Tenosynovial Giant Cell Tumor masked as de Quervain Tenosynovitis.

We present a patient who initially presented with clinical symptoms consistent with de Quervain tenosynovitis, resistant to conservative treatment. MRI revealed a cystic mass surrounding the radial styloid within the first extensor compartment (abductor pollicis longus (APL) and extensor pollicis brevis (EPB)), associated with scaphotrapezoid (STT) arthritis. However, upon surgical exploration, the lesion was identified as a localized tenosynovial giant cell tumor (TSGCT), which had caused erosion of the trapezium bone and compressed the EPB tendon. Histopathology confirmed the diagnosis. Postoperative recovery was uneventful, with functional improvement and no recurrence. This exceptional case highlights the importance of accurate diagnosis and the potential for atypical presentations of benign tumors. Informed consent was obtained from the patient after surgery to use her case details in publication for educational and research purposes.