Details

Autor(en) / Beteiligte

• Renaud, James

Titel

On the Development of Absorbed Dose Calorimeter Systems for Absolute Clinical Dosimetry

Ort / Verlag

ProQuest Dissertations & Theses

Erscheinungsjahr

2017

Quelle

ProQuest Dissertations & Theses A&I

Beschreibungen/Notizen

• The aim of this work is to develop and evaluate the feasibility of absolute dose to water measurements in clinical high-energy photon, electron, and proton beams using a probe format graphite calorimeter (GPC; a.k.a. Aerrow), a sealed electron water calorimeter (ESWcal), and a short range water calorimeter (SHREWcal).Measurements were performed using these calorimeters, all of which were designed and built in-house, and ionization chambers with calibrations traceable to national primary dose standards. A sealed glass vessel constructed at the National Research Council of Canada (NRC) was used as part of the water calorimeter experiments. Calorimeter-based dose results were validated in high-energy photon beams against established dose standards. A finite element analysis software package was used to numerically solve the heat transport equation in models of the calorimeters used throughout this project. Monte Carlo radiation transport codes were used to calculate the perturbation factors accounting for the presence of non water (or graphite) detector materials in the path of the beam. For the GPC, absolute dose output measurements were performed using its two independent modes of operation for several clinical high-energy photon and electron beams, in addition to a relative characterization of the detector. For the ESWcal, electron beam quality conversion factors were directly measured for two types of ionization chambers. For the SHREWcal, dose measurements were performed for clinical short-range electron beams and cyclotron-based monoenergetic and modulated proton beams.Absorbed doses measured using both GPC modes of operation were found to agree with chamber-derived doses to well within the combined uncertainty of about 1.5 %. Moreover, the detector was characterized as having a strong linear response in the range of 80 cGy to 470 cGy, and no dependence upon dose rate in the range of 0.5 Gy/min to 5.4 Gy/min. For photon and electron beam qualities in the range of 58.4 % < %dd(10)× < 86.8 % and 2.33 cm < R50 < 8.27 cm, respectively, no statistically significant energy response trend was exhibited and a maximum deviation of ±1% from the average across all beam qualities was observed.The ESWcal measured dose with a relative combined standard uncertainty of 0.5 % for electron beams with energies of 9 MeV and greater, and about 1.0 % for the 6 MeV beam. Validation measurements against the NRC water calorimeter in a high-energy photon beam were found to agree with the combined uncertainty of 0.43 %. Non-statistically significant differences of up to 0.7 % were found between the measured electron beam quality conversion factors and values listed in published protocols.Absorbed doses to water were measured with the SHREWcal with an associated type A standard uncertainty of approximately 0.4 % and 0.2 % for the electron and proton beam experiments, respectively. In terms of thermal stability, drifts were on the order of a couple of hundred µK per minute, with a short-term variation of 5–10 µK. Relatively large heat transfer correction factors were calculated to be between 1.021 and 1.049. The overall combined standard uncertainty on the absorbed dose to water was estimated to be 0.6 % for the electron beams, as well as for the monoenergetic protons, and 0.7 % for the modulated proton beam.In conclusion, this project establishes the foundations of several new calorimeter-based dose standards for use at the level of the clinic, as well as the national standards laboratory. The practical significance of these calorimeter devices is that they will allow for more accurate knowledge of the clinically delivered dose through reduced dosimetric uncertainties, thereby contributing to improved patient outcomes in terms of better tumour control and lower complication rates for those treated using photon, electron, and proton modalities.