15-month-old male with history of left ankle swelling. SOFT TISSUES: Immediately inferior and slightly anterior to the distal fibula, there is a soft tissue lesion demonstrating low T1 and high T2 signal abnormality measuring 11 x 6 mm in maximal axial dimensions. The lesion abuts, but is separate the underling peroneus tendon sheath which is abnormally thickened and of heterogeneous signal intensity. Surrounding the lesion there are inflammatory changes and edema extending medially into the ankle joint space in an infiltrative manner. Heterogeneous enhancement is appreciated both within the lesion itself as well as within the inflamed soft tissues. TENDONS: In the region of the aforementioned mass, the peroneus tendons are abnormally thickened and of heterogeneous signal intensity.LIGAMENTS: No significant abnormality noted.ARTICULAR SURFACES AND BONE: The underling bone marrow signal is normal. ADDITIONAL
Solid, enhancing soft tissue mass adjacent to the fibular tip as above. Thickening and heterogeneous appearance of the adjacent peroneus tendon is thought to be reactive in etiology. The differential diagnosis should include the spectrum of fibromatoses ranging from benign to malignant. Another consideration would be a ruptured ganglion or synovial cyst with surrounding reactive changes and inflammation, although this is felt to be less likely given the physical and clinical presentation.