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.

Immune-related adverse events (irAE) present a unique challenge in modern oncology. In patient treated with immune checkpoint inhibitors (anti-PD-1, anti-CTLA-4, anti-PD-L1) for solid tumors irAEs rate ranges from 60 to 85 % [1]. irAEs lead to complete treatment discontinuation in approximately 40 % of patients [2]. The most common irAEs are endocrinopathies (up to 10 %), pneumonitis (5 to 10 %), autoimmune hepatitis (up to 20 %), colitis (up to 30 %), and skin toxicity (up to 50 %) [3]. Other forms of adverse events, such as hematologic and cardiovascular toxicities, are very rarely reported. We present a clinical case of two rare adverse events in a patient treated with pembrolizumab for PD-L1‑positive advanced non-small cell lung cancer.