Accuracy of patient positioning during radiotherapy for bladder and brain tumours

We report the results of a prospective study to quantify inaccuracies in pa tient set-up during routine radiotherapy for tumours of the brain and bladd er, which took place as part of our departmental development. Knowledge of these inaccuracies is required to put into practice the ICRU 50 recommendat ions regarding clinical target volume and planning target volume. We measur ed inaccuracies in two dimensions by comparing portal beam films with the s imulator check film. Our method used manual measurements, proved to be a ve ry laborious technique, and demonstrated the need for portal imaging. Ninety-five brain and 97 bladder portal films from 30 brain and 30 bladder patients were examined. Displacements greater than 0.5 cm were seen in 13% of brain treatments in the supero-inferior direction and 1% in the anteropo sterior direction. With bladder treatments, displacements greater than 0.75 cm were seen in 12% in the supero-inferior direction and 5% in the lateral direction. These results are consistent with other previous studies. We id entified a very small systematic error in the department, which was conside red to be clinically significant. These results are discussed with referenc e to other similar studies and the ICRU 50 recommendations.