Advanced Accurate Dosimetry Techniques in Radiation Therapy using Calorimetric Techniques and Monte Carlo Simulations

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Introduction In radiation therapy (RT), dosimetric uncertainties in treatment delivery are directly associated with variations in tumor control and complication rate and a dosimetric accuracy of 3.5% is traditionally considered required. The absorbed-dose delivered to the target and critical structures is the result of a number of steps involving machine calibration, localization of target volume and critical structures, treatment planning dose calculation and radiation delivery. This research program addresses the first step in the chain leading to the determination of dose to the tumour, i.e., reference dosimetry. For conventional RT beams, the calibration chain starts with air-filled ionization chambers calibrated in terms of absorbed dose to water in a standard reference field of 10 x 10 cm^2 of a high-energy photon beam (usually Co-60). While these calibration conditions are appropriate for the field types and depths used in three-dimensional conformal radiation therapy, in modern radiation therapy, small fields or dynamically combined small fields are used in stereotactic body radiation therapy (SBRT) or intensity modulated radiation therapy (IMRT). To address accurate reference dosimetry of these beams, a new dosimetry formalism was proposed by a working group of the International Atomic Energy Agency in which the PI of this proposal played a key role. To put this new dosimetry formalism in practice, data is required on correction factors for detectors used in the calibration of these beams. A second area of dosimetric uncertainty is that of intraoperative kV radiation sources that are characterized using detectors, which are calibrated in terms of air kerma with an uncertain dose conversion process, while the quantity of interest is absorbed dose to water. A third area of emerging importance for radiation treatment, are heavy charged particle beams (proton, carbon) where also considerable uncertainties are involved. The overall objective of this research program is to address uncertainty in reference dosimetry in these challenging conditions using calorimetric techniques and Monte Carlo simulations. The long-term goal of this work is to improve success of radiation treatments through improved dosimetry in these nonstandard fields. Objectives 1. Address dosimetric uncertainties in small and composite fields: (i) Study the source parameters (spot size, beam modifiers, etc) that determine the accelerator output in small field conditions. (ii) Measure and calculate correction factors for output measurements using available small field detectors for application in the small field protocol, (iii) test the criteria for selection of suitable plan-class specific reference fields. 2. Address dosimetric uncertainties for the intraoperative kV source: (i) develop and commission a probe calorimeter with improved sensitivity; (ii) compare the calibration of the kV source with other detector measurements and Monte Carlo simulation. 3. Address dosimetric uncertainties in carbon and proton beams: (i) develop the water calorimeter for measurements in carbon and proton small beams; (ii) compare the water calorimeter with graphite calorimetry and ionization chamber detectors. Impact Modern radiation therapy techniques increasingly use very high doses in reduced fractions for improved local control. Radiation delivery is highly complex, through a large number of small fields the dosimetry of which presents significant challenges. The impact of this program is to bring overall better consistency, conceptual clarity and improved accuracy in the dosimetry of nonstandard beams. Improved accuracy in the long term will lead to better treatment outcomes.