SU-E-T-536: A Simple Method for Optimal Field Abutment Between Photon and Electron Beams in Mixed Beam Radiation Therapy

Abstract

Purpose: Significant efforts have been made to develop practical methods for the clinical implementation of advanced mixed beam therapy. One of the problems with the use of multiple adjacent electron radiation fields is the possibility of extreme dose inhomogeneity in the junction region. Because radiation beams are divergent, adjacent fields can overlap at depth and give rise to regions of excessive dose. Overlaps can be avoided by separating the electron fields but this in turn can give rise to areas of reduced dose at shallow depths. Thus the aim of this work is to find the optimal gap for different combined electron and photon beam situations. Method: Monte Carlo simulation was conducted for our Varian treatment head using the MCBEAM code, and the MCSIM code was used for modeling a water phantom to derive the dose distribution. Simulated PDDs and profiles were compared to measurements for photon and electron beams. Then isodose distributions were generated for adjacent fields with different separation gaps for different abutment situations such as photon‐photon, electron‐electron and photon‐electron combinations. Results: Good agreement was achieved between Monte Carlo simulation and measurement for all studied photon and electron beam energies. Separation gaps can be chosen that would Result in a flatter profile at a desired depth. Optimal gaps can be selected that compromise between the cold and hot spots inside the target. Conclusion: The Monte Carlo code used was capable of modeling our photon and electron beams accurately. We addressed a simple method to solve the field abutment problem in combined photon and electron treatments for advance mixed beam radiation therapy.