Evaluation of Two Intensity Modulated Radiotherapy Techniques for Total Body Irradiation: Initial Clinical Experience From an Ongoing Pilot Study

Abstract

A multi-institutional pilot study is initiated to compare two IMRT delivery techniques (volumetric modulated arc therapy (VMAT) and helical tomotherapy (HT)) for total body irradiation as conditioning regimen in patients undergoing hematopoietic stem cell transplant with an updated lung dose limit based on recently published clinical data on lung toxicities. We report comparison and initial clinical experience in dosimetric plan quality, patient setup correction techniques, and dose delivery efficiency.We reviewed treatment planning and delivery record for the initial seven patients that were prospectively enrolled in this study. TBI was given to each patient in 8 fractions at 165 cGy per fraction. Patient height ranged from 154.9 to 193.5 cm. The planning target volume (PTV) is the whole body excluding the lung volume. The total lung and 15 additional normal organs were included in plan optimization and dosimetric evaluation. In planning, mean lung dose (MLD) limit was 8 Gy and maximum dose (Dmax) to normal organs was required to be < 130% of the prescribed dose with a recommended goal of < 115%. Four patients received TBI with HT and the other three on a conventional Linac. Patients treated with HT had an upper body HT plan with a rotating gantry using a 5-cm jaw and a lower extremity HT plan using static AP/PA HT fields. Patients treated on a conventional Linac had an upper body VMAT plan using 9 to 10 VMAT fields and a lower body plan with either VMAT fields or static AP/PA fields. The virtual bolus technique was used in VMAT plans to reduce dose variation due to setup uncertainty. The VMAT fields used a 6 MV photon beam with a 90° collimator angle and had an overlap of at least 2 cm between adjacent fields along the longitudinal direction. All the plans were normalized so that 85% of the PTV received at least 13.2 Gy in total dose.In the upper body HT plans the average PTV dose was 13.6 Gy (range: 13.5 - 13.7 Gy), the MLD averaged at 7.71 Gy (range: 7.21 - 7.95 Gy), and Dmax was < 115% of prescribed dose for at least 14 normal organs. In the upper-body VMAT plans the average PTV dose was 13.95 Gy (range: 13.86 - 14.02 Gy), the MLD averaged at 7.12 Gy (range: 6.85 - 7.63 Gy), and Dmax was < 115% of prescribed dose with 1 to 9 normal organs. The Dmax limit of 130% of prescribed dose was achieved in all the plans. In HT treatments, an MVCT scan was used for daily patient setup corrections in both the upper and lower body treatments and the overall treatment time per fraction was 1.23 ± 0.21 hours (range: 0.90 - 1.88 hours). In VMAT treatments, both cone beam CT scans and orthogonal kilovoltage images were used for daily patient setup on a conventional Linac and the overall treatment time per fraction was 1.39 ± 0.20 hours.Both the HT and VMAT plans achieved adequate lung sparing under our new institutional lung dose limit in TBI. VMAT TBI is clinically feasible and could provide access to more patients for TBI treatments. The two IMRT techniques also allow selective sparing of additional organs in TBI treatments.