Program Information
Towards Absolute Dose Measurement in MRI-Linac and GammaKnife: Design and Construction of An MR-Compatible Water Calorimeter
N Entezari1*, J Renaud2 , G Peterson3, A Sarfehnia4 (1) Sunnybrook Health Science center, Toronto, Ontorio,(2) McGill University, Montreal, Qubec, (3) Sunnybrook Health Sciences Centre, Toronto, ON, (4) Sunnybrook Health Science center, Toronto, Ontario,
Presentations
TH-CD-BRA-9 (Thursday, August 4, 2016) 10:00 AM - 12:00 PM Room: Ballroom A
Purpose: The purpose of this work is to design and optimize a portable Water Calorimeter (WC) for use in a commercial MRI-linac and Gamma-knife in addition to conventional radiotherapy linacs. Water calorimeters determine absorbed dose to water at a point by measuring radiation-induced temperature rise of the volume (the two are related by the medium specific heat capacity). In this formalism, one important correction factor is heat transfer correction k_ht. It compensates for heat gain/loss due to conductive and convective effects, and is numerically calculated as ratio of temperature rise in the absence of heat loss to that in the presence of heat loss. Operating at 4⁰C ensures convection is minimal.
Methods: A commercial finite element software was used to evaluate several WC designs with different insulation materials and thicknesses; channels allowing coolant to travel around WC (to sustain WC at 4⁰C) were modeled, and worst-case scenario variation in the temperature of the coolant was simulated for optimization purposes(2.6 mK/s).
Additionally, several calorimeter vessel design parameters (front/back glass thickness/separation, diameter) were also simulated and optimized. Optimization is based on minimizing long term calorimeter drift (24h) as well as variation and magnitude of k_ht.
Results: The final selected WC design reached a modest drift of 11μK/s after 15h for the worst-case coolant temperature variation. This design consists of coolant channels being encompassed on both sides by cryogel insulation.
For the MRI-linac beam, glass thickness plays the largest effect on k_ht with variation of upto 0.6% in the first run for thicknesses ranging between 0.5-1.7mm. Subsequent runs vary only within 0.1% with glass thickness. Other factors such as vessel radius and top/bottom glass separation have sub 0.1% effects on k_ht.
Conclusion: An MR-safe 4⁰C stagnant WC appropriate for dosimetry in MRI-linac and Gamma-Knife was designed, optimized, and construction is nearly completed.
Funding Support, Disclosures, and Conflict of Interest: NSERC Discovery Grant RGPIN-435608
Contact Email: