\emph{Objective}. Optical fibre dosimeters offer great promise for real-time in vivo dose measurement in radiation-based treatment modalities such as radiotherapy and brachytherapy. This is attributed to their many useful qualities such as high spatial resolution and sensitivity. However, there are several requirements that an optical fibre dosimeter must meet to be acceptable for dose measurement in a specified treatment modality. In this work, the dosimetric performance of a novel optical fibre dosimeter for use in external beam radiotherapy is presented.
\emph{Approach}. The dosimeter was characterised for photon beam energies between 6-15 MV using a Varian TrueBeam Linac at dose rates between 100-2400 MU/min and assessed based on its repeatability, dose dependence, dose rate dependence, energy dependence and dose-per-pulse dependence.
\emph{Main Results}. The results demonstrated excellent short-term repeatability of 0.3 $\%$, good linearity in response (R$^{2} >$ 0.9997), and minor dose rate dependence between 0.53-2.49 $\%$ for all beam qualities investigated. As the scintillator of the optical fibre dosimeter is non-water equivalent, Monte-Carlo-TOPAS simulations were used to calculate the absorbed dose energy dependence. A dose-per-pulse dependence was also investigated and compared with dosimetry measurements made with an ionisation chamber and simulated from the treatment planning system. An over-response of 20 $\%$ was found at the lowest investigated dose-per-pulse, and an under-response of 34 $\%$ was found at the highest investigated dose-per-pulse. This is believed to be due to an intrinsic energy dependence making this type of optical fibre dosimeter unsuitable for external beam radiotherapy dosimetry.
\emph{Significance}. The optical fibre dosimeter evaluated in this work was primarily designed for high-dose-rate
brachytherapy whereas this study includes the first measurements made in external beam radiotherapy and highlights the challenges of transferability of the dosimeter to a different radiation source.
Keywords: dosimetry; inorganic scintillator; linac; optical fibre dosimeter; radiotherapy.
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