DOSIMETRY OF BRACHYTHERAPY SOURCES IN MILLIMETER RANGE

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DESCRIPTION: (Adapted from the applicant's abstract): Restenosis is the major limitation of coronary angioplasty. Recently brachytherapy has emerged as a potential treatment for restenosis using photon emitters such as 192Ir, 125I, 103Pd and beta emitters such as 32P, 90Sr, and 90Y. Two approaches under investigation for intravascular brachytherapy are: (i) use of a radioactive source at the end of a catheter, (ii) permanent implantation of a radioactive stent at the occlusion site. The first is an example of temporary intracavitary brachytherapy where radioactive sources are placed in a body cavity near the target lesion; the other is an example of permanent brachytherapy where radioactive sources are implanted in the target lesion. It is well known that dose gradients in the immediate vicinity of the radioactive sources are very high because of the geometric and tissue attenuation effects. Traditionally, the dose to the target is specified at a distance of 1 cm from the source. At this reference distance the dosimetry of brachytherapy sources is reasonably well established. However, the intended target for irradiation in intravascular brachytherapy is much smaller, in the range of 1 - 3 mm. At these short distances, the dosimetry is highly uncertain and needs improvement. One of the major reasons for dose uncertainty at short distances may be the contribution from low energy secondary radiations, such as fluorescent x rays, beta particles, secondary electrons etc., which are primarily absorbed in the source encapsulation or the first few mm of tissue around the source. Their effects are largely ignored in traditional brachytherapy dosimetry because only a small fraction of the target volume is affected by them. This is, however, not true for intravascular brachytherapy where the entire target may be within millimeters of the source. With many investigations currently underway, mostly with commercial support, to determine the efficacy of intravascular brachytherapy, there is a tremendous need to not only standardize the prescription of dose, but importantly to also determine the dose delivered over short distances. This dosimetry may well be significantly different depending on specific radionuclide as well as design of source and applicators. In this project, the physics of intravascular brachytherapy dosimetry for treatment of restenosis will be investigated using thermoluminescent dosimeter chips and sheets, radiochromic film, polymer gel dosimeters, Rossi-type proportional chamber for microdosimetry, and Monte Carlo simulations. Finally, effects of tissue heterogeneity and self-shielding effects of catheters and stents for photons and beta particles will also be investigated.