Evaluating the impact of ionization chamber-specific beam quality correction factor in dosimetry of filtered and unfiltered photon beams

Abstract

Aim: The response of ionization chamber changes when used at beam quality Q which is different from beam quality Q o (usually 60 Co) that was used at the time of its calibration. Hence, one needs to apply beam quality correction factor (k Q, Qo) during dosimetric measurements. However, k Q, Qo data are unavailable for novel ion chambers in the literature. Moreover, most of such data do not differentiate between filtered (flat) and unfiltered (unflat) beams. In addition, literature-based data do not differentiate among different pieces of the ion chambers of the same make and model. Hence, the purpose of our study was to determine the ion chamber-specific experimental values of k Q, Qo and to evaluate their impact in dosimetry. Materials and Methods: In this work, the value of k Q, Qo were measured for six ionization chambers of three different types in 6, 10, and 15 MV filtered (with flattening filter [WFF]) as well as 6 and 10 MV unfiltered (flattening filter free [FFF]) photon beams. The measured values of k Q, Qo were compared with Monte Carlo-calculated values available in the literature. The uncertainties in measurement of k Q, Qo values were also evaluated. Results: For 6 MV FFF beam, the measured value of k Q, Qo was found to be consistently lower than 6 MV WFF beam for all Sun Nuclear Corporation ion chambers, while it was higher as per the theoretical data. The inter-chamber variation in k Q, Qo values was observed for the same model of the ion chambers. The maximum difference between absolute dose values on using the theoretical and experimental k Q, Qo values was up to 3.23%. Conclusion: The measured absolute dose values by the ion chamber of a given make and model were found different due to the use of its theoretical and experimental k Q, Qo values. Furthermore, the variation in response of different pieces of ion chambers of the same make and model cannot be accounted for theoretically, and hence, the use of theoretical k Q, Qo data may introduce an inherent error in the estimation of absorbed dose to water. This necessitates the use of measured value of k Q, Qo for each ionization chamber.