Canal Filling Ratio Predicts Hearing Decline After Standardized Gamma Knife Radiosurgery for Vestibular Schwannoma

Objective: To identify pretreatment predictors of hearing decline after standardized Gamma Knife radiosurgery (GK) for vestibular schwannoma (VS). Study design: Retrospective cohort study with parallel competing proportional-odds models and robustness assessment across multiple specifications, including inverse-probability-of-follow-up weighting (IPFW). Setting: Single tertiary referral center. Patients: A total of 306 patients with sporadic VS and pre- and post-treatment audiometry. Intervention: GK radiosurgery under a standardized 12-Gy protocol. Main outcome measure: Ordinal hearing decline defined by Gardner-Robertson grade change (0–4). Results: At a mean 5.1-year follow-up (median 4.0; range: 2.0–16.5), 71 patients (23.2%) experienced hearing decline. Canal filling ratio (CFR)—the proportion of IAC volume occupied by tumor—was the only significant morphometric predictor (OR 1.50 per SD; 95% CI: 1.07–2.10; P =0.018), independent of baseline hearing, outperforming intracanalicular tumor volume (OR 1.31; P =0.064; ΔAIC=+2.7) and 4 other candidates (all P >0.07). CFR remained significant across outcome definitions and strengthened after IPFW correction (OR 1.63; 95% CI: 1.15–2.30). Age and sex were nonsignificant. All 12 plan-level dosimetric parameters were nonsignificant; cochlear dose was unavailable. CFR operated as a continuous risk gradient with no useful binary threshold. Conclusions: Accessible on routine imaging before treatment planning, CFR refines hearing prognosis within the GK pathway as a continuous risk gradient. External validation in multicenter cohorts with observation comparison arms is warranted.