Advantage of Adding Non-Coplanar Arcs for Scalp-Avoidance Whole-Brain Irradiation with Volumetric-Modulated Arc Radiotherapy to Reduce Scalp Dose in Pediatric Patients with Medulloblastomas

Abstract

With increasing long-term survival, permanent alopecia has received much attention as a crucial late adverse effect in pediatric patients with medulloblastoma. Therefore, we performed scalp-avoidance whole-brain irradiation with volumetric-modulated arc therapy (SAWB-VMAT) as a component of craniospinal irradiation. We have previously reported that SAWB-VMAT significantly reduced the scalp equivalent doses in 2 Gy per fraction (assuming α/β = 2) to 50% of the areas (EQD50%) compared to conventional WBRT, and the estimated cut-off EQD50% was 19.9 Gy for severe permanent alopecia. In SAWB-VMAT with two coplanar arcs, there were cases with the high-risk disease in which radiation oncologists and medical physicists had difficulty in reducing the scalp dose below the cut-off EQD50%. Therefore, to investigate the utility of adding non-coplanar arcs to reduce the scalp dose, we conducted a planning study to compare the scalp EQD50% and the dose to the organ at risk (OAR) in VMAT with two coplanar arcs (Co2arcVMAT), VMAT with three coplanar arcs (Co3arcVMAT), and VMAT with two coplanar and two non-coplanar arcs (NcVMAT). Co2arcVMAT, Co3arcVMAT, and NcVMAT plans were created for 10 pediatric patients with medulloblastoma. The planned target volume (PTV) for SAWB-VMAT included regions of the whole brain, cervical spinal cord, cerebrospinal fluid space, and intervertebral foramen. The prescribed dose for the PTV was 35.2 Gy in 22 fractions, and all plans were normalized to ensure that V90 = 99%. The scalp EQD 50% was evaluated separately for four areas (top, back, left, and right) according to anatomical structures in each case. In addition, we also calculated normalized scalp EQD50% values (NV_EQD50%scalp), and Co2arcVMAT was set to unity. The eye, lens, hypopharynx, cochlea, thyroid, oral cavity, and parotid gland were defined as other OARs. The median EQD50% of the top area of the scalp was 21.8 Gy, 22.0 Gy, and 18.2 Gy for Co2arcVMAT, Co3arcVMAT, and NcVMAT, respectively. The scalp EQD50% of the top area in NcVMAT was significantly reduced than in Co2arcVMAT and Co3arcVMAT (P-value < 0.05). The median EQD50% of the top area of the scalp for NcVMAT was < 19.9 Gy, which was the cut-off dose for severe permanent alopecia. The NV_EQD50%scalp index of the back area was reduced in a similar manner with statistical significance (Co2arcVMAT, 1.0; Co3arcVMAT, 1.0; and NcVMAT, 0.94) (P-value < 0.001). When considering the dose of indices of other OARs and coverage of PTV, there were no significant differences between the three techniques. SAWB-VMAT with non-coplanar arcs can reduce the scalp EQD50% of the top area below the cut-off dose of 19.9 Gy.