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Development of a radiochromic film scanner to measure steep dose gradients around brachytherapy sources
Dissertation

Development of a radiochromic film scanner to measure steep dose gradients around brachytherapy sources

Paul Sullivan
Doctor of Philosophy (Ph.D.), Medical College of Pennsylvania and Hahnemann University
2001
DOI:
https://doi.org/10.17918/00008211
pdf
Sullivan_Paul_20013.85 MB
PDF Access upon request, Email title, URL, or DOI to archives@drexel.edu

Abstract

Radiography
The emergence of sophisticated treatment procedures and new radioactive isotopes in radiation oncology has challenged medical physicists to measure dose in the near-zone of brachytherapy sources. To accurately measure dose in these complex regions the dosimeter should possess high sensitivity, good spatial resolution, energy and dose-rate independence, and tissue equivalence. Gafchromic MD-55 film possesses many of these dosimetric characteristics and is currently viewed as the "gold standard" for measuring dose in the near-zone of brachytherapy sources. This project has custom-built a radiochromic film scanner to extract dose information from Gafchromic film dosimeters. The laser-based scanning system operates at 670 nm or 633 nm and provides accurate control of light power, light polarization, and film positioning during scanning. Gafchromic MD-55 film was used in conjunction with the laser scanner to measure the radial dose of both 90Sr and 125I brachytherapy sources. The 90Sr source investigated was a custom-designed planar applicator used to treat a variety of ophthalmic diseases. The 125I source was a Nycomed-Amersham (Model 6711) design that is in widespread clinical radiation oncology use. Results indicate that the manufacturer stated dose rates for the 90Sr applicator significantly underestimate dose in the near-zone. At typical calibration distances of 2 mm, the Gafchromic dose-rate results are 40% higher than those quoted by the manufacturer. 125I radial dose profiles are in agreement with the benchmark Monte Carlo data of Williamson. These results extend reliable near-zone data for 125 I from 3 mm down to 1 mm. This project has also discovered that laser-based densitometers using laser diodes (670 nm) and helium neon lasers (633 nm) may further polymerize Gafchromic film. The light induced coloration is irreversible and shows dependence on laser power, wavelength, dwell time and prior radiation dose of the film. The results demonstrate that laser powers as low as 0.1 mW produce a non-linear increase in the optical density of the film. Laser light-induced coloration can have a significant impact on practical Gafchromic film dosimetry.

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