The IAEA is currently coordinating a multi-year project to update the TRS-398 Code of Practice for the dosimetry of external beam radiotherapy based on standards of absorbed dose to water. One major aspect of the project is the determination of new beam quality correction factors, k Q , for megavoltage photon beams consistent with developments in radiotherapy dosimetry and technology since the publication of TRS-398 in 2000. Specifically, all values must be based on, or consistent with, the key data of ICRU Report 90.Data sets obtained from Monte Carlo (MC) calculations by advanced users and measurements at primary standards laboratories have been compiled for 23 cylindrical ionization chamber types, consisting of 725 MC-calculated and 179 experimental data points. These have been used to derive consensus k Q values as a function of the beam quality index TPR 20,10 with a combined standard uncertainty of 0.6%. Mean values of MC-derived chamber-specific f ch factors for cylindrical and plane-parallel chamber types in 60 Co beams have also been obtained with an estimated uncertainty of 0.4%.
The calculated kQmsr,Q0fmsr,fref values in this work will enable users to apply the appropriate correction for their own specific phantom material only knowing the electron density of the phantom material.
Purpose The purpose of this study is to provide a calibration methodology for radiation therapy machines where the closest field to the conventional reference field may not meet the lateral charged particle equilibrium (LCPE) condition of the machine‐specific reference (msr) field. We provided two methodologies by extending the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 code of practice (COP) (Palmans et al. TRS‐483: Dosimetry of small static fields used in external beam radiotherapy: an international code of practice for reference and relative dose determination; 2017) methodology for the calibration of radiation therapy machines with 6 MV flattening filter free (FFF) beam and with field sizes down to 10 × 2 cm2. Methods Two methods of calibration were provided following the TRS‐483. In calibration Method I, the generic correction factors kQA,Q0fA,fref were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm2 to 10 × 10 cm2. In calibration Method II, we extended the methodology in TRS‐483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm2. The beam quality specifier for a hypothetical 10 × 10 cm2 field was derived by extending the methodology provided in the TRS‐483. Since the beam quality correction values for the conventional reference field ( kQ,Q0fnormalref) tabulated in TRS‐483 are provided only for large reference chambers, we calculated the kQ,Q0fnormalref values analytically for our beam quality specifier and chambers used, using interaction data in TRS‐398 (Andreo, et al. TRS‐398: Absorbed dose determination in external beam radiotherapy: an international code of practice for dosimetry based on standards of absorbed dose to water; 2001). Results The kQA,Q0fA,fref correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS‐483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm2 field size. However, this had a negligible impact on the kQA,Q0fA,fref values (less than 0.1%). For chambers with C552 and Al electrode material, the correction factors determined using the two methods of calibration were in agreement to within 0.5%. However, for the chambers with electrode made of higher atomic number (Z), the difference between the two methodologies could be as large as 1.5%. It was shown that this difference can be reduced to less than 0.5% if central electrode perturbation effects and kQAFFF,QFFFfA,fref values introduced in TRS‐483 were taken into account. Conclusions In this study, applying the kQA,Q0fA,fref correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absor...
Currently, the American Association of Physicists in Medicine (AAPM) TG-21 is the conventional protocol currently used for the calibration of the Leksell Gamma Knife® (LGK) (despite the publication of the AAPM TG-51 protocol). However, this protocol is based on the air-kerma standards requiring an elaborate conversion process resulting in an increase in the possibility of errors in the clinic. The International Atomic Energy Agency (IAEA) Technical Reports Series (TRS)-483 Code of Practice provides new recommendations on the dosimetry of small static fields and correction factor data for the calibration of the LGK unit. The purpose of this study is to experimentally validate previously calculated factors for the calibration of the LGK Perfexion/Icon unit in the context of the TRS-483 protocol. An experimental comparison between three protocols (TG-51, TG-21 and TRS-483 with the aforementioned correction factors) for calibration of the LGK unit is provided. Dose-rate measurements were performed on a LGK Icon unit using three ionization chambers and three phantoms with different orientations of the chambers with respect to the LGK unit. The dose rate was determined following the three calibration protocols. The standard deviation on the mean dose rate over all phantoms and chambers in different orientations determined using TG-51, TG-21 and TRS-483 protocols were 0.9%, 0.5% and 0.4%, respectively. The mean dose rate calculated using TG-51 protocol was 1.6% and 1.2% lower comparing to the TG-21 and TRS-483 protocols respectively. Applying the values calculated in Mirzakhanian et al () to the measured dose rates in LGK unit for all chambers and phantoms resulted in dose rates that are consistent to within 0.4%. The TRS-483 protocol improves the consistency of the results especially when the chamber was positioned in different orientations with respect to the LGK (from 1.6% when using TG-51 or TG-21 protocols to 0.2% when using TRS-483 protocol) since the other protocols do not correct for the different chamber orientations.
The results are in reasonable agreement with previous experimental and theoretical results which shows the applicability of the Geant-DNA toolkit in nanodosimetry calculations which benefits from the open-source accessibility with the advantage that the DNA models used in this work enable us to save the computational time. Also, the results showed that the simpler geometry is suitable for direct break calculations, while for the indirect damage yield, the more precise model is preferred.
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