Irregular field dose determination with the weighted beam-zone method

For successful radiotherapy, exact application of a certain dose to the target volume is as necessary as optimal sparing of included or neighbouring organs at risk. A new simple but versatile method for rapid dose determination at relevant reference points in target volumes or shielded organs at risk irradiated with irregularly shaped photon fields has proved suitable for clinical routine. The weighted beam-zone method does not need extensive individual dosimetrical measurements or sophisticated treatment planning systems. All essential influences on the absorbed dose, delivered to a regarded dose reference point can be derived from existing dose measurements in square fields, only. Since this method shows the way to include arbitrarily shaped fields into the principle of equivalent square fields with known dose, dose determination at free or shielded on or off-axis points is reduced to simple counting of dose contributing or non contributing weighted beam-zones. The influence of shielded areas on the dose distribution, according to their extent and distance, can obviously be seen. Instant dose estimation in shielded regions shows the efficiency of sparing organs at risk. The practical experiences allow standardization for typical shapes of target volumes, giving the mean reduction of the effective field size, the relative dose to organs at risk and the correct timing for shielding.