Validation of dose-volume calculation accuracy for intracranial stereotactic radiosurgery with volumetric-modulated arc therapy using analytical and clinical treatment plans

Abstract

Background: Stereotactic radiosurgery (SRS) poses challenges in calculating dose volume histograms (DVHs) due to the inherent spatial discretization uncertainties. Purpose: To develop a procedure for quantifying discretization errors and assessing DVH accuracy in intracranial SRS applications. Methods: The capability of Monaco Treatment Planning System (TPS) to calculate structure and isodose volumes using slice thickness (ST) and dose grid (DG) of 1 mm was validated against analytical values, and in-house calculations performed for 15 patients with brain metastases diameters ranging from 6 to 30 mm treated with a VMAT technique. For these patients, STs of 1.5 and 2 mm, and DGs of 2 and 3 mm were also explored to establish clinically acceptable thresholds, using isodose volume calculations and clinically relevant DVH indices. Results: Monaco TPS presented an excellent performance in calculating structure and isodose volumes for the 1-mm ST and DG, with an average percentage difference compared to analytical and in-house calculations within 2.1%. For the clinical treatment plans, switching to 2 and 3-mm DGs led to statistically significant differences compared to 1-mm DG across all the considered indices, yet the variations between the 1 and 2-mm DGs remained under 5% when target diameters exceeded 20 mm. Although no statistical differences were observed between the calculated indices when different STs were considered, clinically significant differences were observed in selected cases with lesion diameters smaller than 20 mm. Conclusion: Monaco TPS demonstrated excellent performance in calculating structure and isodose volumes pertinent to SRS applications using 1-mm ST and DG, while adherence to 1-mm ST and DG should be maintained in clinical cases, unless target diameters surpass 20 mm, where STs and DGs up to 2 mm could be also utilized. The method developed in this study could act as a quality assurance procedure in order to establish clinically relevant discretization thresholds for SRS platforms.