SU-E-T-731: The Effect of Thermoplatic Mask to the Surface Dose in Sterotactic Brain Treatments with Flattening Filter Free Beams with EBT3 Gafchromic Film and OSL Dosimeter




The surface dose of flattening filter free beams is higher than conventional flattening filter beams. In this study, The effect of thermoplatic mask to the surface dose in sterotactic brain treatments was compared using EBT3 Gafchromic film and OSL dosimeter.


Treatment planning were done using 2 full arc VMAT technique with 6 FFF beam on the CT scan of Randophantom simulating the treatment of sterotactic treatments for one brain metastasis.1×1 cm2 Gafchromic EBT 3 film pieces were placed under the thermoplastic mask on the lens of the phantom. Then OSL dosimeter was also placed at the same way in the same place. Then phantom was irradiated without mask with film and OSL again. The surface doses with or without mask was compared for film and OSL dosimeter seperately.


The calibration of film and OSL dosimetry were done at first. Surface dosemeasurements made using the EBT3 film showed good agreement with results of the OSL dosimetry for standard fields of FFF beams within the measurement uncertainty of 3.1%. The surface dose determined using EBT3 film was found to change from 57 % to 93 % to with the mask. The surface dose determined using OSL dosimeter was found to change from 56 % to 93 % to with the mask. This study also demonstrates the suitability of Gafchromic EBT3 film and OSL dosimeter for surface dose measurements in FFF beams.


The skin sparing effect FFF beams are lower than conventional beams. Significant bolus effect on skin dose in the presence of thermoplastic mask was seen. The skin reactions resulting from thermoplastic masks should be monitored especially for FFF beams and corrective measures should be taken during treatment to avoid skin toxicity.