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Quantitative Investigation of Dosimetric Uncertainties Associated with Small Electron Fields

E Kendall*, S Ahmad , I Ali , Oklahoma university Health Science Ctr., Oklahoma City, OK


(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: To investigate dosimetric uncertainties associated with small electron fields using both the electron Monte Carlo dose calculation algorithm (EMC) and measurement with different detectors.

Methods: The dose distributions of small fields were calculated on solid-water and MapCheck2 phantoms using the EMC dose calculation algorithm in the Eclipse treatment planning system. The corresponding doses were measured with a single point edge diode and 2D-dose distributions using the MapCheck2 made from a multiple-diode-array-detectors with a Varian Trilogy machine. Small electron cutouts with diameters of 1.5, 2.0, 3.0, 4.0 and 5.0 cm mounted on a 6cmx6cm cone with beam energies from 6-20 MeV were used both in dose calculation and measurement.

Results: Large deviations exist between the dose calculated with the EMC algorithm and the dose measured with MapCheck2 and edge detector for small electron fields up to 50% for 6 MeV. The discrepancy between the calculated and measured doses decreases from 50% to 10% with increasing electron beam energy from 6 MeV to 20 MeV for a 1.5 cm diameter circular field. For electron fields with cutouts of a diameter equal or larger than 2.0 cm, the measured and calculated doses agree within 5% for all electron energies from 6-20 MeV. For small electron fields, the maximal depth dose shifts upstream and becomes more superficial as the electron beam energy increases from 6 MeV to 20 MeV. The electron beam profiles changes as the electron field decreases with increased dose gradients as beam energies increases due to lack of charge particle conditions.

Conclusion: The EMC underestimates dose up to 50% for small circular electron fields with a small diameter of 1.5 cm in comparison with the dose measured with a small diode detectors. Furthermore, beam profiles and depth dose curves changes, while dose extrapolation by EMC fails for small fields.


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