Abstract
PURPOSE: We performed scalp-avoidance whole-brain irradiation with volumetric-modulated arc therapy (SAWB-VMAT) as a component of craniospinal irradiation. In SAWB-VMAT with two coplanar arcs, radiation oncologists and medical physicists sometimes experience difficulty in reducing the dose to the scalp to below the cut-off equivalent dose in 2 Gy per fraction (assuming α/β = 2) to 50% (EQD50%(scalp) ). To investigate the advantage of adding coplanar or non-coplanar arcs in reducing the dose to the scalp in SAWB-VMAT, we conducted a planning study to compare the EQD50%(scalp) , the dose to other organs at risk (OARs), and target coverage in VMAT with two coplanar arcs (Co2arcVMAT), VMAT with three coplanar arcs (Co3arcVMAT), and VMAT with two coplanar and two non-coplanar arcs (NcVMAT). METHODS: Co2arcVMAT, Co3arcVMAT, and NcVMAT plans were created for 10 pediatric patients with medulloblastoma. The planned target volume (PTV) included the regions of the whole brain, cervical spinal cord, cerebrospinal fluid space, and intervertebral foramen. The EQD50%(scalp) was evaluated separately for four areas (top, back, left, and right) in each case. The prescribed dose for the PTV was 35.2 Gy in 22 fractions. RESULTS: The median EQD50%(scalp) of the top area was 21.9 , 22.1 , and 18.3 Gy for Co2arcVMAT, Co3arcVMAT, and NcVMAT, respectively. The EQD50%(scalp) of the top area was significantly reduced in NcVMAT compared to those in Co2arcVMAT and Co3arcVMAT (p < 0.05). The median EQD50%(scalp) of the top area for NcVMAT was < 19.9 Gy, which is the cut-off dose for severe permanent alopecia. There were no significant differences in EQD50%(scalp) in the three other areas, the dose to other OARs, or the dose coverage of PTV among the three techniques. CONCLUSION: NcVMAT could reduce the EQD50%(scalp) of the top area below the cut-off dose of 19.9 Gy. NcVMAT appears to be a promising treatment technique for SAWB-VMAT.