A method of scaling the 3D electron pencil-beam dose calculation to obtain accurate monitor units for irregularly-shaped electron beams

A method for determining monitor units (MU) for electron beams using a 3D pencil beam dose algorithm is described. A set of correction factors (OF_c) to the pencil beam dose is generated that transforms the output into dose per MU. The OF_cs depend on cone selection, field size, and source-to-surface distance (SSD) for a given electron energy. The dose per MU is determined by scaling the dose by the OF_c. The OF_c value is determined using a measured set of relative output factors (OF_rel) for various field sizes and SSD. The pencil-beam algorithm is used to compute the "raw" value (OF_p) to each of the OF_rel measurement points. The OF_c is the ratio of the measured OF_rel to the calculated OF_p. The OF_c value for irregularly-shaped electron fields or for electron fields at extended SSD may be interpolated from the OF_c table. The interpolation over SSD is performed linearly using the central axis SSD. The interpolation over field size is more complicated and uses the minimum area-circumscribing rectangle around the field shape. Due to the relative flatness of the OF_c, the interpolation is less error prone than the more common direct interpolation of the output factors. Computations were performed in the ADAC Pinnacle³ Planning System. Measurements were obtained using a Varian 2300c for 6 and 15 MeV. The results show that this method can predict the dose per MU within 1% to 2% for clinical fields and within 3% to 4% for extreme field shapes.