Poster — Wed Eve—55: Automated Image Guidance for Prostate IMRT Using Low Dose Cone Beam CT

Currently, we are using Varian's On Board Imager to acquire cone beam computed tomography (CBCT) images prior to prostate intensity modulated radiotherapy (IMRT). The images are used to determine the post‐setup prostate position, and thus, to shift the couch towards the situation assumed during treatment planning. This “manual forward” image guided radiotherapy (IGRT) technique is time consuming and requires significant imaging dose over the course of treatment. Therefore, we present two low‐dose IGRT methods that are entirely automatic. The dose is reduced by lowering the mAs during CBCT acquisition at the cost of increased noise in the final images. In the “auto forward” IGRT technique, the CBCT image is registered to the planning CT and the result is used to shift the patient on the couch. The “auto reverse” technique involves non‐rigid registration of the planning CT to the CBCT. Both techniques were evaluated using a retrospective analysis of data from ten patients, including one planning CT and five CBCT images acquired at evenly spaced fractions per patient. At the standard imaging dose, IGRT error was 4.3 ± 1.6, 3.9 ± 1.2, 3.8 ± 1.8, and 3.1 ± 1.1 mm for no correction, manual forward, auto forward, and auto reverse techniques, respectively. Errors exceeded seven mm in 4% of the fractions when employing the auto reverse technique, highlighting a possible need for manual surveillance. The imaging dose could be reduced to 20% without an appreciable loss in accuracy of the automatic methods.