Improved Organ Sparing with Autoplanned VMAT TBI

Abstract

To evaluate dosimetric differences between Volumetric Modulated Arc Therapy (VMAT) and conventional 2D AP/PA Total Body Irradiation (TBI) techniques.Ten pediatric patients treated with a VMAT TBI technique on a C-arm Linac from November 2019 to August 2020 were included in this study. The VMAT TBI plans were generated using an in-house developed autoplanning script using a treatment planning system. For each VMAT TBI plan a corresponding 2D plan was created replicating our institution's current conventional AP/PA technique with the patient positioned standing in a booth at extended SSD with custom compensators to account for patient thickness variations in 8 regions on central axis, lung blocks with 50%-transmission and electron chest-wall boosts. The clinically relevant metrics in this study, including plan global Dmax, PTV V110%, lungs, and lungs contracted by 1cm (lungs-1 cm) Dmean were analyzed and compared between the VMAT TBI plans and simulated 2D plans. All dosimetric comparisons between the VMAT and 2D plans were made with the dose expressed as a percentage of the prescription dose (2Gy or 12 Gy) and the volume expressed as a percentage of the PTV volume. Paired t-test was used to compare the dosimetric indices between VMAT and 2D TBI plans.All VMAT TBI plans achieved D90%≥100% of prescription. PTV coverage, D90%, was reduced significantly (-6.2% ± 2.4%, P < 0.001) with the 2D plans, whereas no significant differences were observed between the 2D and VMAT global Dmax (P < 0.226) and PTV V110% (P < 0.444). Compared to 2D plans, VMAT TBI plans produced a significant decrease in the Dmean to the lungs and lungs-1cm volumes of -25.6% ± 11.5% (P < 0.001) and -34.1% ± 10.1% (P < 0.001), respectively. In addition to lungs, VMAT TBI technique enabled sparing of other organs: for 12 Gy prescription, kidneys Dmean of 64.7% ± 3.3% of prescription dose; for 2 Gy prescription, testes/ovaries Dmean of 31.6% ± 10.7%, brain Dmean of 74.8% ± 1.6% and thyroid Dmean of 72.5 ± 3.5% of prescription dose. Kidneys, gonads, brain and thyroid received the full prescription dose with 2D plans.Improved lung sparing with superior target coverage and similar global Dmax were observed with the VMAT plans as compared to 2D plans. In addition, VMAT TBI plans provided dose reduction in gonads, kidneys, brain and thyroid. Furthermore, VMAT TBI treatments were tolerated well by the patients in more comfortable supine position.