SUPERHEATED DROP DETECTORS FOR DOSIMETRY IN RADIOTHERAPY

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The superheated drop detector (SDD) consists of a collection of superheated drops suspended in a gel. Its operation is based on the same principle as that of the bubble chamber; namely, the initiation of vapor bubbles by energetic ions in superheated liquids. Since the drops are kept in a "perfectly smooth" container, i.e. another liquid, the sample can be maintained in a superheated state for a long period of time. It has already been shown that direct readability, simplicity of preparation and low cost make it potentially useful in the measurement of neutron dose equivalent for patients underling high energy x-ray or electron radiotherapy. New photon-sensitive SDDs have recently become available that open up new possibilities for dosimetry applications. A major objective of this project is to generalize the SDD technology to detect radiations above a selected linear energy transfer (LET) by varying the degree of superheat. In the first phase of this project photon-sensitive SDDs will be developed, tested and characterized for 2-and 3-dimensional dosimetry of brachytherapy sources using optical tomography and MRI to quantitatively characterize the bubble distribution produced in the detector. Using this new technology, anisotropy of dose distributions around photon-emitting brachytherapy sources will be determined, thereby permitting better dose estimates to patients and therefore improved source design and patient treatment. The source-to-source shielding effects in multisource configurations simulating clinical implants will be investigated using SDDs. Also, the 3-dimensional SDDs will be used to investigate the effects of customized shields in brachytherapy applicators. In the next phase of this project, the generalized SDD systems will be developed to measure radiations with LET above a specified threshold value, which clinical applications in radiation therapy modalities using high energy proton beam, neutron beams, boron-neutron capture therapy and neutron brachytherapy; with special attention to the dosimetry of high energy proton beams for radiotherapy. One of the secondary specific aims of this project is to update the information on photoneutron yields from currently available radiotherapy accelerators.