Planar dose calculation of electron therapy

Abstract

Purpose:
The purpose of this study is to compute the planar dose distribution of irregularly shaped electron beams at their maximum dose depth (zmax) using the modified lateral build-up-ratio (LBR) and curve-fitting methods.
Methods:
Circular and irregular cutouts were created using Cerrobend alloy for a 14×14 cm2 applicator. Percentage depth dose (PDD) at the standard source-surface-distance (SSD = 100 cm) and point dose at different SSD were measured for each cutout. Orthogonal profiles of the cutouts were measured at zmax. The data were collected for 6, 9, 12, and 15 MeV electron beam energies on a VERSA HDTM LINAC using the iba Blue Phantom2 3D water phantom system. The planar dose distributions of the cutouts were also measured at zmax in solid water using EDR2 films.
Results:
The measured PDD curves were normalized to a normalization depth (d0) of 1 mm. Each cutout's lateral-buildup-ratio (LBR), lateral spread parameter (σR(z)), and effective SSD (SSDeff) were calculated using the PDD of the open applicator as the reference field. The modified LBR method was then used to calculate the planar dose distribution of the irregular cutouts inside the field at least 5 mm from the edge. A simple curve-fitting model was developed based on the profile shapes of the circular cutouts around the field edge. This model was used to calculate the planar dose distribution of irregular cutouts in the region from 3 mm outside to 5 mm inside the field edge. Finally, the calculated dose distribution was compared with the film measurement. 
Conclusions:
The planar dose distribution of electron therapy for irregular cutouts at zmax was calculated using the improved LBR method and a simple curve-fitting model. The calculated profiles were within 3% of the measured. The gamma passing rate with 3%/3mm was more than 96%.