Background and Aims

Range uncertainties remain the largest source of uncertainty in proton therapy and prevent taking full advantage of its superior dose conformity. To ensure optimal patient safety, daily quality assurance (QA) procedures are carried out each day before treatment begins, which are often time-consuming. In FLASH proton therapy, short treatment delivery times and high dose-rates mean that typical ionisation chamber-based dosimetry methods become unusable. FLASH dose-rates are currently estimated to be approximately 40 Gy/s or 600 nA to the patient.

The Quality Assurance Range Calorimeter (QuARC) is currently under development at UCL to provide fast, accurate, water-equivalent proton range measurements to speed up daily QA, which could also be used for range QA at FLASH dose-rates. The detector is a series of optically isolated plastic scintillator sheets that sample the proton energy deposition along its path length.

Methods

Each scintillator sheet is coupled to a photodiode that measures its light output directly. An analytical depth-light model is used to fit the data and recover the proton range. A preliminary beam test was conducted at UCLH using pencil beams between 70-110 MeV.

Results

The QuARC was found to consistently recover proton ranges with good accuracy, even with low levels of light. Live fitting of the captured data enables stable real-time range reconstruction at 40 Hz. Given its large dynamic range, measurements at FLASH dose rates are feasible, with an estimated factor 200 increase in light output.

Conclusions

Further measurements are required to fully characterise detector performance and determine light output with FLASH dose-rates.