MUC5AC immunoreactivity in scattered tumor cells is useful for diagnosing CIC-rearranged sarcoma.

CIC-rearranged sarcoma is an aggressive round cell sarcoma, and an alternative ATXN1/ATXN1L fusion has been reported. Diagnosis may be difficult, and molecular assays may suffer from imperfect sensitivity. Characteristic histology and ETV4 immunohistochemical positivity are diagnostically helpful. However, ETV4 staining is unavailable in most laboratories. Here, we explored the diagnostic utility of MUC5AC immunohistochemistry in CIC-rearranged sarcomas. All 30 cases, except one, of CIC-rearranged sarcomas and 2 ATXN1-rearranged sarcomas were positive for MUC5AC, although the number of immunopositive cells was generally low (< 5%) in most samples, representing a characteristic scattered pattern. The only MUC5AC-negative case had the lowest tumor volume. Among the 110 mimicking round cell malignancies, 12 tumors showed MUC5AC positivity, including occasional cases of synovial sarcoma and small cell carcinoma, whereas the remaining 98 samples were negative. Despite its lower specificity than that of ETV4 and sparse reactivity that requires careful interpretation, MUC5AC may serve as a useful marker for CIC/ATXN1-rearranged sarcoma because of its wider accessibility.