Author: Yunzhi Ma, Ling Zhuang 👨🔬
Affiliation: Northwestern Medicine 🌍
Purpose: Multiple layers of brass mesh bolus (RPD, Inc., Albertville, MN) may be needed to provide better body conformity and sufficient dose build up on patient surface during electron treatment. This study was to evaluate the dosimetric characteristics of multi-layers of brass mesh bolus (BMB) under Varian TrueBeam (Varian Inc., Palo Alto, CA) electron beam.
Methods: Solid water phantom with Edge detector (Sun Nuclear, Melbourne, FL) at Dmax was used in the phantom setup. Electron energy 6E, 9E and 12E were delivered with 10x10 cone using 100MU to the phantom without BMB and multiple layers of BMB (1-3 layers) on top of phantom using SSD setup. BMB water equivalent thickness (WET) was calculated through comparing calculated PDD with PDD from commissioning measurement. In addition, Dmax-measure, Surface depth dose (SDD)measure, R90-measure were generated based on comparing WET measurement with commissioning PDD and compared with Dmax-calculation, SDDcalculation, R90-calculation extracted from Eclipse treatment planning (Varian Inc., Palo Alto, CA) calculations.
Results: Based on measurement, the WET of 1-3 layers of BMB were 0.45cm/0.76cm/1.0cm for 6E, 0.35cm/0.67cm/0.96cm for 9E and 0.31cm/0.70cm/1.00cm for 12E. Bolus placement led to Dmax shift to surface with an average of 0.09cm based on measurement and 0.18cm based on calculation. SDDmeasure was on average 5.5±2.3% higher than SDDcalculation which was likely due to the hollow centers of BMB leading to repeatability variations with multi-layer placement. Based on paired t-test, R90-measure was not significantly different from R90-calculation (p=0.2) which indicated the WET from measurement was clinically acceptable.
Conclusion: Our results indicated that multiple layers of BMB could be used on patients to build up skin dose. However, the WET of multiple layers of BMB was energy specific and showed non-linear relationship with the physical thickness of the bolus thus should be used with special consideration on patients.