrence, such distant and anatomically unexpected presentations are rare. This case underscores the need for vigilance against recurrence even after prolonged disease-free intervals. It contributes to the understanding of atypical metastatic patterns in neuroendocrine tumors, potentially informing long-term surveillance strategies.