SUPERHEATED DROP DETECTORS FOR DOSIMETRY IN RADIOTHERAPY

Yale University, New Haven CT

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Abstract

Micro-droplets Of halocarbons can serve as radiation dosimeters when maintained in a metastable superheated state suspended in a gel matrix. As ionizing radiation crosses the emulsion of droplets, it deposits energy along the tracks of released charged particles. This triggers the sudden vaporization of the superheated droplets, creating bubbles which can be detected visually and/or electronically. A unique advantage of these detectors in dosimetry for radiation oncology is that their response depends on both absorbed dose and linear energy transfer (LET). In fact, the detectors can be designed to respond above specific LET thresholds by controlling their degree of superheat. Several neutron-sensitive and photon-insensitive probes for in- vivo measurements of the photoneutron contamination of radiotherapy x-ray beams have been developed. Moreover, some photon sensitive halocarbons were identified and a large superheated emulsion chamber (SEC) based on these materials was developed. Our hypothesis is: can the SEC become a reusable 3D dosimeter, which can ultimately become a powerful tool for measuring complex three-dimensional dose distributions produced by modern radiotherapy. Our initial studies opened the way to further developments which will allow benchmark dosimetry measurements for brachytherapy sources and for high-LET particles. Specifically, we propose: (1) to improve the photon energy response of the SEC by using emulsions containing only elements with tissue-like atomic numbers; (2) to optimize the speed of image acquisition and spatial resolution of magnetic resonance imaging of bubbles using partial k-space sampling and echo-planar techniques; (3) to develop a micro-droplet emulsion chamber which can be scanned using optical transmission tomography techniques; (4) to perform selected benchmark brachytherapy dosimetry studies; (5) to build and employ SECs for the dosimetry of high-LET particle beams and for the spectrometry of neutron fields created at depth in tissue by x-ray, electron, proton or boron neutron capture therapy beams. The research plan will attempt to exploit the many unique features of the SEC, namely its reusability, high sensitivity and LET discrimination.