Looking Behind the Curtain: Changes in Physician Prescribing Patterns After Implementation of Electron Monte Carlo for Breast Boost Planning

Abstract

The aim of this study was to determine the effect of electron monte carlo (EMC) treatment planning on physician selection of electron energy for breast lumpectomy cavity treatments. A total of 208 women treated with a breast boost following whole breast radiation from 2015-2016 were included in this study. This included 103 women treated with plans generated from hand calculations (HC) and 105 with EMC in Varian Eclipse. Each patient underwent CT simulation with Phillips a CT scanner with 3 mm slices. Cavity depths were measured from the skin surface to the deepest portion of the lumpectomy cavity. For the HC group, electron energy was chosen based on the deepest point in the cavity using depth dose curves. Full 3D plans with isodose lines were generated for the EMC group. The average lumpectomy cavity depth was 3.5 cm (HC) vs 3.4 cm (EMC) (p=<0.01) Higher energy electrons were more commonly prescribed with HC plans than with EMC (Table 1). EMC resulted in a treating the same lumpectomy cavity with a wider range of energies and typically with a higher energy, reflected in an increase of the ratio of electron energy to depth of cavity from 3.5 (+/- 0.4) to 3.8 (+/-0.7). EMC based planning for breast boosts better reflects the actual dose being delivered as compared to 2D planning. Despite a higher energy to depth ratio, physicians prescribed 15 and 18 MeV electrons less frequently. This may reflect physician visualization of poor isodose coverage of the deepest portion of the lumpectomy cavity on 3D images or the low dose to critical structures which is not visualized using a depth dose curve. With EMC planning, volume normalization can also be used to force good coverage at a larger range of energies in patients who are on the edge of acceptable coverage. We hypothesize that after EMC implementation patients who would have previously been treated with higher energy electrons are likely being switched to photon boosts after reviewing the 3D dose calculation. In conclusion, implementation of EMC results in a change in energies prescribed for breast boosts reflecting physician utilization of 3D information provided using this new method of electron planning.Abstract 3631; Table 1Distribution of Electron Energy Prescribed with HC and EMC methodsEnergy (MeV)69121518HC311164028EMC92231275 Open table in a new tab