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

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

Seven proton therapy centers collaborating within INSPIRE project participated in the mailed dosimetry audit of proton therapy treatment units utilizing spot scanning beams organized by EURADOS WG9 'Radiation dosimetry in radiotherapy'.

Methods

Prototype assembly for positioning the detectors in the water phantom has been developed and tested during simultaneous irradiation at IFJ PAN in Krakow and PTC in Prague in June 2020. Alanine, radiophotoluminescent and thermoluminescent detectors from five European institutes have been irradiated with 10 x 10 cm² single layer field and 10 x 10 x 10 cm³ plan.

Results

Very good agreement between the two centers has been obtained for alanine, RPL and TL detectors. Based on the results of the initial experimental campaign, the assembly for detector positioning has been modified. As a next step, one assembly for each participating center has been produced including the corresponding number of water-proof detector holders. In the frame of INSPIRE join research activity, audit manuals, detectors and holder assemblies were distributed to participating centers. Audit irradiations took place in all centers during March-April 2021.

Conclusions

The obtained results provide an indication of dosimetric agreement between centers and bases for definition of standardization procedures in dosimetry of active scanning proton beams. After a successful initial experience, mailed audit of scanning proton beams may become a part of clinical routine helping in standardization of active proton beam dosimetry.

This work was partially supported from the European Union's H2020 Research and Innovation Programme, under Grant Agreement No: 730983 (INSPIRE project).

Background and Aims

We present the current status and outcomes of the joint radiobiological experiment performed at eight European proton therapy centers or research institutes. The study aims to spot the potential differences in the in vitro proton RBE values measured by different groups sharing a similar setup and identify its causes.

Methods

A phantom and a protocol for sample preparation and post-processing are shared among the participants to ensure minimal differences in the biological part of the experimental procedure. In this phantom, V79 cells grow on the polyester slides that can be inserted at different depths, which enables their simultaneous irradiation at multiple positions within the radiation field. The setup is irradiated with proton beams with two SOBP configurations (6 cm, 6 Gy, and 4 cm, 8 Gy), followed by the reference photon irradiation (LINAC or x-ray), and the biological effect is evaluated using a colony-forming assay.

Results

The study is still ongoing, and the spread of data for measured cell survival is yet to be evaluated. However, some non-obvious differences in the experimental procedures and setups are already revealed, e.g. post-processing timing or varying dose distributions in the beam plateau/fall-off regions.

Conclusions

As an outcome of the experiment, we plan to summarize the details of the experimental procedure for biological experiments with proton beams, differing between the centers across Europe. Accounting for these details would help to harmonize future studies in the field.

This work was supported by EU Horizon2020 grant 730983 (INSPIRE).