ESTRO 2024 - Abstract Book

S3169

Physics - Detectors, dose measurement and phantoms

ESTRO 2024

Results:

The consitency of the EGSnrc MC code was benchamrked based on a Fano test. Generally self consistency is achieved within 0.1% of expected (analytical) values for small volume-type chambers. Agreement within 1% or better was found between the MC and measured lateral and depth doses profiles of all field sizes, validating the beam model. To benchmark the detector models and the experimental setup, the MC calculated and the measured output factors for both detectors were compared. Results show that for all the different setup configuration and for field sizes ≥2x2 cm2 the agreement is within 1%. For field sizes <2x2 cm2, however, the deviation was found to range between 4% and 7%. The unusual discrepancies between the MC and the experimental data for field sizes <2x2cm2, could be partially attributed to the intra-type detector variation. This assumption was tested with further intra-type detector response measurements and a variation of up to 4% were observed between different same type detectors. Figure 1 shows MC calculated and measured output correction factors for a PTW pinpoint 3D and PTW icrodiamond detector in a MRIdian MRI-linac. For the smallest field size (0.82x0.82 cm2) the difference between MC and measured data was up to 7%, for the rest of the field sizes the difference was within 2%.

Conclusion:

This study shows that MC simulations can be used to calculate output correction factors for MRgRT systems. However, care should be taken when performing simulations with field sizes lower than 2x2 cm2, as the intra-type detector variation may introduce significant discrepancies between the MC and the measured data. It should also be noted that this study used conventional ways to validate the beam model (lateral and depth dose profiles), and does not consider the detector dose response, which play a crucial role for accurate small field dosimetry simulations. This may increase the uncertainties on the calculated output correction factors and should be addressed by using the methodologies described in Francescon et al (2008) and Duchaine et al (2020).

Keywords: Small field, dosimetry, MRI guided Radiotherapy