RADIOLOGY: ABDOMEN: Case# 33054: PSOAS/FLANK ABSCESS 2? TO ACTINOMYCOSIS. Progress evaluation of cystic right retroperitoneal mass. There is a large right retroperitoneal abscess. It extends from the level of the right renal hilum inferiorly to the right iliacus. From this level it extends posteriorly into and around the gluteal muscles. At the level of the gluteal muscles, the collection measures 6 x 5 cm. There is no associated bone destruction.