Unusual Signal Patterns of Break-apart FISH Probes Used in the Diagnosis of Soft Tissue Sarcomas.

Break-apart FISH probes are the most popular and reliable type of FISH probes used to confirm certain pathological diagnoses. The interpretation is usually easy, however, in some instances it is not so unequivocal. Our aim was to reveal and elucidate the problems occurring in the process of evaluation of the break-apart probe results. Altogether 301 soft tissue sarcomas with confirmed molecular tests using break-apart probes were assessed to reveal the frequency and type of unusual signal pattern. Among 89 synovial sarcoma (SS18) 11%, 12 alveolar rhabdomyosarcoma (FOXO1) 50%, 53 myxoid liposarcoma (DDIT3) 7.5%, 6 low grade fibromyxoid sarcoma (FUS) 67%, 93 Ewing sarcoma (EWSR1) 3%, 12 clear cell sarcoma (EWSR1) 8%, 5 desmoplastic small round cell tumor (EWSR1) 0%, 9 extraskeletal myxoid chondrosarcoma (EWSR1) 0%, 2 myoepithelial carcinoma (EWSR1) 50%, 14 dermatofibrosarcoma protuberans (COL1A1) 86% and 6 nodular fasciitis (USP6) 17% atypical break-apart signals were detected. Despite the unusual signal pattern type, the fusion genes were detected using either metaphase FISH, interphase FISH with translocation/TriCheck probe or RT-PCR methods. Although the interpretation problems in the process to evaluate the break-apart probe results is well known from sporadic case reports, a systemic overview to detect their frequency has not been performed so far. In our work we highlighted the relative frequency of this problem and pinpointed those signal-patterns which, despite their unusual appearance, can still confirm the diagnosis.