Purpose: Reference dosimetry is the most important task that links to radiation dose.Dosimetry is performed through seminal guidelines such as TG-51 and IAEA-398 to provide ±2% accuracy.Addendum to TG-51 protocol by McEwen et al introduced a new term Prp to account for radial dose distribution of the photon beam over the detector volume that may be present in any beam, but mainly in FFF beams.In this investigation,we provided the implication of Prp in reference dosimetry and its magnitude in reference condition.
Methods: Prp,the radial dose distribution was studied on Elekta and Truebeam with FFF beam.Measurements were made in reference condition with two reference detectors; PTW60013 and SI-A12.Beam energies evaluated were 6MV,6FFF,10MV,10FFF on an Elekta Versa HD and a Varian Truebeam on their respective dose rate.The detectors were placed in a small water phantom and the position was set to center of the sensitive volume.Prp was studied by rotating the table by 15-degree interval from -90 to 90 degree.Also,in zero position Prp was studied with varying x and y position to the length and diameter of the detector.Readings were normalized to zero position to see the effect of beam flatness and radial distribution.
Results: Prp is detector, machine and energy dependent. The magnitude of Prp between ±90 deg angle are 1.0018±0.0018,1.0011±0.0011,0.0015±0.0007,and 1.0004±0.0010 for 6MV,6FFF,10MV and 10FFF for Truebeam for A-12 respectively. The values for PTW60013-Truebeam is 1.0010±0.0009,1.0027±0.0013,1.0018±0.0009,and 1.0006±0.0012.Similar data for Elekta for A-12 are 1.0005±0.0017,1.0001±0.0019,0.9995±0.0011,and 0.9998±0.0013; and for PTW60013 are 1.0008±0.0011,1.0009±0.0011,0.9998±0.0007,and 1.0015±0.0010 or 6MV,6FFF,10MV and 10FFF respectively.
Conclusion: The radial dose distribution does impact the dosimetry however, its magnitude in <0.3% in extreme cases in both machines. For clinical cases and references dosimetry Prp seems to have very limited impact on the calibration in standard condition except FFF beams.
Dosimetry Protocols, Ionization Chamber, Linear Accelerator
TH- External Beam- Photons: Calibration protocol and primary standards