Parameterization of electron beam output factor

Empirical formulation for small circular electron fields

An empirical model for small circular electron fields was developed. This can be of great help in the treatment planning process for small circular electron fields. A complete dosimetric analysis of the circular fields defined by electron cutouts diameters (2 cm-9 cm) was done for nominal electron energies ranging between 6 MeV and 20 MeV using a 3D water phantom and a pin-point ion chamber. Properties studied included depth dose, in-air inverse-square fall-off, and beam profiles. The Varian Clianc 2100 C accelerator was modelled, benchmarked and Monte Carlo simulations were performed using the EGSnrc/BEAMnrc code for the small circular cutouts. A simple exponential model was found to accurately predict the very important therapeutic depth (90% of Dmax) for the small circular field size within an accuracy of better than 2 mm in most cases. The model has only two parameters (d₉₀and 'b'). Also, the penumbra widths (90% of the off-axis profiles) of these small circular electron fields were studied and least square fitted to a simple quadratic model. Full dosimetric profiles of these small circular electron fields were further studied using the benchmarked Monte Carlo simulations. This study presents a simple model to predict the very important therapeutic depth (90% of Dmax) and a recipe to develop such an electron treatment model for any linear accelerator system. Such predictions can be extremely valuable and time saving prior to treatment planning involving not only small circular shaped electron fields but also irregularly shaped electron fields.

Monitor unit calculation in electron therapy using Monte Carlo Simulation: a GUI for the phase-space field trimming

Purpose: We developed a graphical user interface (GUI) for electron phase-space field trimming using Monte Carlo simulations. This GUI can be used for monitor unit (MU) calculation in electron therapy. Methods: The GUI and electron field trimming algorithm were developed using MATLAB and C code. Phase-space files for the electron fields were generated using the EGSnrc code based on a Varian 21EX Linac with variables of applicator size, field size and energy. Verification of the alogrithm was carried out by comparing the relative output factor, which was used for MU calculation, predicted by Monte Carlo simulations and from actual measurements. Results: Our electron field trimming algorithm was found to be about five times faster than the original Monte Carlo simulation. Clinically, the GUI performed best when using voxel size ≥ 0.3 × 0.3 × 0.3 cm3, and field size larger than 2 cm in radius based on an acceptable deviation of 2%. Conclusion: A GUI for generating irregular field for MU calculation using Monte Carlo simulations was created as a user-friendly tool in electron therapy.