Calorimeter for Real-Time Dosimetry of Pulsed Ultra-High Dose Rate Electron Beams

Abstract

An aluminum calorimeter was investigated as a possible real-time dosimeter for electron beams with an ultra-high dose per pulse (DPP), as used in FLASH radiation therapy (a few Gy/pulse). Ionization chambers, the most widely used active dosimeter type in conventional external beam radiation therapy, suffer from large ion recombination losses at these conditions. Passive dosimeters, such as alanine, are independent of dose rate but do not provide real-time read-out. In this work it is shown that the response of alanine is independent of the DPP in the investigated ultra-high DPP range (up to 2.3 Gy/pulse). Alanine dose measurements were then used to determine the ion recombination correction for an Advanced Markus plane-parallel ionization chamber at ultra-high DPP. Ion collection losses larger than 50% were observed. Therefore, ionization chambers are not considered suitable for accurate dosimetry in FLASH radiation therapy. As an alternative, in a second (independent) experiment an aluminum open-to-atmosphere calorimeter, operated in the quasi-adiabatic mode was investigated at ultra-high DPP electron radiation. The beam pulse charge, and thus the DPP, was varied to evaluate the linearity of the calorimeter response in the DPP range between 0.3 and 1.8 Gy/pulse. On average, the standard deviation of the calorimeter response was 0.1%. The response was proportional to the DPP in the investigated range. The average deviation of the linear fit of the calorimeter dose as a function of the beam pulse charge was <0.5%. This preliminary investigation suggests that a simplified calorimeter design is suitable as a dosimeter with real-time read-out for clinical FLASH radiation therapy beams.

Technical note: Characterization and practical applications of a novel plastic scintillator for online dosimetry for an ultrahigh dose rate (FLASH)

“…15 However, these protocols are not directly applicable to UHPDR, particularly due to the high recombination factor that is difficult to estimate and introduces increased dosimetric uncertainty. 19,33,34 Instead, nearly all investigators use passive detectors insensitive to dose rate, whether they are thermoluminescent detectors, 9 alanine detectors, 9 optically stimulated luminescence detectors, 23 or radiochromic film. 9,12,23 The dose-response measurements of these passive detectors are measured at conventional dose rates using reference dosimetry traceable to national standards and then used to measure the radiation dose at UHPDR.…”

Radiobiology Experiments With Ultra-high Dose Rate Laser-Driven Protons: Methodology and State-of-the-Art

“…In the attempt to further reduce dose uncertainties with laserdriven protons, an alternative approach employing graphite calorimeters as absolute dosimeters for laser-driven protons is under investigation at the National Physical Laboratory (NPL) in the framework of the European-Joint Research Project "UHDPulse" [57], aimed at establishing protocols and procedures for the dosimetry of ultra-high-pulse electron and proton beams [57,64]. The measurement of the temperature increase in the calorimeter core upon irradiation is a direct measurement of the energy and dose deposited [65].…”

Design, realization, and characterization of a novel diamond detector prototype for FLASH radiotherapy dosimetry

“…However, a noticeable decrease in their ion collection efficiency has been observed when the DPP value exceeds 0.1 Gy/pulse, 34–38 due to direct recombination and polarization effects. Recently, the suitability of a transportable aluminum calorimeter as real‐time detector for the accurate dosimetry of electron beams with UH‐DPP has been demonstrated 27,32 . However, the use of this dosimetric system in clinical environments is often impractical.…”

“…However, these somewhat extreme irradiation conditions are indeed challenging in terms of dosimetry. 11 , 20 , 22 , 26 , 27 , 28 , 29 , 30 As a matter of fact, only passive dosimetric systems (basically alanine, 31 , 32 Gafchromic films, 33 and thermo‐luminescent dosimeters 28 ) have been successfully used at present, provided that specific irradiation protocols are adopted. A delay of hours or even days can be needed in order to get the response from the passive detector irradiation.…”

“…Recently, the suitability of a transportable aluminum calorimeter as real‐time detector for the accurate dosimetry of electron beams with UH‐DPP has been demonstrated. 27 , 32 However, the use of this dosimetric system in clinical environments is often impractical. As far as solid‐state detectors are concerned, silicon diodes exhibit response nonlinearities as a function of the DPP.…”