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Dosimetric Uncertainties Associated with Small Electron Field Dosimetry

S Ahmad*, I Ali, Oklahoma Univ. Health Science Ctr., Oklahoma City, OK


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: To evaluate quantitatively the dosimetric uncertainties associated with dose calculation and measurement of small fields used in electron therapy.
Methods: The dose distributions of small field from circular cutouts of diameters ranging from 1.5-5cm were calculated for different electron energies (6-20MeV) using the electron Monte Carlo dose calculation algorithm (eMC) in the Eclipse treatment planning system. The corresponding planner dose distributions were measured with multiple-diode-array detector that irradiated with a Varian Trilogy machine. The calculated dose profiles, depth dose curves and scatter factors were compared with measured values using small diode detectors.
Results: The dose discrepancies between the measured and calculated electron small fields varied with the cutout sizes and energy of the electron beams. The calculated and measured doses agreed within 5% for electron cutouts with 2.0cm diameter and larger for all electron energies (6-20MeV). For fields smaller than 2.0cm, large discrepancies of 50% between the calculated and the measured doses particularly for low energy electron beams (6MeV) were observed. These discrepancies between measured and calculated doses resulted from dose extrapolation using large field data acquired in the commissioning of eMC and failure of the dose calculation algorithm to model the variations in beam quality and lack of charge-particle-equilibrium in the small electron fields. Large deviations between the scattering factors measured with different detectors produced large dose discrepancies in the dose calculation from the very small electron fields < 1.5 cm diameter. This is due to the use of large ionization chamber detectors to measure the scattering factor for the data input in the treatment planning system.
Conclusions: The dose extrapolation by eMC algorithm deviated significantly from the measured dose using different detectors. The eMC should include small field data that can account for variations in beam quality and other dosimetric artifacts associated with electron small field dosimetry.


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