(1) Background: To investigate the contralateral neck failure (cRF) rates and outcomes among patients with well-lateralized locally advanced oral cavity squamous cell carcinoma (OSCC) with/without ipsilateral or bilateral neck adjuvant irradiation. (2) Methods: Patients with lateralized OSCC diagnosed between 2007 and 2017 were retrospectively enrolled. Patients who had undergone curative surgery with pathologically proven pT3/4 or pN0-2b without distant metastasis were included, while those with cross-midline, neck-level 1a involvement and positive extra-nodal extension (ENE) were excluded. The primary endpoint was the cumulative incidence of 5-year cRF as the first site of failure. The secondary endpoints included cancer-specific survival (CSS), local-regional recurrence-free survival (LRRFS), distant-metastasis-free survival (DMFS), and contralateral-regional recurrence-free survival (cRRFS). (3) Results: In total, 149 patients were analyzed with a median follow-up time of 5.2 years (range, 2.91–7.83). Pathological stages T3 and T4 were 22.7% and 56.8%, respectively. Pathologically negative and positive lymph nodes were 61.4% and 38.6%, respectively. The cumulative 5-year cRF rate was 3.6% (95% CI, 1.3–7.7%). No significant differences in the 5-year CSS, LRRFS, DMFS, and cRRFS were observed among those undergoing unilateral or bilateral neck irradiation. Five patients (3.4%) had contralateral neck recurrence, all simultaneously with local recurrence. No isolated contralateral neck recurrence was identified. (4) Conclusions: The cRF rate was acceptably low in patients with well-lateralized advanced OSCC with the initially uninvolved contralateral neck. Omitting contralateral neck irradiation with active surveillance could be considered without compromising the cure rate in locally advanced OSCC patients.
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