Bolus effect of immobilization mask during CyberKnife treatment

PurposeTo investigate the effect of thermoplastic masks on the surface dose of CyberKnife (CK) treatment.MethodsThe head of RANDO phantom was used to measure the percentage increase in the surface dose using two different thermoplastic mask brands. Surface doses on the phantom were measured for three different scenarios using the optically stimulated luminescence dosimeter and EBT3 films. The increase of percentage surface dose with the application of mask was calculated.ResultsSince the CK had a noncoplanar and nonisocentric beam delivery, the application of immobilization mask increased surface dose up to 58%. In the brain SRS treatment generated using fixed cone, the mean percentage surface dose decreased approximately 36% with respect to reference setup for both mask brands. This decrease was approximately 30% in MLC treatment.ConclusionThe bolus effect of the immobilization mask is important for CK delivery. To avoid skin toxicity, surface doses should be taken into consideration.