(Sunday, 7/29/2018) 3:00 PM - 6:00 PM
Room: Exhibit Hall
Purpose: To investigate quantitatively dosemetric uncertainties associated with heterogeneity correction, detectors used in beam data measurement and bremsstrahlung in the electron Monte Carlo dose calculation (eMC) algorithm in the Eclipse treatment planning system
Methods: Dose distributions for different field sizes were calculated using eMC and measured with a multiple-diode-array-detector (MapCheck2) for field sizes ranging from 6-25 cm. The dose distributions were calculated in the MapCheck2 and water-equivalent phantoms. The CT-numbers form the images of the MapCheck2 phantom were used to correct for tissue heterogeneity in the Eclipse treatment planning system. The calculated treatment plans were delivered on a Varian Trilogy machine and measured with the MapCheck2.
Results: The eMC algorithm uses the electron and material densities extracted from the CT-numbers which have large uncertainties (10%) in the material densities and corresponding stopping-power when used to calculated dose for MeV electron beams. CT-numbers are usually used for electron densities for photon dose calculation. In the penumbra region, the eMC overestimates dose up to 40% for high energy 20 MeV electrons compared to the measured dose with small diodes. This is primarily due to lack of consideration of volume averaging of the ionization chamber used in beam data commissioning which is input to the eMC dose calculation algorithm. In the umbera region outside the treatment field, the eMC underestimate dose up to 300% for 20 MeV electron beams due to lack of consideration of bremsstrahlung emitted laterally and not accounted by eMC.
Conclusion: This study shows large dose discrepancies in the eMC algorithm due to use of stopping power ratios that are not well represented by the CT-numbers. Large dose uncertainties in the penumbra and umbra regions due to detector volume averaging used in commissioning data and lack of consideration of dose from lateral emission of bremsstrahlung produced by energetic electrons, respectively.
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