Study of three-dimensional dose distribution measurement of ultra-high dose rate electron beam.

FLASH radiotherapy which employs an ultra-high dose rate (>40 Gy/s), effectively protects normal tissues and organs while maintaining robust tumor control. However, it also presents serious challenges for quality assurance (QA). Currently, dosimeters face various limitations under FLASH radiotherapy conditions which make it difficult to meet the demand for rapid and high-precision three-dimensional dose verification. In this paper, CMOS cameras were used to capture luminescence images generated by irradiated water and for three-dimensional dose reconstruction. 
Approach: A tri-orthogonal view camera imaging setup was designed and constructed to collect radioluminescence signals in water phantom. The ordered subsets expectation maximization (OSEM) iterative algorithm was adopted to obtain three-dimensional dose reconstruction. The experiments were performed by 6 MV and 10 MV conventional dose rate X-rays, as well as 9 MeV electron with an ultra-high dose rate (UHDR) of 120 Gy/s. The dose distribution was verified by treatment planning system calculations and film measurements, respectively. Quantitative evaluations were performed through percentage depth dose (PDD), lateral dose profiles, and two-dimensional gamma analysis (3%/3 mm, 10% threshold). 
Results: The luminescence intensity of water phantom shows high linearity with the irradiation dose and exhibits independent characteristics with dose rate. The PDD curve of conventional X-ray reconstruction underestimates the dose in the dose fall-off region, and the transverse dose distribution shows high consistency with the reference distribution. Under different irradiation fields, the difference in the full width at half maximum (FWHM) of the lateral dose distribution curves at each depth is less than 1.25 mm. The PDD curve of the UHDR electron beam reconstruction result underestimates the dose in the shallow region. The deviation of the half-peak dose depth from the film measurement result is less than 0.7 mm. Calculations of the gamma passing rate on each plane of the reconstruction result indicate that the method used shows high accuracy for the transverse distribution. 
Significance: The proposed three-dimensional dose distribution measurement method is applicable to both conventional dose rate radiotherapy and UHDR radiotherapy conditions. It shows high accuracy in the evaluation of lateral parameters such as field shape, field size, flatness and uniformity, and can meet the basic requirements for field consistency verification in clinical UHDR radiotherapy QA.