Supplementary Material for: Adjuvant sequential chemotherapy for salivary duct carcinomas: a retrospective comparative analysis

Introduction: This study aimed to report the efficacy and safety of adjuvant sequential chemotherapy after definitive treatment of salivary duct carcinoma (SDC) compared with the standard treatment alone (surgery with postoperative radiation therapy). Methods: This was a retrospective study of pathologically confirmed 135 SDC patients (study period 2009 to 2022). After curative surgery and adjuvant radiation therapy, 55 of 135 patients decided to receive additional chemotherapy (OP+RT+Chemo group), while 80 opted for surgery and radiation (OP+RT group). Treatment outcomes of overall survival (OS), disease-free survival (DFS), and distant-metastasis-free survival (DMFS) were compared using a propensity score matching (PSM) analysis. Results: Adjuvant chemotherapy consisted of three cycles of cisplatin-based regimen, which was well tolerated in most patients with minimal adverse events. Multivariable analyses indicated that the addition of chemotherapy did not improve OS (p = 0.05), DFS (p = 0.386), and DMFS (p = 0.735), although there was a trend toward favoring adjuvant chemotherapy in terms of OS. With PSM analysis, OS (OP+RT+Chemo to OP+RT, Hazard ratio [HR] = 0.40, 95% confidence interval [95%CI] = 0.12-1.29, p = 0.126), DFS (HR = 0.69, 95% CI = 0.30-1.56, p = 0.367) and DMFS (HR = 0.96, 95% CI = 0.46-1.99, p = 0.903) were not statistically different. Conclusions: Current cisplatin-based adjuvant chemotherapy did not significantly improve treatment outcomes of SDC patients over the surgery and adjuvant radiation. Further development or clinical studies are required to improve the outcomes of SDC, including chemotherapeutic, biomarkers, immune checkpoint inhibitors, or treatment strategies.