Development of a dual wavelength microdensitometric dosimetry system for analyzing radiochromic films

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Development of a dual wavelength microdensitometric dosimetry system for analyzing radiochromic films


Author: Lee, Kit-yee
Title: Development of a dual wavelength microdensitometric dosimetry system for analyzing radiochromic films
Degree: Ph.D.
Year: 2005
Subject: Hong Kong Polytechnic University -- Dissertations.
Radiation dosimetry.
Department: Dept. of Health Technology and Informatics
Pages: xv, 209 leaves : ill. (some col.) ; 30 cm.
Language: English
InnoPac Record:
Abstract: Verification of radiation treatment is essential to ensure the correct delivery of the radiation to a patient. Verification is usually done in a phantom undergoing the same treatment as a surrogate of the patient. However, when the treatment involves a very small irradiation field, verification using conventional dosimetric methodologies fails to give an accurate result in terms of absolute dose and two-dimensional dose distribution mainly due to the relative large size and the undesirable features of the dosimeters when applied in a very small irradiation field. The radiochromic film (RCF) is a kind of dye films that offers a very high spatial resolution suitable for use in the dosimetric work of small irradiation field, however, its applicability is compromised by its low sensitivity and the lack of commensurate densitometers designed for it. The aim of this project is to develop a readout device (microdensitometer) to extract the high spatial resolution that the RCF can offer and to improve the film sensitivity by reading the film transmittance at the two absorption peaks with maximum sensitivities. Another aim of the project is to apply the developed microdensitometer in verifying a small-field stereotactic radiosurgery (SRS) treatment plan of which the verification has been a challenge to the conventional methodologies. The microdensitometer consists of a monochromator which provides an analyzing light of variable wavelength, a film tray on a high-precision scanning stage, a transmission microscope coupled to a thermoelectrically cooled CCD camera, a microcomputer and corresponding interfaces. The high spatial resolution of the microdensitometer, on the order of micrometers, achieved through the use of the microscope is combined with a measure-and-step technique to cover the area of interest on the film. The improved sensitivity has been derived from the dual-peak measurement which is only attainable by this novel microdensitometer. A new algorithm of dose measurement for the RCF has also been proposed. The doses obtained at the peaks are fed into a weighted objective function (OF) and an optimum dose is searched by minimizing the OF to give the best estimate of the dose deposited on the film. The weighting factors used in the OF are derived from the film calibration curves of the RCF. The weighting factor governs the relative significance of the measurements made at the two absorption peaks and aids in giving the best estimate of the dose. Performance of the instrument in regard to the positional accuracy, system reproducibility and dual-peak film calibration, and the dual-peak algorithm were evaluated. The results show that the instrument and the algorithm developed fulfill the design objective of providing a designated readout device for the RCFs with commensurate capabilities. With the advent of this invention, high-resolution radiochromic film dosimetry (Hr-RCFD) has been realized and become an easily accessible, reliable and practical technique in dosimetric tasks where high resolution and film sensitivity are required. The Hr-RCFD has been successfully applied in verifying a small-field SRS of which the verification always presents problematic issues on conventional methodologies.

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