Dosimetric comparison between volumetric modulated arc therapy on truebeam and halcyon of boost plan for total body irradiation

Total Body Irradiation (TBI) is a special treatment technique with the aim of immunosuppression. The TBI treatment technique was restricted due to the possibility of the radiation therapy facility. The purpose of this study was to compare the dosimetric results of volumetric modulated arc therapy (VMAT) plans of TBI treatment between TrueBeam and Halcyon treatment machines.

The pilot study of 5 TBI patients was planned by 6 MV of two lateral opposing with extended SAD technique to 5 m to receive the prescribed dose of 1200 cGy in 6 fractions at head and legs. A single fraction of boost dose at a thick region ranging from shoulder to mid-thigh was optimized and calculated in Eclipse treatment planning system using VMAT by TrueBeam (6 isocenters) and Halcyon (4 isocenters) machines based on the dose from the original opposing plan. The prescribed dose was normalized at D50%.

The results showed that D98%, D95%, and D2% of PTV were 949.0 ± 21.8 cGy, 989.7 ± 18.5 cGy, and 1351.6 ± 16.9 cGy for TrueBeam and 935.3 ± 29.3 cGy, 966.7 ± 10.5 cGy, and 1379.6 ± 41.1 cGy for Halcyon, respectively. The Dmean of lungs and Dmean of kidneys were 999.6 ± 22.6 cGy and 976.5 ± 14.9 cGy for TrueBeam and 1004.5 ± 26.0 cGy and 980.6 ± 12.0 cGy for Halcyon, respectively. The monitor unit (MU) of TrueBeam was 7455.4 ± 251.6 MU and Halcyon was 7386.2 ± 247.7 MU. The Homogeneity Index of TrueBeam was 0.34 ± 0.02, and Halcyon was 0.37 ± 0.05.

Both plans pass the clinical goals except for the mean lungs dose. TrueBeam plans are slightly better in both PTV coverage and OAR dose sparing.