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Unusual Extraarticular Myxoid Malignant Tenosynovial Giant Cell Tumor Mimicking Myxofibrosarcoma: A Report of 2 Cases
Authors:  Bin-cai Pan, M.Phil., Zhi Li, Ph.D., Chun-hua Wang, M.Phil., Gui-fang Huang, M.B.B.S., Ying-wen Liang, M.B.B.S., Hui Fu, M.B.B.S., and Xiao-yun Zheng, M.B.B.S.
  BACKGROUND: Malignant tenosynovial giant cell tumor (MTGCT) is an extremely rare pathological condition whose existence may be debatable. The present study is aimed to investigate the clinicopathological features of extraarticular MTGCT and to discuss the key points of MTGCT diagnosis to improve the accuracy of the current diagnostic modalities. CASES: Two rare cases of extraarticular MTGCT are presented: one in the left thigh muscles of a 46-year-old man and the other in the skin dermal and subcutaneous adipose tissue of the left lower leg of a 62-year-old woman. Histologically, the tumors showed pronounced pleomorphism and atypia of the tumor cells, active mitoses, and extensive necrosis. However, in contrast to most of the previously reported MTGCTs, our cases demonstrated a prominent myxoid change in tumor and displayed an appearance mimicking myofibrosarcoma. The coexistence of conventional tenosynovial giant cell tumor (TGCT) and irregular clefts could also be observed in the tumors. Despite the gross total resection and marginal resection, the tumors showed very aggressive malignancy with rapid tumor recurrence.
We speculate that MTGCT is a neoplastic entity as a malignant counterpart of TGCT. Myxoid variant might be a distinctive morphological feature of MTGCT. Awareness of MTGCT and its various histological features is important to avoid a diagnostic pitfall caused by its histological similarities to the common TGCT and other soft tissue sarcomas in intra-articular or extraarticular locations.
Keywords:  differential diagnosis, extra-articular lesion, giant cell tumor of tendon sheath, myxoid change, tenosynovial giant cell tumor
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