Dosimetric characterization of whole brain radiotherapy of pediatric patients using modulated proton beams

This study was designed to investigate dosimetric variations between proton plans with (PPW) and without (PPWO), a compensator for whole brain radiotherapy (WBRT). The retrospective study on PPW and PPWO in Eclipse and XiO systems and photon plans (XP) using controlled segments in Pinnacle system was performed on nine pediatric patients for craniospinal irradiations. DVHs and derived metrics, such as the homogeneity index (HI), the doses to 2% (D(2%)) and 5% (D(5%)) volumes, and mean dose (D(mean)) of the whole brain (i.e., PTV), and the organs at risk (OARs) such as lens and skull, were obtained. The PPW plans from both Eclipse and XiO systems uncovered the following advantages: (1) encompassing a cribriform plate area with the 100% isodose line was better than either PPWO or XP, according to calculated two-dimensional distributions of one patient; (2) the mean value of D(5%) for lens was reduced to 23.6% of D(P) from 54.1% for PPWO or 41.6% for XP; and (3) the mean value of D(mean) for skull was reduced to 94.8% of D(P) from either 98.4% for PPWO or 98.3% for XP. However, the PPW plans also exposed several disadvantages including: (1) the HI of PTV increased to 7.7 from 4.7 for PPWO or 3.7 for XP; (2) D(2%) to PTV increased to 108.8% of D(P) from 104.8% for PPWO or 105.1% for XP; and (3) D(5%) to the skull increased to 104.9% of D(P) from 101.6% for PPWO or 103.4% of for XP. One-half of the observed variations were caused by different penumbra on lateral profiles and distal fall-off depth doses of protons in Eclipse and XiO. Because the utilization on the sharp proton distal fall-off was limited for WBRT, the difference between PPW and PPWO or XP indicated no distinguishable improvement by using a compensator in proton plans.