Actinomycosis of the retroperitoneum and an extremity: CT features.

A patient presenting with thigh and flank masses and back pain for 3 months proved to have actinomycosis involving the retroperitoneum and quadriceps muscle. Retroperitoneal involvement without intraperitoneal disease is rare. Computed tomography (CT), however, showed disease transgressing adjacent anatomic compartments with direct extension through the body wall and involvement of adjacent bony structures which is characteristic of actinomycosis.