Comparison of broad beam central axis depth dose curves from different accelerators using the universal depth dose curve model.

Electron beam central axis depth dose distribution can be transformed to fluence distributions with geometric depth replaced by a measure of the state of angular dispersion of the beam. A transformed central axis depth dose distribution was called a 'fluence curve.' The transformation was applied to a set of central axis depth dose curves calculated by Monte Carlo code for broad electron beams of energies ranging from 1 to 60 MeV in homogeneous phantoms of water, aluminium, and copper. For the energies and compositions likely to be encountered in external beam radiation therapy, the resulting fluence curves, were found to belong to a single parameter family. A collimator scatter parameter was introduced to take into account the initial angular dispersion produced by the collimator of an accelerator. Given the energy of the beam, the medium in which the beam is passing through, and the collimator scatter parameter, the fluence curve associated with the beam can easily be transformed back to a calculated depth dose curve. The collimator scatter parameter necessary to fit the depth dose curves measured on different accelerators was investigated. The results for the Clinac 18, LMR-13, Mevatron XII, Mevatron 80, Microtron, Sagittaire, Siemens Betatron, and the Therac 20 are presented.