Dosimetry with a clinical linac adapted to FLASH electron beams

Abstract

Purpose: To assess dosimetric properties and identify required updates to commonly used protocols (including use of film and ionization chamber) pertaining to a clinical linac configured into FLASH (ultra-high dose rate) electron mode.

Methods: An 18MV photon beam of a Varian iX linac was converted to FLASH electron beam by replacing the target and the flattening filter with an electron scattering foil. The dose was prescribed by entering the MUs through the console. Fundamental beam properties, including energy, dose rate, dose reproducibility, field size, and dose rate dependence on the SAD, were examined in preparation for radiobiological experiments. Gafchromic EBT-XD film was evaluated for usability in measurements at ultra-high dose rates by comparing the measured dose to the inverse square model. Selected previously reported models of chamber efficiencies were fitted to measurements in a broad range of dose rates.

Results: The performance of the modified linac was found adequate for FLASH radiobiological experiments. With exception of the increase in the dose per MU on increase in the repetition rate, all fundamental beam properties proved to be in line with expectations developed with conventional linacs. The field size followed the theorem of similar triangles. The highest average dose rate (2 × 10⁴ Gy/s) was found next to the internal monitor chamber, with the field size of FWHM = 1.5 cm. Independence of the dose readings on the dose rate (up to 2 × 10⁴ Gy/s) was demonstrated for the EBT-XD film. A model of recombination in an ionization chamber was identified that provided good agreement with the measured chamber efficiencies for the average dose rates up to at least 2 × 10³ Gy/s.

Conclusion: Dosimetric measurements were performed to characterize a linac converted to FLASH dose rates. Gafchromic EBT-XD film and dose rate-corrected cc13 ionization chamber were demonstrated usable at FLASH dose rates.

Keywords: FLASH; gafchromic film; ionization chamber; ultra-high dose rate.

Fig. 1

A diagram of the components in the linac head in the configuration used to deliver FLASH electron beams. The locations where the dose was measured are also shown (not in scale).

Fig. 2

Verification of the FLASH‐beam energy through interpolation of the depth‐dose ratios of conventional electron beams (16 MeV scattering foil).

Fig. 3

Reproducibility of the delivered dose with FLASH beam as measured with the cc13 chamber for various MUs. The lines indicate the corresponding averages.

Fig. 4

Dose vs. MU measured at SAD = 49 cm using film (absolute dose), cc13 chamber (relative dose), and OSL (absolute dose).

Fig. 5

Dependence of the charge measured with the cc13 chamber on the repetition rate in the FLASH electron mode (16 MeV foil).

Fig. 6

Proposed model explaining how the dose depends on the repetition rate in: (a) and (c) FLASH and in: (b) and (d) conventional beam.

Fig. 7

The dose profiles at SAD = 49 cm in the FLASH beam.

Fig. 8

The dependence of the field size on the distance from the nominal beam center.

Fig. 9

The relative dose distribution in the coronal plane of the 3D mouse phantom irradiated with the FLASH electron beam (18‐MeV beam, 9‐MeV foil, SAD = 49 cm) and the conventional electron beam (16 MeV, at the isocenter).

Fig. 10

The dose rate measured with film (at SAD = 49 cm) computed using the time obtained from counting the video frames of Cherenkov glow.

Fig. 11

The dose rate at various distances from the nominal beam center for the FLASH electron beam. The measurements (film, OSL, and cc13 chamber) are shown together with the effective IVSL model (scaled to the film data at SAD = 408 cm).

Fig. 12

Measured (symbols) and modeled (lines) efficiency of the cc13 chamber at various dose rates (i.e. at various dose‐per‐pulse) for the repetition rate of 600 MU/min. The FLASH data measured at various distances from the beam center is combined with the data measured with the conventional 16‐MeV electron beam.