Re-evaluation of histopathological factors for the outcome of salivary duct carcinoma patients: A multi-institutional retrospective study of 240 cases in a Japanese cohort.

AIMS: Salivary duct carcinoma (SDC) is a relatively common, high-grade salivary gland malignancy that can often occur as a carcinomatous component of carcinoma ex pleomorphic adenoma (CXPA). This study aimed to elucidate the histological factors which are related to the outcome of SDC. METHODS AND RESULTS: We conducted a comprehensive histological review of 240 SDC cases and we analyzed the association between the histomorpholgical parameters and the clinical outcomes to identify new histological prognostic factors. The majority of cases involved the parotid glands (n = 197 cases). SDC showed a marked male predilection (M/F = 5.3:1), and the median age was 66 years-old. This study included 110 de novo cases and 130 CXPA cases. Multivariate analysis revealed that only the pathological stage was significantly associated with overall survival (OS), whereas previously reported histological parameters, such as poorly differentiated clusters, nuclear polymorphism, and mitotic index were not significantly associated with OS and progression-free survival (PFS). Vascular invasion (V [+]) was significantly associated with PFS, and lymphatic invasion was associated with late lymph node metastases. Even in the same pathological stage, V (+) cases always had the worse PFS than V (-) cases. CONCLUSIONS: The histopathological review determined that as distant metastasis relapse was the most important prognostic factor in patients with SDC, V (+) status was also a significant outcome indicator.