Volumetric and Linear Imaging Response After Stereotactic Radiotherapy for Vestibular Schwannoma

Vestibular schwannomas (VS) are benign tumors of the vestibulocochlear nerve. Advances in magnetic resonance imaging (MRI) have increased the detection of smaller lesions, shifting management toward functional preservation. Stereotactic radiotherapy (SRT) delivered as single-fraction stereotactic radiosurgery (SRS), hypofractionated stereotactic radiotherapy (HFSRT), or conventionally fractionated stereotactic radiotherapy (FSRT) achieves high tumor control, but standardized progression criteria and regimen-stratified outcomes remain underexplored.

We retrospectively reviewed adults with VS treated with SRT (2013-2022). Baseline features, symptoms, and treatment variables were collected. Tumor control was assessed volumetrically (≥20% increase) and linearly (maximum linear diameter (MLD) ≥2 mm) at 12 months, 24 months, and five years. Statistical analyses explored associations with clinical and imaging factors.

Among 143 patients (median age 58; 84 (59%) female), Koos stage III-IV predominated, present in 104 (73%) patients, 44 (31%) had prior surgery, 86 (60%) had cystic component, and 17 (12%) had T2-weighted hypersignal. Regimens were SRS in 23 (16%), HFSRT in 59 (41%), and FSRT in 61 (43%) patients. The median follow-up was 6.5 years. Baseline symptoms were frequent, reported in 141 (99%) patients, mainly hearing loss (127, 89%) and tinnitus (75, 52%). Volumetric progression declined over time, documented in 25 (22%) patients at 12 months, 18 (15%) at 24 months, and 14 (17%) at five years (p=0.030), while linear rates were stable, occurring in 16 (14%) patients at 12 months, 15 (13%) at 24 months, and 10 (12%) at five years. Both measures correlated strongly (ρ=0.76-0.87). Volumetric progression predicted higher clinical worsening, especially hearing decline at 12 months and five years (p<0.05). Outcomes were comparable at five years across regimens, though SRS showed earlier pseudoprogression at 24 months. Salvage surgery or re-irradiation occurred in eight patients (6%).

SRT achieved durable control of VS across SRS, HFSRT, and FSRT in this single-institution cohort, with few salvage interventions. The three-axis volumetric criterion (20% increase) outperformed the MLD 2 mm threshold and correlated strongly with contoured gross tumor volume (GTV). Early progression subsided by 24 months, and stability at 12 and 24 months predicted five-year control. Between-regimen divergence at 24 months was consistent with SRS-driven pseudoprogression, with five-year progression remaining similar across regimens. Baseline imaging offered limited prognostic value, whereas clinical worsening, mainly hearing decline, correlated with volumetric progression. Annual MRI after year 1 is generally adequate, with tighter intervals reserved for selected patients. Prospective, audiometry-standardized studies are warranted for validating hearing-related endpoints.