Monte Carlo calculated output correction factors for Gafchromic EBT3 film for relative dosimetry in small stereotactic radiosurgery fields

To calculate small field output correction factors, k fclin,fmsr Qclin,Qmsr, for Gafchromic EBT3 film using Monte Carlo simulations. These factors were determined for a Novalis Trilogy linear accelerator equipped with Brainlab circular cones with diameters of 4.0 to 30.0 mm. The BEAMnrc Monte Carlo code was used to simulate the Novalis Trilogy linear accelerator and the Brainlab cones with diameters 4.0 to 30 mm. The DOSXYZnrc code was used to simulate Gafchromic EBT3 film with the atomic composition specified by the manufacturer. Small field correction factors were calculated according to new IAEA TRS-483 Code of Practice for small field dosimetry. The depth of calculation was 10 cm and a source to surface distance of 100 cm. The X-ray beam used in the simulations was a 6 MV SRS. The correction factors were then used to determine field output factors with Gafchromic EBT3 film. These field output factors were validated using three solid state detectors and applying correction factors from the TRS-483 Code of Practice. The solid state detectors were IBA SFD diode, PTW 60018 SRS diode and PTW 60019 microDiamond. The Monte Carlo calculated output correction factors, kfclin,fmsr Qclin,Qmsr, for Gafchromic EBT3 film ranged between 0.998 to 1.004 for Brainlab circular cones with diameters between 4.0 and 30.0 mm. The uncertainty for these factors was 2.0%. The field output factors obtained with Gafchromic EBT3 film were within 2% of the mean results obtained with the three solid state detectors. For field sizes 4 mm diameter and above, Gafchromic EBT3 film has field output correction factors within 1% of unity. Therefore, Gafchromic EBT3 film can be considered to be correction less and supports the assumption made about this film in the TRS-483 Code of Practice.