Temporal Volumetric Response Patterns After LINAC-Based Radiosurgery for Vestibular Schwannoma

Purpose: Tumor control after stereotactic radiosurgery (SRS) for vestibular schwannoma (VS) has traditionally been defined as either stability or reduction in tumor size. However, the definition of treatment failure remains unclear, mainly because of high incidence of pseudo progression. This study aims to evaluate patterns of tumor volumetric changes following LINAC-based SRS for VS, as well as hearing preservation rates and neurological functional outcomes. Methods: Eighty-two consecutive patients with either primary (75%) or residual (25%) VS underwent LINAC-based SRS at our center over eight years. Volumetric follow-up assessment was performed using high-resolution gadolinium-enhanced T1-weighted 3D MRI. Audiometric function as well as other SRS-related complications were assessed throughout the follow-up. Results: Radiological control was achieved in 93% of patients in ⁓4 years of follow-up. Three distinct patterns of tumor volume response were identified: Most patients (75.6%) had a biphasic pattern with initial increase and then gradual decrease in tumor volume, 17.7% demonstrated a stable volume, and 6.7% exhibited a twofold increase in tumor volume. Functional hearing preservation rate (Gardner-Robertson score I or II) after ⁓3 years was 50%. Transient facial and trigeminal neuropathy were recorded in 7 (8%) and 3 (3.7%) patients, respectively. Functional and radiological outcomes for patients with primary vs. residual/recurrent VS were comparable. Conclusions: Early post-SRS volumetric dynamics, rather than absolute tumor size at last follow-up, appear to differentiate true from pseudo-progression. In particular, early tumor expansion exceeding 100% may serve as a marker of treatment failure, whereas more modest early enlargement likely reflects transient post-radiation effects.