Treatment Results of Postoperative Radiotherapy on the Salivary Duct Carcinoma of Major Salivary Gland

Salivary duct carcinoma (SDC) is a high grade pathologic type and rare malignancy. Currently, there are no confirmed prognostic variables in the management of SDC. The objective of this study is to evaluate the clinical outcome and prognostic factors of 35 cases of SDC treated with postoperative adjuvant radiation, and to investigate postoperative adjuvant radiotherapy (RT) role of SDC. A total of 35 cases of SDC underwent complete surgical resection were retrospectively reviewed. Neck dissection was performed in 31 patients (88.6%). All patients received postoperative adjuvant RT to tumor bed and ipsilateral neck node. Prescribed median dose was 59.4 Gy (range, 50.4 - 71.4). The evaluated prognostic factors were gender, age, symptom duration, tumor site, tumor size, TNM classification, and pathologic features; perineural invasion (PNI), lymphovascular invasion (LVI), extra-parenchymal invasion, and resection margin status. Overall survival (OS), locoregional control (LRC), and disease-free survival (DFS) rates were estimated using Kaplan-Meier method. The median follow-up period was 43 months (range, 7 - 155). The median age at the initial diagnosis in SDC was 62 years (range, 38 - 75), and it was observed in 30 male patients (85.7%). Parotid gland was mainly affected in 22 cases (62.9%). Eighteen patients (51.5%) were a pathologic T3/T4 tumor, and pathologic nodal involvement was presented in 26 patients (74.3%). The actuarial 3-year locoregional control rate was 75.8%, disease free survival rate was 55.7%, and the 3-year overall survival rate was 79.5 %. Cause specific death rate was 31.4% (n = 11). Pathologic nodal involvement was correlated with distant metastasis (p = 0.011). Lymphovascular invasion was significant prognostic factor of distant metastasis-free survival (p = 0.049), locoregional control (p = 0.012), and overall survival (p = 0.003) in the Cox proportional hazard model. Perineural invasion was significant prognostic factor only of overall survival (p = 0.005). Despite high nodal involvement rate, loco-regional control was successful. Surgery and postoperative radiotherapy were effective for locoregional control. Lymphovascular invasion and perineural invasion were significant prognostic factors for patients with SDC.