Room: Exhibit Hall | Forum 3
Purpose: Electron Monte Carlo (eMC) offers more accurate dose calculation over the conventional pencil beam algorithm (PBA), taking full account of lateral scattering in presence of tissue inhomogeneity and improving accuracy with oblique incidence and surface irregularity. This study investigated the dosimetric difference between eMC and PBA for accelerated partial breast irradiation (APBI) using mixed photon and electron beams.
Methods: Eleven prior mixed-modality APBI patients originally planned in Elekta XiO (v4.34) using collapsed cone convolution (CCC) for photons and PBA for electrons were reproduced in RayStation (v4.7.2) using eMC and CCC, with the same energy, aperture and monitor units (MU). Five patients received 36 Gy in nine fractions, and six with 40 Gy in ten fractions. In all plans, 80% of the prescription was delivered by tangent photon fields, and 20% by an enface electron field. Target coverage and organ dose were compared between the two plans. A Wilcoxon signed-rank test was performed.
Results: When replacing PBA with eMC, the averaged PTV V100 (volume covered by 100% prescription) decreased from 98.3% to 92.2% (p<0.01). The global hotspot decreased by 1.4 Gy (p<0.01). The seroma V100 dropped from 99.8% to 98.8% (p=0.013). The mean dose to normal breast outside PTV increased by 0.4 Gy (p<0.01), and the mean dose to the first 5-mm skin directly irradiated by electrons increased by 1.5 Gy (p<0.01). The difference in ipsilateral lung dose was not statistically significant.
Conclusion: The more accurate eMC calculation suggested our prior practice had provided less than expected PTV and seroma coverage but higher than expected breast and skin doses. While our decade long clinical outcome with excellent tumor control and limited toxicity was based on the less accurate PBA calculation, we recommended our physicians be mindful on distal target coverage and not significantly tune up MU to maintain similar coverage.