PhD Defence: ADVANCEMENTS IN WATER CALORIMETRY DESIGN AND TECHNIQUES, AND APPLICATIONS TO MODERN RADIOTHERAPY

Student: Mark D'Souza

Supervisors: Dr. Arman Sarfehnia and Dr. Carl Kumaradas

Abstract

The overarching goal of this work is to develop new construction techniques and designs that are used to build novel water calorimeters (WCs) that can successfully operate in a wide range of novel radiation delivery modalities in clinics. To that end, a framework to optimize vessel design was developed. This framework uses Finite Element Method analysis to study the dimensions of a vessel and optimize them for specific applications. This framework was applied to clinical electron beams where optimal designs were determined. Next, a novel portable WC was presented, and its performance was validated. The calorimeter was designed to be positioned using onboard imaging allowing for rapid setup without loss of thermal stability. To characterize its performance, the novel WC was used to measure the absolute dose in an Elekta Versa HD under a 6 MV FFF beam with an overall uncertainty of 0.54% (k = 1). Dose measurements using our calorimeter were compared to those from an NRC calibrated ionization chamber, achieving an agreement within 0.25%, demonstrating our calorimeter's accuracy. Subsequently, the validated calorimeter was used to determine the combined beam quality and magnetic field correction factor (𝑘mag Q ) of an ionization chamber in a high energy high magnetic field MR-linac. The calorimeter was positioned using portal and MR imaging, following which measurements were performed. Cross-calibration measurements of an A1SL ionization chamber against the water calorimeter yielded a 𝑘mag Q of 0.985 ± 0.98%. To our knowledge, to date, only two other groups in the world have used WCs in MR-linacs, and our original work was the first to characterize an A1SL chamber in this beam.

Finally, we investigated the suitability of 3D-printed vessels for WCs by studying the associated heat defect correction factor 𝑘hd. Our results revealed that uncoated 3D-printed vessels are unsuitable due to varying 𝑘hd, with a 2.8% difference between two vessels. However, applying Parylene C/Parylene N coatings to 3D-printed vessels showcased a consistent 𝑘hd. This study confirmed that coated 3D-printed vessels are suitable for WCs, enabling the accurate creation of specialized shapes like spherical vessels for GammaKnife® ICONTM.