Retroperitoneal actinomycosis with intraperitoneal spread stellate pattern on CT

We report a case of pathologically proved actinomycosis that presented unusual computed tomographic (CT) features: a soft-tissue retroperitoneal mass encasing the adjacent ureter resulting in hydronephrosis and spreading into the intraperitoneal space with a stellate pattern. The role of CT in monitoring the response to antibiotic treatment is also discussed.