UBC Theses and Dissertations
Study of wedged asymmetric photon beams Qiu, Yue
In radiation therapy, many recent advances have been made in the technology used for dose delivery. However, conventional physical wedges are still in clinical use. The combination of asymmetric field collimation and physical wedge presents a challenge for accurate dose calculation. Algorithms for calculating monitor units (MUs) in wedged asymmetric photon beams as implemented in treatment planning systems have their limitations. In this work, the dose calculations for rectangular wedged asymmetric fields by the Eclipse treatment planning system were tested by direct comparison to ion chamber measurements and up to 6.5% discrepancy was found. Monte Carlo simulation by BEAMnrc was used for independent dose calculations. Finally, a correction method was developed for accurate wedged asymmetric dose calculations. The difference in dose between a wedged asymmetric field and the corresponding wedged symmetric field is accounted for by a correction factor that is a function of field sizes, off axis distance and depth of measurement. For both 6MV and 18MV photon beams at d max [subscript] and 10cm, the correction factor is within 1% of the measurement in most cases and the maximum difference is 2%. The dose at the asymmetric field center, which is based on wedged symmetric profiles and the correction factor, is within 2% of the measured dose in most cases and the maximum difference is 4%. It can be concluded that our simple correction factor is able to calculate dose at the center of wedged asymmetric fields with acceptable accuracy.
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