Arc therapy dose perturbation due to the center bar in a Clinac 4/100 couch assembly

Dose changes due to an isocentric arc beam intercepted by a metallic centerspine bar of the treatment couch assembly in a Clinac 4/100 are investigated. For taking experimental data a cylindrical phantom, fitted with a PTW ion chamber at its center, was placed on the treatment couch above the bar and irradiated. Measurements were taken at the isocenter for a 60 degree rotation of the source for different collimator settings. A simple theory which makes use of the measured attenuation values for the 4 MV beam in the metallic bar is proposed. A good agreement between the calculated doses and experimental observations have been found suggesting that the model could be used to estimate doses in clinical situations. The influence of the bar in the plane of rotation is qualitatively analyzed by mapping isodoses using exposed films.