The Accuracy Of Dose Calculations For The Brainlab Imaging Couch Top With The Eclipse Treatment Planning System

PURPOSE To evaluate the accuracy of the Brainlab imaging couch top model within the Eclipse treatment planning system for dose calculations. METHODS A 25cm3 polystyrene phantom was irradiated at 6MV with an ion chamber (IC) approximately 12cm deep. A second setup was constructed for film collection of depth dose data to the surface of the phantom. Irradiations were performed with and without the couch in the beam path and compared to the calculated dose using Eclipse version 10 and AAA (Varian Medical Systems, Palo Alto, CA). Dose and water equivalent depth (WED) were calculated using the standard Brainlab couch model (-1000HU interior, -300HU surface) and a modified model (-890HU interior, +700HU surface) taken from the iPlan treatment planning system. A comparison was performed using triple channel dosimetry in FilmQA Pro (Ashland, Wayne, NJ) with a gamma metric using 2%/2mm as a tolerance. RESULTS The mean HU values on CBCT were -925HU in the interior and -425HU on the surface of the couch. WED through the couch at the couch-phantom surface was 0.47cm without the adjusted HU values and 1.4cm with the modified values. This agreed better with the Brainlab published WED of 1.2cm. IC agreement to the treatment plan was 1.58% for the AP field and -1.78% for the PA field with the default HU and 1.16% with the adjusted HU values. Film depth dose measurements showed a decrease in gamma from 99.8% to 94.4% points passing with the default values. Dose at the couch - phantom interface was approximately 20% different on film for the default values and 1% for the adjusted HU values. CONCLUSION The default couch HU values were found to be similar to what was observed on CT. However, for our configuration they proved to be insufficient in accurately calculating dose, especially dose to the phantom surface.