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

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 volumetricmodulated 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 VMATₚa. 2. VMAT plan using avoidance sectors, namely VMATₐs. 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% (VMATₐs) and 97. 99% (VMATₚa) (p = 0. 121) while V107% are 0. 733 and 0. 742 for VMATₐs and VMATₚa, 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 VMATₐs and VMATₚa 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 treatment 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.