Actinomycosis of the thigh presenting as a soft-tissue neoplasm

A 14-year-old boy presented with a mass measuring 6 cm x 9 cm, proximal to the patella on the anterior aspect of the knee. Magnetic resonance imaging revealed a 6 x 8 x 4 cm smoothly marginated slightly heterogeneous ellipsoidal mass which displaced musculature, the adjacent cutaneous fat and the quadriceps tendon. At surgery, the mass was noted not to penetrate the superficial surface of the quadriceps tendon and was full of slimy fluid. Histological examination revealed multiple basophilic lobulated granules with homogeneous centres and eosinophilic peripheral clubbed projections. The organisms grown anaerobically were identified as Actinomyces species. The anatomical site of the lesion is very unusual for actinomycosis and the exact route of infection in this patient remains unclear.