Tenosynovial Giant Cell Tumor (TGCT) in the Hip Joint With an APOB Mutation.

A diffuse-type tenosynovial giant cell tumor (D-TGCT) is a rare, locally invasive tumor that mainly involves large weight-bearing joints, such as the knees, hips, and ankles. Herein, we report a rare case of D-TGCT of the hip joint in a 14-year-old girl. The patient presented with pain and limited movement without a history of trauma. Owing to the continuous progression of hip synovitis, whole exome sequencing was performed, which identified pathogenic variations in the frizzled class receptor 4 (NM_012193.3:c.379C>T(p.Arg127Cys)) and apolipoprotein B (NM_000384.2:c.10579C>T(p.Arg3527Trp)) genes. APOB mutations are associated with hypercholesterolemia. Magnetic resonance imaging of the right hip joint showed thickening of the synovial membrane with irregular edges, low signal intensity on T1-weighted images, and high signal intensity on PDWI+FS images. Small nodules with low signal intensity and clear boundaries were also observed on both types of images. An arthroscopic examination revealed numerous white synovial, chondromate-like tissues under the articular capsule, and D-TGCT was diagnosed based on the histopathological results. One year after the surgery, the patient had good functional recovery and no tumor recurrence.