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. 2022 Sep;49(9):6171-6182.
doi: 10.1002/mp.15844. Epub 2022 Jul 14.

Ultrahigh dose rate pencil beam scanning proton dosimetry using ion chambers and a calorimeter in support of first in-human FLASH clinical trial

Eunsin Lee  1   2 , Ana Mónica Lourenço  3   4 , Joseph Speth  5 , Nigel Lee  3 , Anna Subiel  3 , Francesco Romano  6 , Russell Thomas  3   7 , Richard A Amos  4 , Yongbin Zhang  1   2 , Zhiyan Xiao  1   2 , Anthony Mascia  1   2
Affiliations

Ultrahigh dose rate pencil beam scanning proton dosimetry using ion chambers and a calorimeter in support of first in-human FLASH clinical trial

Eunsin Lee et al. Med Phys. 2022 Sep.

Abstract

Purpose: To provide ultrahigh dose rate (UHDR) pencil beam scanning (PBS) proton dosimetry comparison of clinically used plane-parallel ion chambers, PTW (Physikalisch-Technische Werkstaetten) Advanced Markus and IBA (Ion Beam Application) PPC05, with a proton graphite calorimeter in a support of first in-human proton FLASH clinical trial.

Methods: Absolute dose measurement intercomparison of the plane-parallel plate ion chambers and the proton graphite calorimeter was performed at 5-cm water-equivalent depth using rectangular 250-MeV single-layer treatment plans designed for the first in-human FLASH clinical trial. The dose rate for each field was designed to remain above 60 Gy/s. The ion recombination effects of the plane-parallel plate ion chambers at various bias voltages were also investigated in the range of dose rates between 5 and 60 Gy/s. Two independent model-based extrapolation methods were used to calculate the ion recombination correction factors ks to compare with the two-voltage technique from most widely used clinical protocols.

Results: The mean measured dose to water with the proton graphite calorimeter across all the predefined fields is 7.702 ± 0.037 Gy. The average ratio over the predefined fields of the PTW Advanced Markus chamber dose to the calorimeter reference dose is 1.002 ± 0.007, whereas the IBA PPC05 chamber shows ∼3% higher reading of 1.033 ± 0.007. The relative differences in the ks values determined from between the linear and quadratic extrapolation methods and the two-voltage technique for the PTW Advanced Markus chamber are not statistically significant, and the trends of dose rate dependence are similar. The IBA PPC05 shows a flat response in terms of ion recombination effects based on the ks values calculated using the two-voltage technique. Differences in ks values for the PPC05 between the two-voltage technique and other model-based extrapolation methods are not statistically significant at FLASH dose rates. Some of the ks values for the PPC05 that were extrapolated from the three-voltage linear method and the semiempirical model were reported less than unity possibly due to the charge multiplication effect, which was negligible compared to the volume recombination effect in FLASH dose rates.

Conclusions: The absolute dose measurements of both PTW Advanced Markus and IBA PPC05 chambers are in a good agreement with the National Physical Laboratory graphite calorimeter reference dose considering overall uncertainties. Both ion chambers also demonstrate good reproducibility as well as stability as reference dosimeters in UHDR PBS proton radiotherapy. The dose rate dependency of the ion recombination effects of both ion chambers in cyclotron generated PBS proton beams is acceptable and therefore, both chambers are suitable to use in clinical practice for the range of dose rates between 5 and 60 Gy/s.

Keywords: FLASH; PBS; dosimetry.

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Conflict of interest statement

Richard A. Amos is on the Clinical Advisory Board of TAE Life Sciences.

Figures

FIGURE 1
FIGURE 1
Schematic cross‐sectional image of the National Physical Laboratory (NPL) proton calorimeter from Ref. [17] (a) and NPL proton calorimeter aligned at isocenter at gantry zero at the Cincinnati Children's Hospital Medical Center‐University of Cincinnati Medical Center Proton Therapy Center (b)
FIGURE 2
FIGURE 2
Measured data of the inverse of amount of collected charges 1/Q by the (a) Physikalisch‐Technische Werkstaetten (PTW) Advanced Markus and (b) the Ion Beam Application (IBA) PPC05 ion chambers, as a function of the inverse of the squared bias voltage 1/V 2 at various dose rates
FIGURE 3
FIGURE 3
The inverse of collected charges versus the inverse of bias voltages at various dose rates fitted by the Niatel model for the Physikalisch‐Technische Werkstaetten (PTW) Advanced Markus (a) and the modified Niatel with charge multiplication correction for the Ion Beam Application (IBA) PPC05 (b) chambers
FIGURE 4
FIGURE 4
The extrapolated values of ks operated at 300 V and ks factors obtained using the two‐voltage technique at the voltage ratio of 3 for the Physikalisch‐Technische Werkstaetten (PTW) Advanced Markus (a) and the Ion Beam Application (IBA) PPC05 (b) chambers as a function of dose rates. Note that some of ks values from the linear and the semiempirical models are below unity after corrected for a possible charge multiplication effect
See this image and copyright information in PMC

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