Abstract P4-11-19: Electron Breast Boost Radiotherapy Planning Using Monte Carlo Based Calculations

Introduction: Patients with early stage breast cancer, usually undergo breast conserving surgery and whole breast irradiation. A radiotherapy boost dose to the tumour bed may be indicated and electron beam therapy is often used for this. The planning of the electron boost has evolved over time. Currently, imaging is used to determine the tumour bed but clinical parameters are frequently used to determine field size and electron energy. Monte Carlo calculations of electron beams (eMC) have been in existence for a long time; however, commercial eMC planning algorithms, such as the Eclipse eMC algorithm (EeMC) are relatively new. Current practice has not yet routinely adopted these calculations for electron beam planning. Purpose: The purpose of this study was to determine if the use of Monte Carlo calculations to optimise electron field planning significantly changes the prescription compared to current practice. Methods and Materials: A retrospective review of the practices of two breast radiation oncologists at our center between 2007 and 2009 revealed that they had treated forty eight patients with external beam whole breast radiotherapy and a tumour bed boost using 6-20 MeV electrons. These physicians outlined the original Clinical Target Volume, Planning Target Volume, and the organs at risk for the original radiotherapy treatment plan. These volumes were subsequently used by a medical physicist, blinded to the actual treatment, to develop an optimal electron boost plan (Plan 1) using the Varian Eclipse electron Monte Carlo algorithm (V 8.1.17). Next, the actual dose delivered to this volume was calculated (Plan 2) using the same planning system. The plans were assessed by an independent physician, who compared dose homogeneity, dose conformality and radiation dose delivered to the adjacent organs at risk. Results: In Plan 1, the mean planning target volume (PTV) receiving 95% of the prescribed dose, deemed PTV95, was 95.8% (range 92-99%). Similarly the mean PTV90 for Plan 1 was 100% (range 100-100%). The mean CTV95 for Plan 1 was 99.1 % (range 96.0-100%). For Plan 2, the mean PTV95 was 56.9% (range 6.0-89.0%). The mean PTV90 was 84.9% (range 35.0-100%) and the mean CTV95 was 64.5% (range 6.0-98.0%). In 85% of the patients, the electron energy used in Plan 1 was higher than the electron energy used in Plan 2. In 100% of the patients, the electron energy used in Plan 1 was equal to or higher than the electron energy used in Plan 2. Conclusions: The electron boost dose given, calculated retrospectively using an EeMC algorithm, frequently revealed suboptimal coverage of the target volume compared to a plan optimised using the algorithm. The generation of electron breast boost plans using an EeMC technique is both feasible and allows for improved coverage of the target volume with greater dose homogeneity. We recommend using this technique routinely for determining electron field size and energy. Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr P4-11-19.