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 Qo (usually 60Co) that was used at the time of its calibration. Hence, one needs to apply beam quality correction factor (kQ, Qo) during dosimetric measurements. However, kQ, 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 kQ, Qo and to evaluate their impact in dosimetry. Materials and Methods: In this work, the value of kQ, 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 kQ, Qo were compared with Monte Carlo-calculated values available in the literature. The uncertainties in measurement of kQ, Qo values were also evaluated. Results: For 6 MV FFF beam, the measured value of kQ, 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 kQ, 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 kQ, 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 kQ, 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 kQ, Qo data may introduce an inherent error in the estimation of absorbed dose to water. This necessitates the use of measured value of kQ, Qo for each ionization chamber.

Keywords

• beam quality correction factor
• flattening filter free
• ionization chamber
• uncertainty