AbstractID: 9385 Title: Performance of laser CT scanning of polymer gels as a tool for
three dimensional dose verification
Results and technical issues with regard to the development of a 3D dosimetry system based on optical CT scanning of gel dosimeters are
presented. The dose verification system consists of BANG polymer gels, a laser-beam optical CT scanner for signal collection, an image
reconstruction program for signal processing, and software for dose comparison between treatment plans and those measured from optical CT
scanning. The performance of the OCT-OPUS-ONE™ scanner (MGS Research, Inc., Madison, CT) is evaluated in terms of signal-to-noise ratio,
spatial resolution, and dosimetry uncertainty. For a comparison between gel experiment and planning system, two treatment plans (a 6 x 6cm
single field and an IMRT) were delivered to 17cm diameter x 13cm height cylindrical gel containers immersed in a 24x24x20 cm water tank. In
the central cylindrical region of approximately 75% radius of the gel cylinder, agreement within 3 to 4 percent is obtained. In regions closer to
the container edge, however, the gel measurement generates erroneous optical density values that arise from reflection and refraction of the laser
rays at the interface between the gel and the container. We have found that the inhibition of radiation-induced polymerization by atmospheric
oxygen permeating through the plastic wall actually has the beneficial effect of making the gel optically uniform in the inadequately sampled
peripheral regions. This reduces significantly the influence of the edge effect on the accuracy of the dose reconstruction in the central region.
Supported by NIH grant R44HL59813.