Rectal GIST (Gastrointestinal Stromal Tumours) constituting 5% of all GISTs and 0.1% of all tumours originating in the rectum is a rare disease. Advanced imaging, histopathological examination and above all immunohistochemistry positivity for CD117, DOG1 and CD34 are mandatory not only for clinching the diagnosis, but also to distinguish between a primary prostatic EGIST (Extra Gastrointestinal Stromal Tumours) and a primary rectal GIST invading prostate.

We report this case of a recto-prostatic mass finally diagnosed as a primary rectal GIST with prostatic invasion taking place in a 60 years old gentleman, clinical feature of which mimicked the symptoms of a primary prostate cancer. While, imaging techniques like Ultrasonography (USG), Contrast Enhanced Magnetic Resonance Imaging and finally Positron Emitting Tomography Scan did not suffice to point out the primary site of the GIST, diagnosis was finally achieved by HPE and IHC from prostatic tissue showing evidence of invasion of disease from rectal origin.

Chemotherapeutic intervention with Imatinib remains successful to bring out favorable response. To the best of our knowledge, probably we are reporting the first case of rectal GIST with prostatic invasion from our country.

Keywords: Rectal Gist, Immunohistochemistry, Imatinib