Accurate Measurements of Absorbed Dose to Water with Dose Rate Independent Detectors In a Clinical Uhdr Beam
Abstract
Purpose
To demonstrate, for the first time in a clinical setting, the accuracy of a novel robust, dose-rate independent and self-consistent calorimeter system to accurately determine absorbed dose to water in UHDR beams from a Mobetron irradiator.
Methods
The detectors used were two solid calorimeters in a single assembly with aluminum and graphite absorbing cores with thermistors calibrated traceable to primary temperature standards. Absorbed dose to aluminum or graphite was determined using the temperature rise during irradiation multiplied by the specific heat capacity of the absorber. Measurements were obtained in 9 MeV electron beams from a Mobetron irradiator with dose per pulse of nominally 2.8 Gy/pulse at the reference depth in water. Finite element analysis simulations to solve 3D heat transport equations were used to show that no heat transfer correction was required, even at the elevated dose rates used for UHDR beams. However, since the calorimeters measure absorbed dose to aluminum or graphite rather than dose to water, absorbed dose conversion factors are required, which were calculated using Monte Carlo (MC) simulations with realistic models of the calorimeter geometries and the source.
Results
MC calculated conversion factors for dose to water to dose to the calorimeter core were 1.625 for graphite and 1.605 for aluminum. Converted results for dose to water were within 2% when comparing results obtained with the graphite and aluminum calorimeters. Uncertainties in the determination of dose to water using the calorimeters are less than 3%. Calorimeter results converted to dose to water using MC simulations are within 7% compared to measurements obtained with film at the reference depth in water, which is within the combined uncertainties of the measurements.
Conclusion
This work demonstrates that absorbed dose to water can be accurately determined using dose-rate independent calorimeters that are typically only employed at primary laboratories.