A POSITION-SENSITIVE SUPERHEATED EMULSION CHAMBER FOR 3-DIMENSIONAL PHOTON DOSIMETRY

A position-sensitive detector chamber is introduced for the three-dime nsional (3D) dosimetry of photon-emitting brachytherapy sources. The d etector is based on an extremely fine suspension of monochloropentaflu oroethane droplets emulsified in a gel. The droplets are highly superh eated at room temperature and their evaporation can be triggered by ph oton interactions, leading to the formation of microscopic bubbles. Th us, when photon-emitting brachytherapy sources are inserted into the d etector, bubble distributions form around them, enabling visualization of the radiation field. The tissue-equivalent emulsifier gel is highl y viscous and keeps the bubbles immobilized at the location of their f ormation. Bubbles can then be imaged by nuclear magnetic resonance or optical scanning techniques. After the imaging, the detector can be pr essurized in order to recondense the bubbles to the liquid phase. In a few minutes, the device is annealed and ready to be used again for re peated measurements improving the counting statistics. The photon sens itivity of the monochloropentafluoroethane droplets was determined wit h highly filtered, quasi-monochromatic x-ray beams and radionuclide ga mma sources. The air-kerma response presents a broad maximum at low en ergies, due to the relatively high effective atomic number of the halo carbon molecule. A prototype chamber was built and successfully tested : bubble distributions deriving from the insertion of a I-125 source w ere imaged by means of a slice-selective 3D gradient-echo technique. T hese experiments confirm the potential and viability of this new appro ach to 3D photon dosimetry.