Apophysitis of the ischial tuberosity mimicking a neoplasm on magnetic resonance imaging.

We present multimodality imaging features of an ischial tuberosity apophysitis in a 13-year-old boy who was an active baseball pitcher. Roentgenography of the pelvis and computed tomography showed mild irregularity in the inferior margin of the left ischial tuberosity. T1-weighted MRI showed a wide area with low signal intensity in the left ischial body; T2-weighted fat-suppression images showed areas with markedly high signal intensity in the ischial apophysis and body and the surrounding periosteum; contrast-enhanced T1-weighted fat-suppression MRI showed that the ischial body, surrounding periosteum, and origin of the hamstring muscles strongly enhanced; technetium-99m scintigraphic scans showed increased isotope uptake in the entire ischial body. Histological specimens obtained from the bone showed increased osteoblastic activity, edema, and proliferation of benign spindle cells and small vessels in the bone marrow spaces. In the present case, because MR imaging demonstrated extensive signal abnormalities involving the apophysis, periosteum, and intramedullary portion of bone, a neoplasm could not be excluded, and a biopsy was undertaken.