Dosimetric And Treatment Efficiency Comparison Among Volumetric Modulated Arc Therapy, Proton Beam Therapy And Intensity Modulated Radiotherapy For Brain Metastases

Part 1: Hypofractionated stereotactic radiotherapy for brain metastases: A dosimetric and treatment efficiency comparison between volumetric modulated arc therapy and intensity modulated radiotherapyObjective: A treatment planning comparison study was performed to evaluate the dosimetric characteristic and treatment efficiency of volumetric modulated arc therapy with step-and-shoot intensity modulated radiotherapy (IMRT) for the hypofractionated stereotactic radiotherapy (HFSRT) in patients with brain metastases.Methods: CT datasets of 10 patients with two to four brain metastases were selected for the comparison. The structures of GTV and OARs were delineated on the CT axial image. Three plans were generated for each case: seven-field step-and-shoot IMRT, single (RA1) and double (RA2) arcs with RapidArc technique (RA, Varian Medical System). The prescribed dose was 50Gy in 10 fractions and plans were all normalized to the mean dose to the planning target volume (PTV). For PTV, plans aim to achieve at least 95% of PTV was encompassed by the prescription dose. Plans were compared in terms of the target coverage, conformity index, homogeneity index, maximum dose to critical organs and the volume of the normal tissue receiving low dose by analysis of dose-volume histogram. The number of monitor unit was scored to measure expected efficiency. Results: The plans generated by three techniques were clinically acceptable. The volume receiving 100% prescribed dose (V100%)were 97.3%±1.6%, 97.7%±1.5% and 98.2%±1.1%, while V107% were 28.1%±16.9%, 19.1%±4.8% and 15.6%±3.3% for IMRT, RA1 and RA2, respectively. The CI was 0.868 (IMRT), 0.863(RA1) and 0.895 (RA2), and HI was 7.7 (IMRT), 7.5 (RA1) and 6.5(RA2), respectively. Our results showed that target coverage, conformity and homogeneity index were slightly improved with RA2 plans compared to both RA1 and IMRT(P<0.05), while there was no significant difference between IMRT and RA1. Compared with IMRT, the maximum dose in RA2 plans to the brainstem, left and right optic nerves, left and right lens was reduced by 1.6Gy, 6Gy, 3Gy, 1.5Gy, 1.3Gy, respectively. V5 of healthy tissue, as equivalent for low-dose area, was lower for IMRT and higher for RA1 and RA2(P<0.05), while V15 and V20 were lower for RA2(P<0.05). With the same prescription dose, the number of MU per fraction reduced by 36.8% and 27.2% compared to IMRT for RA1 and RA2, respectively. The pure beam-on time needed per fraction was 6.5±1.2min (IMRT), 1.25 min (RA1) and 2.5 min (RA2), respectively. The beam-on time for RA1 and RA2 was approximately 80% and 40% less compared to IMRT.Conclusion: Dose distributions of three treatment planning protocols were acceptable clinically. RA2 show slightly improvement in target coverage, conformity and homogeneity, and also in sparing of OARs with less monitor units compare to conventional IMRT.Part 2: Dosimetric study of hypofractionated proton beam radiotherapy for brain metastases.Objective: A treatment planning study was performed to evaluate the potential benefit of proton beam therapy (PBT) by comparing the dosimetric characteristics with volumetric modulated arc therapy and intensity modulated radiotherapy (IMRT) for the hypofractionated stereotactic radiotherapy (HFSRT) in patients with brain metastases.Methods: CT datasets of 8 patients with oligo brain metastases were selected for the comparison. The structures of GTV and OARs were contoured on the CT axial image. Three treatment plans were generated in Shandong tumor hospital for each case: seven-field step-and-shoot IMRT, single (RA1) and double (RA2) arcs with RapidArc technique. Simultaneously the above CT image and structures were transferred to Eclipse Proton treatment planning system of proton therapy center in Zibo Wanjie cancer hospital to generate the PBT treatment plans. The prescribed dose was 50Gy in 10 fractions for all plans. For PTV, plans aim to achieve at least 95% of PTV was encompassed by the prescription dose. By analysis of dose-volume histogram, four plans were compared in terms of the target coverage, conformity index, homogeneity index, maximum dose to critical structures and the volume of the surrounding normal tissue receiving low dose.Results: For all cases, the PBT treatment plans were clinically acceptable. For PTV, the CI was 0.863 (IMRT), 0.870(RA1), 0.896 (RA2) and 0.690(PBT), and HI was 7.7 (IMRT), 7.4 (RA1), 6.4(RA2) and 3.5(PBT), respectively. Our results showed that compared to both IMRT and RapidArc, the maximum dose to targets and homogeneity index were improved significantly with PBT(P<0.05), while the conformity index was decreased. With PBT, the dose to the brainstem, optic nerves, and lens was reduced significantly, and the same trend was observed in the volume of healthy tissue receiving 5Gy to 20Gy(P<0.05).Conclusion: For the irradiation of brain metastases, PBT has the potential to achieve superior dose distributions compared to IMRT and RapidArc. PBT provides significant improvement in target coverage and homogeneity, and also the superior sparing of OARs and surrounding tissues.