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Transmission dosimetry with a scanning liquid filled electronic portal imaging device for megavoltage photo beams

University of British Columbia

An important question with respect to radiation therapy is to what degree of accuracy, dose delivery can be achieved in clinical practice. It is well understood that accurate delivery of a specific radiation dose to a target volume is critical to the success of radiation therapy. The use of portal imaging in radiotherapy for verification of field size, shape, orientation and patient setup is of vital importance in delivering successful radiation treatment. One of the modalities used in improving tumor localization and geometric verification of field sizes is the on-line electronic portal imaging device (EPID). In this thesis, the dosimetric characteristics of a scanning liquid - filled ionization chamber electronic portal imaging device (SLIC-EPID) have been investigated. To assess the system's response in relation to radiation beam intensity, a series of characteristic curves were obtained for various field sizes and energies. It was shown that the response of the imaging system was dependent on incident radiation intensity and could be characterized by a square root function to within 1% accuracy. Exit dose measurements with the SLIC - EPID on the central axis of the beam using homogeneous attenuating phantom materials revealed that the Portal Vision™ is capable of measuring the transmission dose to within 3% of standard dosimeters, such as the ionization chamber. For dosimetry applications, additional build-up material on top of the detector cassette of the portal imager is required to calibrate the system prior to dose measurements. Open field and wedged dose profiles of the portal imager show an accuracy of better than 3.5% when compared with those obtained using an ionization chamber in regions near field edges. SLIC-EPIDs, when configured appropriately have the potential of being used as exit dosimeters. They are capable of determining the relative dose at the exit surface of the patient with an accuracy of better than 3.5%.

Thesis/Dissertation

application/pdf

2009-03-10

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http://hdl.handle.net/2429/5838

Physics

University of British Columbia

UBCV

DSpace