Dosimetric comparison of short and full arc in spinal PTV in volumetric-modulated arc therapy-based craniospinal irradiation

Since 2011 when it was first described, the volumetric-modulated arc therapy (VMAT) technique for craniospinal irradiation (CSI) has always seen the use of large arc lengths for the spine fields ranging from 200 degrees to 360 degrees. This study was aimed to do a dosimetric comparison between the large and shorter spinal arc for CSI. For a cohort of 10 patients, 2 VMAT_CSI plans were created for each patient, one using the conventional full 360 degrees arc (VMAT_FA) for the spine and the other using 100 degrees posterior arc (VMAT_PA) for 23.4 Gy and 35 Gy prescriptions. In both the plans, 360 degrees arc fields were employed for treating cranial volume. Spillage dose (DBody-PTV) to Body-PTV (DBody-PTV: dose to body excluding planning target volume) was compared with VMAT_FA and VMAT_PA plans. In addition to these VMAT plans, a 3-dimensional conformal radiotherapy plan was also created for all these patients to compare the DBody-PTV and target volume related dose constraints. Mean D95% difference between the two VMAT plans did not exceed 1.3% for cranial and spinal targets for both prescription levels. The conformity index (Cl) was averaged over both prescription doses. Average Cl shows a similar value for VMAT_FA (0.84 +/- 0.04) and VMAT_PA (0.82 +/- 0.05) plans. D95%, V110% and Cl did not exhibit a statistically significant difference between partial and full-arc VMAT plans. However, the VMAT_PA plan exhibited a lower DBody-PTV compared to VMAT_FA plans (0.007 <= p < 0.05) in the 1 to 5 Gy range. Nevertheless, partial arc plans could not offer a statistically significant dose reduction for delineated organs compared to full arc plans, except for bilateral kidneys. (C) 2019 American Association of Medical Dosimetrists. Published by Elsevier Inc. All rights reserved.