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Developing a Graphite Probe Calorimeter for Accurate Clinical Dosimetry

J Renaud

J Renaud1*, D Marchington2, J Seuntjens1, A Sarfehnia1, (1) McGill University, Montreal, QC, (2) National Research Council of Canada, Ottawa, ON

TH-E-BRB-7 Thursday 1:00:00 PM - 2:50:00 PM Room: Ballroom B

Purpose: To optimally design, construct and validate an innovative graphite probe calorimeter (GPC), comparable in size to a 0.6 cm┬│ cylindrical ionization chamber, for use as an absolute clinical dosimeter.

Methods: A numerical finite element method (FEM) based heat transfer study was conducted using COMSOL Multiphysics® to explore the feasibility of the GPC and to optimize its design in terms of shape, dimensions and materials. Through the use of a novel aerogel-based material as the thermal insulation between the graphite bodies, the portability and robustness of the GPC design is made suitable for routine clinical use. A functioning prototype was constructed in-house and used to perform dose to water measurements under a 6 MV stereotactic radiosurgery (1000 MU/min) photon beam from a Novalis Tx, under reference conditions. Heat loss correction factors were determined using FEM analysis and graphite to water absorbed dose conversion factors were calculated using Monte Carlo techniques.

Results: The dose to water measurements agreed well with the TG-51 derived values, with an average measured dose output of 95.7 (s = 1.4) cGy/100 MU, compared to an expected output of 96.6 cGy/100 MU. Heat loss correction factors ranged between 1.005 and 1.013, while the graphite to water dose conversion factor was 1.099. The combined relative uncertainty was estimated at 2.7%, with the largest contributions being the repeatability (type A, 1.4%) and thermistor calibration (type B, 2.1%).

Conclusions: This proof of concept demonstrates the feasibility of using the GPC as a practical clinical absolute photon dosimeter and lays the foundation for a miniaturized version suitable for small and composite field dosimetry. It is expected that through active thermal stabilization provided by a temperature controller and electrical calibration, the uncertainty due to repeatability and calibration will be decreased, such that the overall uncertainty is better than 1%.

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