Manual ABC/2 Ellipsoid Approximation Versus Gold-Standard Delineation for Volumetric Assessment of Residual Vestibular Schwannoma Following Subtotal Resection

Background: In the modern management of vestibular schwannoma (VS), function-preserving subtotal resection (STR) is a frequently employed strategy, necessitating accurate surveillance of residual tumor. The ABC/2 ellipsoid approximation is a time-efficient method for volumetric assessment, but its validity for quantifying irregular, post-surgical tumor remnants remain unestablished. This study aimed to validate the manual ABC/2 method against gold-standard manual delineation for volumetric assessment of residual VS after STR and to determine its impact on clinical classification. Methods: Patients who underwent subtotal resection (STR) for vestibular schwannoma at a single tertiary center between 2006 and 2024 were retrospectively identified. For the ABC/2 method, maximal anteroposterior and mediolateral dimensions were measured on axial images and craniocaudal dimension on coronal images from contrast-enhanced T1-weighted MRIs. For manual delineation, volumes were independently quantified by 3 institutional raters (SY, RR, LK) using semi-automated software (BrainLab) on MRI scans. Agreement was assessed using Pearson correlation and Bland-Altman analysis. Results: Among 285 patients with preoperative data, the ABC/2 method demonstrated a strong correlation with manual delineation (r = 0.898, p20 cm 3 ; r = 0.572). For 278 postoperative residual volumes, the correlation was substantially weaker (r = 0.679, p