Room: AAPM ePoster Library
Purpose: calorimetry (OC) is based on interferometry and provides a direct measurement of absorbed dose to water by measuring refractive index changes induced by radiation. The phase change of a laser passing through an irradiated cell is recorded by a digital camera and absorbed dose is reconstructed mathematically. Accurate dose reconstruction relies on minimizing noise. The purpose of this work was to optimize and characterize a prototype OC tailored for ultra-high dose rate applications.
Methods: radiation dosimeter using the principles of OC was designed in optical modelling software. Traditional image quality instruments, fencepost and contrast phantoms, were utilized to investigate noise reduction techniques and to test the spatial and phase resolution of the system. Absolute dose uncertainty was assessed by measurements in a 6 MV flattening filter free (FFF) photon beam with dose rates in the range 0.2 - 6 Gy/s achieved via changing the distance from the source.
Results: dosimeter was improved by isolating the system from external vibrations and controlling the system’s internal temperature. Mathematical noise reduction techniques were also applied. Simulations showed that these improvements should increase the spatial resolution from 22 to 35 lp/mm and achieve a phase resolution equivalent to 0.05 Gy, these predictions were confirmed experimentally. In the FFF beam the absolute dose uncertainty was dose rate dependent and decreased from ±0.8 to ±0.2 Gy for dose rates of 0.2 and 6 Gy/s, respectively.
Conclusion: radiation dosimeter utilizing the principles of optical calorimetry has been constructed. Optical modelling software and image quality phantoms have allowed for iterative refinement resulting in a prototype system capable of measuring absorbed dose to water in a 6 MV FFF beam. Reduced uncertainty at higher dose rates indicates the potential for OC as a dosimetry system for high dose rate techniques such as microbeam and FLASH radiotherapy.