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 (NM₀12193. 3: c. 379C>T (p. Arg127Cys) ) and apolipoprotein B (NM₀00384. 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.