[A case of rectal gastrointestinal stromal tumor in an elderly patient who was successfully treated with imatinib mesylate with no significant adverse events].

An 89-year-old male patient was found to have a mass with a diameter of 54 mm in the pelvic cavity, connected to the rectum, and was diagnosed with a gastrointestinal stromal tumor (GIST)of the rectum by transrectal biopsy. The patient was treated continuously with 400mg/day of imatinib mesylate with no significant adverse events, and the tumor gradually reduced in size. The tumor reduced in size to a diameter of 24 mm at 57 months post-treatment, and a partial response has been maintained for 60 months. Colorectal GIST is rare, comprising 5% of all GIST cases, and surgical resection is the first choice of treatment. In this case, due to a lack of consent, we chose imatinib mesylate as the treatment. However, imatinib mesylate has been reported to induce adverse events more frequently in older patients, and thus we took care to reduce the treatment dosage. We report this case to highlight that a normal quantity of imatinib mesylate can be administered, with no significant adverse events.