Ulcerative colitis-associated rectal cancer resected and diagnosed by rectovesical fistula: a case report.

Ulcerative colitis (UC), which mainly consists of mucosal lesions, rarely form colovesical or rectovesical fistulas, although few cases of fistula formation associated with comorbidities have been reported. We report a case of UC-associated rectal cancer diagnosed following symptoms associated with rectovesical fistula. A 40-year-old man with a 31-year history of extensive UC presented with difficulty in defecation. Two years before the current presentation, he had experienced pneumaturia, and the examination then had revealed a rectal neoplastic lesion and rectovesical fistula; however, tissue biopsy showed no malignancy. Therefore, he requested for observation with no further treatment. Current examination suggested the rectal tumor had grown to invade the bladder. Tissue biopsy showed no malignancy. However, the clinical symptoms and examination findings strongly indicated UC-associated rectal cancer with bladder invasion; thus, open total proctocolectomy with partial cystectomy was performed. Histopathological evaluation of the rectal neoplastic lesion revealed UC-associated rectal cancer originating from the inflammatory mucosa, and the rectovesical fistula was found to be caused by the rectal cancer invading the bladder. Therefore, other colorectal cancers should be considered even though tissue biopsy does not reveal malignant lesions in UC patients with fistula.