Gastrointestinal stromal tumor of the rectum mimicking a prostatic mass: case report.

We report a rare case of a gastrointestinal stromal tumor (GIST) of rectal origin presenting as a prostatic mass in a 50-year-old man. The patient presented with recurrent lower urinary tract symptoms for several months, including frequency, nocturia, weak urinary stream, and a sensation of incomplete bladder emptying. Physical examination revealed a slightly enlarged, smooth prostate without nodularity. Serum prostate-specific antigen (PSA) was elevated to 6.5 ng/mL. Ultrasound showed a large encapsulated pelvic mass posterior to the prostate. MRI demonstrated a well-defined heterogeneous mass measuring 83 × 98 × 80 mm, showing areas of hemorrhagic necrosis, heterogeneous enhancement after contrast, and mass effect with anterior displacement of the bladder and posterior bulging into the rectal lumen. Transrectal biopsy confirmed the diagnosis of a gastrointestinal stromal tumor (CD117+, CD34+, Ki67 ≈ 10%). The patient was treated with imatinib, resulting in marked reduction in tumor size and improvement in urinary symptoms. This case highlights the importance of considering rectal GIST in the differential diagnosis of prostatic masses, especially in patients with atypical clinical and imaging findings.