Purpose: Absorbed dose in water can be determined by monitoring the concentration of hydrated electrons â€“ short-lived radicals produced by water radiolysis â€“ utilizing fast absorption spectrophotometry. However, the technique has not yet been employed for dosimetry purposes in radiotherapy since the optical path required to measure this effect is on the order of metres, making the dosimeter size impractical. The aim of this study was to assess the suitability of hydrated electron spectrophotometry for radiotherapy dosimetry.
Methods: To verify the operation mechanism, a macroscopic water-based multipass absorption experiment was developed. A 45 mW laser diode (660 nm) was used as the light source and biased silicon photodetectors were used for rapid transmission readouts. The optical path was 60 cm in an absorption cell filled with 3 L of deionized pure water. The cell was irradiated with a 10 MV FFF photon beam using a Varian TrueBeam linear accelerator and absorbance was monitored over time.
Results: Examination of the absorbance profiles indicates clear absorption changes in the water when radiation is delivered to the system. Both the amplitude and the decay time of the signal are consistent with the characteristics of hydrated electrons stated in literature. The effect is reproducible, and tests were conducted to confirm that the observed phenomenon is radiation-induced.
Conclusion: This work confirms the potential of hydrated electron absorption spectrophotometry as a dosimetry method in radiotherapy. In future work, we aim to miniaturize this technique, enabling detection of absorption changes in few microns by folding the optical path back on itself thousands of times in a resonant, fibre-coupled microcavity, facilitating in-vivo compatibility and excellent spatial resolution. A new avenue of research for precision in-vivo dosimetry will be opened by adapting technologies from frontiers of quantum optics research â€“ fibre cavities, in particular â€“ to medical physics.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by the Collaborative Health Research Projects (grant 523394-18), Natural Sciences and Engineering Research Council (grant 241018) and CFI JELF (grant 34987).