Development of an ultra-thin parallel plate ionization chamber for dosimetry in FLASH radiotherapy

Faustino Gómez, Diego M. Gonzalez-Castaño, Nicolás Gómez Fernández, Juan Pardo-Montero, Andreas Schüller, Alessia Gasparini, Verdi Vanreusel, Dirk Verellen, Rafael Kranzer, Giuseppe Felici, Jose Paz-Martin

Research outputpeer-review


Background: Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra-high dose per pulse as used for FLASH radiotherapy. Purpose: We present the design, development, and characterization of an ultra-thin parallel plate IC that can be used in ultra-high dose rate (UHDR) deliveries with minimal recombination. Methods: The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 (Formula presented.) s pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra-high pulse dose rate electron beams. Results: It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra-high dose per pulse range, at standard voltage of clinical grade electrometers, well into several Gy per pulse. The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so-called ultra-thin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 (Formula presented.) s pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 (Formula presented.) s pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch-Technische Bundesanstalt, PTB). Conclusions: This work demonstrates the ability to extend the dose rate operating range of ICs to ultra-high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.
Original languageEnglish
Pages (from-to)4705-4714
Number of pages10
JournalMedical Physics
Issue number7
StatePublished - Jul 2022

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