SU-FF-T-578: Assessment and Management of Dose Perturbations Due to the Treatment Couch

Abstract

Purpose: To develop a model which can predict beam intersection with highly attenuating couch elements and determine the impact of the treatment couch on dose distribution. Method and Materials: Our method is based on fusing a CTimage set of the couch with the patient CT data set, importing regions of interest characterizing couch elements and assigning appropriate densities in the TPS. A retrospective study was performed on patient plans that posed difficulties in beam‐couch intersection during setup. The impact of the treatment couch was assessed through a comparison between clinical plans that excluded and included the couch model. The percentage of PTV covered by 95% of the prescribed dose and the mean CTV coverage were compared. Dose compensation strategies for IMRT treatments with beams passing through couch elements were investigated using a four‐field IMRT plan with 3 beams passing through couch elements. Results: The model agrees with positioning measurement to within 1° gantry rotation; prediction of dose is accurate to within 2% (Table 1, Figure 1). Inclusion of the couch resulted in an up to 3% reduction in PTV coverage and an up to 1% reduction in mean CTV coverage. Using film dosimetry, we were able to show that ignoring couch effects can result in plan deviations of 8 ± 3%; including compensation for the presence of the couch reduced the deviations to 2 ± 2% (Figure 2). Conclusion: We have a couch model that can be easily incorporated into planning procedures and which allows full assessment of the impact of the couch on the dose distribution.