Extra-articular diffuse-type tenosynovial giant cell tumor with benign histological features resulting in fatal pulmonary metastases.

Diffuse-type tenosynovial giant cell tumor (TS-GCT) is categorized as a locally aggressive but non-metastasizing neoplasm according to the WHO classification. Herein, we report an extremely rare case of a 41-year-old woman who developed multiple metastases from diffuse TS-GCT with benign histological features. The patient complained of a painful buttock mass and imaging studies revealed a soft tissue tumor of the buttock and multiple pulmonary nodules. The buttock tumor was excised and the final diagnosis was extra-articular diffuse-type TS-GCT. By video-assisted thoracic surgery, pulmonary nodules were pathologically identical to the primary tumor. Residual pulmonary nodules progressively grew, and she developed a muscle metastasis and a subcutis metastasis. She died of respiratory dysfunction due to multiple pulmonary metastases 1 year after primary surgery. Very few reports on histologically benign metastases from TS-GCT have been published, and the natural course of this rare condition remains unclarified. This rare case could provide important information for further clinical evaluation of this tumor.