Extending the IAEA-AAPM TRS-483 methodology for radiation therapy machines with field sizes down to 10 × 2 cm²

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 .

Two methods of calibration were provided following the TRS-483. In calibration Method I, the generic correction factors $$ were calculated using Monte Carlo (MC) for seven detectors and rectangular physical field sizes ranging from 10 × 2 cm² to 10 × 10 cm² . In calibration Method II, we extended the methodology in TRS-483 for deriving the equivalent square msr field sizes for rectangular field sizes down to 10 × 2 cm² . The beam quality specifier for a hypothetical 10 × 10 cm² field was derived by extending the methodology provided in the TRS-483. Since the beam quality correction values for the conventional reference field ( $$ ) tabulated in TRS-483 are provided only for large reference chambers, we calculated the $$ 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).

The $$ correction values calculated using the first method for chambers with an electrode made of C552 almost did not vary across the different field sizes studied (within 0.1%) while it varied by 1.6% for IBA CC01 with electrode made of steel. Extending the equivalent field and beam quality specifier determination methodology of TRS-483 resulted in a maximum error of 1.3% on the beam quality specifier for the 2 × 2 cm² field size. However, this had a negligible impact on the $$ 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 $$ values introduced in TRS-483 were taken into account.

In this study, applying the $$ correction values calculated using the calibration Method I to the chamber reading improved the consistency on an absorbed dose determination from 0.5% to 0.1% standard deviation (except for the Exradin A16). For this reason we recommend using calibration Method I. If the $$ values are not available for the user's detector, calibration Method II can be used to predict the correction factors. However, the second methodology should not be used for chambers with electrode made of high-Z material unless the electrode perturbation effects and $$ values are taken into account.