Characterization of distant metastases in salivary gland cancer patients undergoing upfront surgery followed by postoperative radiation therapy: a multicenter analysis

BACKGROUND: This study aimed to clarify the incidence and outcomes of distant metastasis (DM) in patients with major salivary gland cancer who received upfront surgery followed by adjuvant postoperative radiation therapy. METHODS: We analyzed 214 patients from a multicenter study. The importance of histologic subtypes on distant metastasis-free survival (DMFS) was analyzed, along with other clinicopathologic prognostic factors. Median total doses and fractional doses were 61 (range, 26-70) Gy and 1.8 (range, 1.8-2.5) Gy, respectively. RESULTS: With a median follow-up of 74 (range, 4-174) months, DM developed as a first (n = 44) or second relapse event (n = 4) in a total of 48 (22.4%) patients. In univariate analysis, histologic subtype (P < .001), salivary gland subsite (P = .002), pT stage (P = .001), pN stage (P < .001), perineural invasion (P = .001), extracapsular extension (P = .001), lymphovascular invasion (P = .001), resection margin status (P = .009), and tumor grade (P = .005) significantly affected DMFS. The 5-year DMFS rates were as follows: 100% (acinic cell carcinoma), 100% (basal cell carcinoma), 100% (epithelial myoepithelial carcinoma), and 96.6% (mucoepidermoid carcinoma) in the low-risk group and 62.5% (adenoid cystic carcinoma), 62.5% (adenocarcinoma), 72.6% (carcinoma-ex-pleomorphic adenoma), 68% (salivary duct carcinoma), and 54.5% (squamous cell carcinoma) in the high-risk group. CONCLUSION: DM risk after postoperative radiation therapy varies substantially by histologic subtype in salivary gland cancer patients. High-risk subtypes demonstrate inferior DMFS, indicating a potential need for improved systemic treatment strategies.