The WHO Classification of Tumors of Soft Tissue and Bone subdivides rhabdomyosarcomas (RMS) into alveolar, embryonal, pleomorphic, and spindle cell RMS. Advances in molecular genetic diagnostics have made it possible to identify new RMS subgroups within traditional morphological entities. One of these subgroups comprises rare tumors characterized by epithelioid and spindle cell morphology, highly aggressive clinical course with pronounced tendency to intraosseous growth, and the presence of pathognomonic recurring genetic aberrations- chimeric genes/transcripts EWSR1::TFCP2, FUS::TFCP2, or MEIS1::NCOA2. Starting from 2018, only 26 reported cases of RMS have been assigned to this subgroup. The rarity of such tumors hampers their correct diagnostics for both anatomic pathologists and molecular oncologists. Here we describe a clinical case of intraosseous spindle cell RMS expressing EWSR1::TFCP2 fusion gene, encountered for the first time in our practice, in a 16-year-old female patient presenting with mandibular lesion. The diagnostic process took considerable time and involved RNA sequencing; a high-throughput method of molecular genetic research. The tumor was extremely aggressive, showing resistance to polychemotherapy, radiation therapy, and crizotinib targeted therapy, with the fatal outcome.
Ewing’s sarcoma is a highly malignant small round cell tumor with a unique rearrangement of the EWSR1 (FUS) gene with partners genes of ETS family. Tumors with Ewing's sarcoma morphological features lacking without specific EWSR1 rear-rangement called undifferentiated small round cell sarcomas of bone and soft tissue. This group includes: sarcomas with СIC gene rearrangement, sarcomas with BCOR gene rearrangement and sarcomas with EWSR1 (FUS) gene rearrangement with non-ETS gene-partner. Clinical, morphological and molecular genetic characteristics of these groups of tumors will be described below
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