On the possibility of determining an effective energy spectrum of clinicalelectron beams from percentage depth dose (PDD) data of broad beams

Experiments have already shown that obvious differences exist between the d ose distribution of electron beams of a clinical accelerator in a water pha ntom and the dose distribution of monoenergetic electrons of nominal energy of the clinical accelerator in water, because the electron beams which rea ch the water surface travelling through the collimation system of the accel erator are no longer monoenergetic. It is evident that, while calculating p recisely the dose distribution of any incident electron beams, the energy s pectrum of the incident electron beam must be taken into consideration. In this note we shall present a method for determining an effective energy spe ctrum of clinical electron beams from PDD data (percentage depth dose data) . It is well known that there is an integral equation of the first kind whi ch links the energy spectrum of an incident electron beam with PDD through the dose distribution of monoenergetic electrons in the medium, as a kernel function in the integral equation. In this note, the integral equation of the first kind will be solved by using the regularization method. The bipar tition model of electron transport will be used to calculate the kernel fun ction, namely the energy deposition due to monoenergetic electron beams in the medium.