An intrapelvic extraintestinal gastrointestinal stromal tumor of undetermined origin: diagnosis by prostate needle biopsy.

We herein report a case of intrapelvic gastrointestinal stromal tumor (GIST) of undetermined origin in a 48-year-old male who presented with dysuria. An enlarged tumor was detected on digital rectal examination. Imaging studies showed a solid and lobular homogenous tumor of 7.0 cm in diameter. The tumor was attached to the right dorsal aspect of the prostate with compression of the seminal vesicles and rectum. It was considered that the tumor had arisen from the prostate, although the patient's serum prostate-specific antigen level was low (0.436 ng/mL). The histological diagnosis by prostate needle biopsy was a spindle cell tumor. At cystoprostatectomy, the tumor was confirmed to be separated from the prostate by a fibrous band, and showed spindle cells with a fascicular growth pattern, but without necrotic areas. Mitotic figures were noted in 12 of 50 high-power fields. The tumor cells were immunoreactive for the KIT protein (CD117), CD34, Discovered on GIST-1 (DOG-1), and vimentin. In contrast, they were negative for desmin, α-smooth muscle actin, pancytokeratin (AE1/AE3), and S100 protein. The Ki-67 labeling index was 5%. The genetic analyses targeting the c-kit gene revealed a point mutation at codon 559 (GTT→GAT). The diagnosis of GIST was confirmed on the basis of the morphological features, immunoprofile, and results of the molecular analyses. Since extraintestinal GIST can resemble a prostatic tumor clinically, KIT (CD117) and DOG-1 should be considered for inclusion in the immunohistochemical panel for spindle cell tumors obtained by prostate needle biopsy.