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Evaluation of an amorphous silicon electronic portal imaging device for use in radiation therapy dosimetry Sibbald, Regan Estcourt

Abstract

The objective of external beam radiation therapy is to deliver a high dose of sterilizing radiation to a diseased tumor site while minimizing the dose to as much of the surrounding normal tissue as possible. This is often accomplished by directing one or more megavoltage xray treatment portals at the diseased area from different directions. In theory, this principle maximizes the dose to the target site while minimizing the dose to surrounding healthy tissue. The efficacy of a course of radiation therapy may however be compromised by several factors. Geometric factors related to the accuracy to which the beam delivery system can reproduce the desired treatment set-up may cause deviations from the desired results. Patient set-up uncertainties are another factor and are related to the alignment of skin markings with respect to the underlying target structures within the body. In addition, a presumably fixed target may be geometrically shifted due to its proximity to neighboring organs or the patient may simply move during treatment thereby shifting the treatment portal with respect to the intended target. All of these factors contribute to a sub-optimal treatment outcome and possibly an increase in the normal tissue complication probability (NTCP). On-line electronic portal imaging devices (EPIDs) have been developed to monitor and help correct the inaccuracies encountered in radiation therapy. EPIDs were developed for verification of geometric accuracy, as they are capable of producing near real-time 2D projection images of the target volume during a radiotherapy treatment. However, it has also become apparent that in addition to geometric verification, EPIDs can provide valuable dosimetric information, indicating points of interest in terms of over and under dosage. In this thesis, the dosimetric characteristics of the amorphous silicon EPID are investigated. First, the EPIDs ability to accurately and reproducibly measure dose as a function of fluence rate is investigated. EPID pixel intensity values were found to vary by up to ±3% from the average value with a standard deviation of 1.3% when measuring the same dose with varying dose rates for both 6 and 18 MV photons. Next the water equivalent depth of the inherent buildup material on the EPID is determined. The inherent build-up of the portal imaging device was found to be 1.4 ± 0.2 cm for 6 MV and 2.2 ± 0.2 cm for 18 MV. A comparison of EPID and film images acquired during enhanced dynamic wedge treatments indicated that the film profiles show an increased response of film compared to the EPID as the distance from the central axis increases. The EPIDS dose response curve was characterized by comparing ion chamber measurements to EPID pixel intensity values. The dose response dependence on incident photon energy and fluence rate was also investigated. The ratio of dose to total pixel value was found to be to be 1.42 x 10⁻⁴ cGy/pix for 18 M V and 1.44 x 10⁻⁴ cGy/pix for 6 M V with confidence intervals of 0.05 x 10⁻⁴ cGy/pix. Dose measurements were found to be reproducible with a standard deviation of 1.3% for both 6 and 18 M V photons. All calibration equations were verified over the dose range of 10 cGy to 100 cGy.

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