Diagnostic Utility of NKX3.1 in Mesenchymal Chondrosarcoma on Needle Biopsy: A Case Report of a Supraclavicular Soft Tissue Mass with Literature Review.

We report a case of young female in her 20s who presented with a supraclavicular soft tissue mass. Diagnostic biopsy showed a malignant round cell tumor with areas of spindling and hyalinized stroma. The utilization of an immunohistochemistry panel revealed positive results for NKX2.2 and CD99 expression. This positivity led to the consideration of a differential diagnosis of Ewing sarcoma, EWSR1::NFATC2- rearranged sarcoma, and mesenchymal chondrosarcoma for further assessment. On further immunohistochemistry with NKX3.1 and EWSR1 break-apart fluorescent in situ hybridization analysis, a diagnosis of mesenchymal chondrosarcoma was rendered which was later on confirmed with biphasic histology on excision specimen. NKX3.1 is a useful immunohistochemistry marker to resolve the differentials when dealing with undifferentiated small round cell sarcoma of bone and soft tissue, especially on a needle biopsy.