A large cystic gastrointestinal stromal tumor of the rectum in the retrorectal space.

A 54-year-old man presented with pain on defecation and rectal bleeding. Colonoscopy revealed a submucosal tumor extending from the lower rectum to the upper rim of the anal canal, which compressed the rectal wall inward by two thirds of its circumference. Magnetic resonance images demonstrated a 70 × 80-mm unilocular cystic mass with a solid portion in the periphery in the retrorectal space, which displaced the rectum anterolaterally. The peripheral solid portion was hypointense on T2-weighted images and not hyperintense on diffusion-weighted images, suggesting low cellularity of the lesion. Cytological examination of the clear and serous fluid obtained by transrectal biopsy showed the presence of normal columnar and squamous epithelial cells and the absence of malignant cells. Therefore, the cystic retrorectal mass was presumed to be tailgut cysts rather than gastrointestinal stromal tumors (GISTs). The mass and rectum were extirpated en bloc with an adequate surgical margin by laparoscopic intersphincteric resection. Pathologically, spindle tumor cells proliferated with nuclear palisading and were strongly immunopositive for c-kit, leading to a final diagnosis of rectal GIST. There are no reports describing a huge, cystic rectal GIST arising in the retrorectal space, which should be considered in the differential diagnosis of cystic retrorectal lesions.