Unusual Clinical Presentation of Gastrointestinal Clear Cell Sarcoma.

BACKGROUND Use of molecular assays is gradually becoming a mandatory part of the clinical management of soft tissue tumors, however the choice and the interpretation of these tests may present a challenge. SUMMARY This report demonstrates an unusual presentation of sarcoma, which was initially diagnosed as a tumor of unknown primary site. Given the presence of vimentin, Fli-1, CD99 and S100 markers, lack of immunostaining for melan A, HMB45, MITF, synaptophysin, CD56, myf4, CKAE1/3 and WT-1, as well as the presence of EWSR1 translocation determined by a break-apart FISH assay, Ewing's sarcoma (ES) diagnosis seemed to be well justified. However, polymerase chain reaction testing for ES-specific rearrangements (EWSR1/FLI1, EWSR1/ERG, EWSR1/ETV1, EWSR1/ETV4, EWS/FEV) failed to confirm the ES origin of the neoplastic tissue. We further considered clinical, morphological, immunohistochemical and molecular diagnostic features of other types of EWSR1-rearranged sarcomas and performed molecular testing for gastrointestinal clear cell sarcoma. The polymerase chain reaction assay revealed EWSR1ex7/ATF1ex5 fusion, thus confirming the latter diagnosis. Subsequent high-precision computed tomography of the abdominal cavity revealed a 5-cm tumor of the small bowel, which was subjected to surgical resection. KEY MESSAGE This report exemplifies that the use of anonymous cytogenetic assays, such as break-apart FISH EWSR1 testing, may not be sufficient even in case of a perfect match with relevant morphological and immunohistochemical tumor features. PRACTICAL IMPLICATIONS Explicit identification of the translocation gene partners is indeed important for proper sarcoma diagnosis management.