Determining optimal gel sensitivity in optical CT scanning of gel dosimeters.

A method for determining the gel sensitivity that is necessary for obtaining optimal image contrast in optical CT scanning of gel dosimeters is presented. The effective dynamic range of the OCTOPUS-ONE research scanner (MGS Research, Inc., Madison, CT) is analyzed. Optical density increments for selected straight-line paths across a gel cylinder to be scanned are calculated based on the optical properties of the polymer gel and the dose distribution from a commercial treatment planning system (Cadplan, Varian Corporation, Palo Alto, CA). Maximum optical density increment across the entire gel is obtained by searching the gel cylinder over a set of transverse planes at different rotational angles. The application of this quantity as a criterion for optimizing the quality of the optical CT scanning is demonstrated through dose verification of two representative treatment plans. When the MU dependence of the dose distribution for a treatment plan is linear, as is the case for static field irradiation, it is possible to scale the treatment plan such that the intensity variation of the signals received by the photodetector spans its entire dynamic range. For treatment plans that are possibly nonlinear, IMRT plans, for example, modification of the sensitivity of the gel material is necessary for the high-dose signals to be collected at a certain signal-to-noise ratio. Results obtained using the optimized CT scanning approach are compared with those from the treatment planning system and the film measurement.