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

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,Q0fref) tabulated in TRS‐483 are provided only for large reference chambers, we calculated the kQ,Q0fref 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 calibrati