Background and Aims

Accurate dosimetry is paramount to study the FLASH effect since dose and dose-rate are critical dosimetric parameters governing its underlying mechanisms. With the goal of assessing the suitability of standard clinical dosimeters in a very high dose rate (VHDR) experimental setup, we evaluated the ion collection efficiency of several commercially available air-vented ionization chambers (IC) in conventional and VHDR proton irradiation conditions.

Methods

The Proteus 235 cyclotron (IBA) at the Orsay Proton therapy Centre was used to deliver VHDR pencil beam scanning irradiation. Ion recombination correction factors (k_s) were determined for several detectors (Advanced Markus, PPC05, RAZOR Nano, CC01) using Jaffé plots. Dose rate independent detectors such as a Faraday cup and alanine dosimetry were employed to cross calibrate absolute dose measurements of the ICs.

Results

Mean dose rates at isocenter ranged from 2Gy/s to 230Gy/s, and instantaneous dose rates were up to 1000Gy/s. The IC and Faraday cup agreed within 3% and differences between alanine and reference measurements were smaller than 1% for doses above 20Gy. Recombination correction factors below 1.5% were obtained for all chambers at VHDR with significant variations among detectors, while k_s values were significantly smaller (0.3%) for the conventional dose rates.

Conclusions

While the collection efficiency of the ICs in VHDR proton therapy is comparable to that in the conventional regime or for dose rates smaller than 150Gy/s, the reduction in collection efficiency cannot be ignored and varies significantly between irradiation conditions and detectors.

This work resulted from the project 18HLT04 UHDpulse which received funding from the EMPIR programme.

Background and Aims

Laser-driven proton acceleration produces short radiation bunches with a continuous energy spectrum up to a sharp cutoff; in previous experiments our group proved the biological effect of a “laser-driven fast-fractionation” modality, where the target dose is deposited by a number of ultra-short radiation pulses at ultra-high instantaneous dose-rate. In a new experiment we aim at reaching the relevant target dose in a single laser pulse (which would be a million times shorter that the accepted limit for FLASH effect) and using multiple spectral components to produce an SOBP for thicker biological samples.

Methods

We used the pico2000/LULI laser facility for proton acceleration, using 12.5um gold targets. The particle transport was ensured by a remotely-controlled beam-line composed by two permanent-magnet quadrupoles (LMU) and a scattering system. Dosimetry measurements were performed with radiochromic films, previously calibrated on the CPO/Institut Curie medical accelerator.

Results

A total charge exceeding 150nC/shot was measured, in a continuous spectrum up to 16MeV. At the biological sample plane a maximum deposited dose of 20Gy/shot could be obtained on a surface of 1cm2 and within an estimated deposition time of 10ns. Dose escalation at the irradiation plane, ensured by variable quadrupole configurations, was applied on monolayer cell cultures and on zebrafish embryos. A precise modelling of the dosimetric data is currently being realised.

Conclusions

A high-energy-laser-driven proton irradiation line capable of producing FLASH-like conditions on a mm thick sample in a sub-us time was demonstrated. Fixed, post-development embryos and cell survival assays are currently under analysis.