MRI findings of peri-anal abscess due to actinomycosis

We present a case of a peri-anal abscess infected by actinomycosis in a 57-year-old male who initially suffered from mild anal pain. There was no fever, body weight loss, night sweats or abnormal findings in laboratory tests. There was also no peri-anal discharge. Our diagnosis at that time was inconclusive. A PET scan was then requested. It successfully detected a left peri-anal lesion, but the initial impression based on the dual phases study, was more in favor of malignancy. An MRI study was then performed, which revealed a cystic lesion posterior to the rectum, with a sinus tract leading towards peri-anal skin but without associated soft tissue mass. The patient returned and reported a dark-reddish discharge from his peri-anus about six weeks later. The pathologist finally confirmed it as actinomyocosis. We concluded that MRI imaging was very advantageous in pre-surgical diagnosis of the peri-anal abscess. The pathologist finally confirmed it as actinomycosis.