Examination of ultra-thin parallel-plate chambers for dosimetry in FLASH radiation therapy

As part of the EMPIR project UHDpulse (http://uhdpulse-empir.eu/), the project partners (Universidad de Santiago de Compostela (USC) and PTW Freiburg GmbH) developed ionization chamber prototypes for dosimetry in FLASH radiation therapy and characterized them in the reference electron beam for ultra‑high pulse dose rate at PTB’s research electron linear accelerator. These prototypes are ultra‑thin parallel‑plate chambers which feature a very small electrode spacing to reduce charge losses due to ion recombination. Under the conditions present in FLASH radiation therapy (ultra‑high dose per pulse), these losses are a significant problem for all available ionization chambers established in clinical dosimetry. The ultra‑thin parallel‑plate chamber prototypes examined show only small losses and thus an almost ideal linear response that necessitates only minor corrections. Ultra‑thin parallel‑plate chambers can therefore play a key role in allowing the continued use of the usual clinical procedures for dosimetry based on established standards and Codes of Practice with only few additions required for FLASH radiation therapy.

Radiation therapy with ultra‑high pulse dose rate is a very promising radiation modality under development for cancer treatment which exploits the so‑called FLASH effect [1]. For this purpose, the prescribed radiation dose is delivered by means of a few radiation pulses but with an ultra‑high dose per pulse (0.6 Gy – 10 Gy). However, the currently available ionization chambers used for dosimetry in conventional radiotherapy show large nonlinearity in their response in the ultra‑high dose per pulse range [2]. They are therefore unsuitable for dosimetry in FLASH radiation therapy.

In conventional radiation therapy, ionization chambers are used for both relative and absolute dosimetry and used in calibration laboratories as secondary standards for the dissemination of the unit of absorbed dose to water D_W.

The current standards for clinical dosimetric procedures (IAEA TRS-398, DIN 6800‑2) are based on measurements with ionization chambers. However, in the ultra‑high dose per pulse range, high charge recombination losses ranging from 20 % to 90 % occur in the currently available ionization chambers, depending on the chamber type and the dose per pulse. In addition, the analytical methods utilized to describe and correct the recombination effects at small dose per pulse fail in the ultra‑high dose per pulse range. Therefore, the conventional dosimetric procedures used in conventional radiation therapy cannot simply be applied in their present state under FLASH conditions. To be able to apply the usual dosimetric procedures used in hospitals also to FLASH radiotherapy, it is necessary to develop ionization chambers that work reliably even at ultra‑high doses per pulse.

As part of the EMPIR project UHDpulse (http://uhdpulse-empir.eu/), the project partners (Universidad de Santiago de Compostela (USC) and PTW Freiburg GmbH) developed ionization chamber prototypes for dosimetry in FLASH radiation therapy and characterized them in the reference electron beam for ultra‑high pulse dose rate at PTB’s research electron linear accelerator. These prototypes are ultra‑thin parallel‑plate chambers in which the distance between the electrodes has been optimized to the smallest possible value (0.2 mm – 0.3 mm). This way, the charge losses due to ion recombination were drastically reduced, without coming below the limit for functioning at standard operating voltages (200 V – 400 V). The prototypes were irradiated with 20 MeV electrons in a water phantom at reference depth, and the generated charge was recorded as a function of the dose per pulse. A beam current monitor calibrated with PTB’s alanine dosimetry system served as a reference. For this purpose, the alanine pellets were placed at the same position as subsequent the prototypes under investigation. In addition, an improved ion recombination model was developed to describe the recombination effects in a parallel‑plate chamber at ultra‑high dose per pulse. This allows the calculation of correction factors even in the ultra‑high dose per pulse range.

As an example, the measurement results of the investigation of an ultra‑thin parallel‑plate chamber with an electrode spacing of d = 0.27 mm are shown in figure 1. The chamber voltage was U = +250 V. The response of this chamber was determined based on measurements in the range of pulse doses smaller than 2 Gy. The blue line represents the ideal behavior, that is to say, a linear extrapolation of the response to higher doses per pulse.

Figure 1: Crosses: Measured charge of an ultra‑thin parallel‑plate chamber with an electrode spacing of d = 0.27 mm as a function of the dose per pulse. Chamber voltage U = +250 V. Blue line: Ideal linear response.

Figure 2 shows the percent deviation of the measurements compared to the calculation done on the basis of the improved ion recombination model. The deviation in the range of up to 5 Gy per pulse is less than 1 %, which is in good agreement with the theoretical value. Ultra‑thin parallel‑plate chambers could therefore play an essential role in enabling the use of ionization chambers in FLASH radiation therapy, thus maintaining the use of usual clinical dosimetric procedures on the basis of established standards and Codes of Practice with only few additions needed for FLASH radiation therapy.

Figure 2: Circles: Percent deviation of the measured charge quantity from the ideal linear response. Red curve: Theoretical deviation calculated with the improved ion recombination model.

The results of this work were presented at the conference titled “FLASH Radiotherapy & Particle Therapy” (FRPT2021) [5, 6] and published in Medical Physics [7].

References

[1]    Favaudon et al., Ultrahigh dose‑rate FLASH irradiation increases the differential response between normal and tumor tissue in mice. Sci Transl Med. 6 (2014) 245ra93.
https://doi.org/10.1126/scitranslmed.3008973

[2]    Petersson et al., High dose‑per‑pulse electron beam dosimetry—a model to correct for the ion recombination in the Advanced Markus ionization chamber. Med Phys. 44 (2017) 1157.
https://doi.org/10.1002/mp.12111

[3]     Schüller et al. The European Joint Research Project UHDpulse—metrology for advanced radiotherapy using particle beams with ultra‑high pulse dose rates. Phys Med. 80 (2020) 134.
https://doi.org/10.1016/j.ejmp.2020.09.020

[4]    Bourgouin et al., Characterization of the PTB ultra‑high pulse dose rate reference electron beam. Phys Med Biol. 67 (2022) 085013.
https://doi.org/10.1088/1361-6560/ac5de8

[5]    F. Gomez et al., Ultra Thin Plane‑Parallel Ionization Chambers: Expanding the Range of Air Ionization Chambers Into Ultra‑High Dose Rate. Phys Med. 94 (2022) S21
https://doi.org/10.1016/S1120-1797(22)01479-X

[6]    Kranzer et al., Vented Ionization Chambers for Ultra‑High Dose Per Pulse Conditions. Phys Med. 94 (2022) S60,
https://doi.org/10.1016/S1120-1797(22)01568-X

[7]    Gomez et al., Development of an ultra‑thin parallel plate ionization chamber for dosimetry in FLASH radiotherapy. Med Phys. 49 (2022) 4705.
https://doi.org/10.1002/mp.15668

Contact

A. Schüller, Department 6.2, Working Group 6.21