IAEA‐AAPM TRS‐483‐based reference dosimetry of the new RefleXion biology‐guided radiotherapy (BgRT) machine

Purpose The purpose of this study is to provide data for the calibration of the recent biology‐guided radiotherapy (BgRT) machine (Hayward, CA, USA) following the International Atomic Energy Agency (IAEA) and the American Association of Physicists in Medicine (AAPM) TRS‐483 code of practice (COP) (Palmans et al. International Atomic Energy Agency, Vienna, 2017) and (Mirzakhanian et al. Med Phys, 2020). Methods In RefleXion BgRT machine, reference dosimetry was performed using two methodologies described in TRS‐483 and (Mirzakhanian et al. Med Phys, 2020) In the first approach (Approach 1), the generic beam quality correction factor was calculated using an accurate Monte Carlo (MC) model of the beam and of six ionization chamber types. The is a beam quality factor that corrects (absorbed dose to water calibration coefficient in a calibration beam quality ) for the differences between the response of the chamber in the conventional reference calibration field with beam quality at the standards laboratory and the response of the chamber in the user’s A field with beam quality . Field A represents the reference calibration field that does not fulfill msr conditions. In the second approach (Approach 2), a square equivalent field size was determined for field A of and . Knowing the equivalent field size, the beam quality specifier for the hypothetical field size was derived. This was used to calculate the beam quality correction factor analytically for the six chamber types using the TRS‐398. (Andreo et al. Int Atom Energy Agency 420, 2001) Here, TRS‐398 was used instead of TRS‐483 since the beam quality correction values for the chambers used in this study are not tabulated in TRS‐483. The accuracy of Approach 2 is studied in comparison to Approach 1. Results Among the chambers, the PTW 31010 had the largest correction due to the volume averaging effect. The smallest‐volume chamber (IBA CC01) had the smallest correction followed by the other microchambers Exradin‐A14 and ‐A14SL. The equivalent square fields sizes were found to be 3.6 cm and 4.8 cm for the and field sizes, respectively. The beam quality correction factors calculated using the two approaches were within 0.27% for all chambers except IBA CC01. The latter chamber has an electrode made of steel and the differences between the correction calculated using the two approaches was the largest, that is, 0.5%. Conclusions In this study, we provided the values as a function of the beam quality specifier at the RefleXion BgRT setup ( and ) for six chamber types. We suggest using the first approach for calibration of the RefleXion BgRT machine. However, if the MC correction is not available for a user’s detector, the user can use the second approach for estimating the beam quality correction factor to sufficient accuracy (0.3%) provided the chamber electrode is not made of high Z material.

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