Comparison of IPSM 1990 photon dosimetry code of practice with IAEA TRS-398 and AAPM TG-51

Analysis of Uncertainties in Clinical High-Energy Photon Beam Calibrations Using Absorbed Dose Standards

We compared the results of absorbed dose measurements made using the TRS-398, TG-51, and DIN protocols and their associated uncertainties to reduce discrepancies in measurement results made using the three protocols. This experiment was carried out on two Varian Medical linear accelerators with 4, 6, 10, and 20 MV photon energies using FC65-G and CC15 (cylindrical) and NACP-02-type (plane-parallel) ion chambers in water phantoms. The radiation beam quality index (Q) was determined from the measurement of percentage depth dose. It was used to determine the photon beam quality factor required with the ionization chamber calibration factor to convert the ion chamber reading into the absorbed dose to water. For the same beam quality, the TRS-398/TG-51 varied from 0.01% to 1.8%, whereas the ratio for TRS-398/DIN 6800-2 varied from 0.1% to 0.88%. The chamber-to-chamber variation was 0.09% in TRS-398/TG-51, 0.03% in TRS-398/DIN, and 0.02% in TG-51/DIN 6800-2. The expanded uncertainties (k = 1) were 1.24 and 1.25 when using TRS-398 and DIN 6800-2, respectively. Using the aforementioned three protocols, the results showed little chamber-to-chamber variation and uncertainty in absorbed dose measurements. The estimated uncertainties when using cylindrical ion chambers were slightly lower than those measured using plane-parallel chambers. The results are important in facilitating comparisons of absorbed dose measurements when using the three protocols.

Validation of Monte carlo Geant4 multithreading code for a 6 MV photon beam of varian linac on the grid computing

Aim

To evaluate the computation time efficiency of the multithreaded code (G4Linac-MT) in the dosimetry application, using the high performance of the HPC-Marwan grid to determine with high accuracy the initial parameters of the 6 MV photon beam of Varian CLINAC 2100C.

Background

The difficulty of Monte Carlo methods is the long computation time, this is one of the disadvantages of the Monte Carlo methods.

Materials and methods

Calculations are performed by the multithreaded code G4Linac-MT and Geant4.10.04.p02 using the HPC-Marwan computing grid to evaluate the computing speed for each code. The multithreaded version is tested in several CPUs to evaluate the computing speed according to the number of CPUs used. The results were compared to the measurements using different types of comparisons, TPR_20.10, penumbra, mean dose error and gamma index.

Results

The results obtained for this work indicate a much higher computing time saving for the G4Linac-MT version compared to the Geant4.10.04 version, the computing time decreases with the number of CPUs used, can reach about 12 times if 64CPUs are used. After optimization of the initial electron beam parameters, the results of the dose simulations obtained for this work are in very good agreement with the experimental measurements with a mean dose error of up to 0.41% on the PDDs and 1.79% on the lateral dose.

Conclusions

The gain in computation time leads us to perform Monte Carlo simulations with a large number of events which gives a high accuracy of the dosimetry results obtained in this work.

Comparison of AAPM Addendum to TG-51, IAEA TRS-398, and JSMP 12: Calibration of photon beams in water

The American Association of Physicists in Medicine (AAPM) Working Group on TG-51 published an Addendum to the AAPM's TG-51 protocol (Addendum to TG-51) in 2014, and the Japan Society of Medical Physics (JSMP) published a new dosimetry protocol JSMP 12 in 2012. In this study, we compared the absorbed dose to water determined at the reference depth for high-energy photon beams following the recommendations given in AAPM TG-51 and the Addendum to TG-51, IAEA TRS-398, and JSMP 12. This study was performed using measurements with flattened photon beams with nominal energies of 6 and 10 MV. Three widely used ionization chambers with different compositions, Exradin A12, PTW 30013, and IBA FC65-P, were employed. Fully corrected charge readings obtained for the three chambers according to AAPM TG-51 and the Addendum to TG-51, which included the correction for the radiation beam profile (Prp ), showed variations of 0.2% and 0.3% at 6 and 10 MV, respectively, from the readings corresponding to IAEA TRS-398 and JSMP 12. The values for the beam quality conversion factor kQ obtained according to the three protocols agreed within 0.5%; the only exception was a 0.6% difference between the results obtained at 10 MV for Exradin A12 according to IAEA TRS-398 and AAPM TG-51 and the Addendum to TG-51. Consequently, the values for the absorbed dose to water obtained for the three protocols agreed within 0.4%; the only exception was a 0.6% difference between the values obtained at 10 MV for PTW 30013 according to AAPM TG-51 and the Addendum to TG-51, and JSMP 12. While the difference in the absorbed dose to water determined by the three protocols depends on the kQ and Prp values, the absorbed dose to water obtained according to the three protocols agrees within the relative uncertainties for the three protocols.