Optimising craniospinal irradiation for medulloblastoma: a dosimetric comparison of two VMAT planning methods

Abstract

Background: Craniospinal irradiation (CSI) poses a challenging planning process because of the complex target volume. Traditional 3D conformal CSI does not spare any critical organs, resulting in toxicity in patients. Here the dosimetric advantages of volumetric-modulated arc therapy (VMAT) using partial arc and avoidance sectors are compared with each other in planning in adult patients undergoing CSI to develop a clinically feasible technique that is both effective and efficient. Patient and methods: Eight adult patients treated with CSI were retrospectively identified. In total 16 plans were made. We generated two plans for each patient: 1. VMAT plan using partial arc, namely VMAT_pa. 2. VMAT plan using avoidance sectors, namely VMAT_as. The dose prescribed was 36 Gy in 20 fractions. The dose-volume histogram for planning target volume (PTV) and organs at risk (OAR) (lens, eye, heart, thyroid, lungs, liver, gonads and kidneys) were analysed and compared. Dose parameters of mean dose, V 95% , and V 107% for the PTV were evaluated. Results: The median length of PTV is 65.58 cm (45.8–79.5). The volume of PTV receiving 95% of the dose (V95%) in both the plans are 97.51% (VMAT_as) and 97.99% (VMAT_pa) ( p = 0.121) while V107% are 0.733 and 0.742 for VMAT_as and VMAT_pa, respectively ( p = 0.969). The doses of OARs such as lens, eye, liver and gonads were comparable. The mean heart dose was 10.4 and 9.0 Gy in VMAT_as and VMAT_pa plans, respectively ( p = 0.005). Significant lower doses to the thyroid, kidneys and lungs were seen in VMAT plans using avoidance sectors. Conclusion: This study provides a practically useful VMAT planning method for the treat-ment of CSI and illustrates the ability of VMAT using avoidance sectors to generate highly conformal and homogeneous treatment plans for CSI, while limiting the dose to the relevant OARs.