Novel SZT2::MAST2 Fusion Detected in Salivary Duct Carcinoma

Salivary duct carcinoma (SDC) is an uncommon neoplasm that often develops early regional and distant metastasis. Standard treatment for SDC is wide surgical resection along with lymph node dissection followed by adjuvant radiation therapy. The role of adjuvant chemotherapy is not clear with an overall low survival rate. SDC resembles high-grade invasive ductal carcinoma (IDC) of the breast both histologically and by immunohistochemistry (IHC). Fusions involving microtubule-associated serine/threonine (MAST) kinases have been suggested to play role in a subset of breast cancer, although the exact mechanism remains unclear. In this report, we present a case of SDC harboring a novel exon 1 of SZT2 joined to exon 4 of MAST2 leading to a likely pathogenic SZT2::MAST2 fusion. Given the reported fusions involving the MAST kinases in breast cancer, this finding demonstrates an additional similarity between these two tumor types.