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Purpose: To evaluate and quantify dose errors in gated breast treatments due to mismatch of adjacent fields caused by variations in the patients' breathing patterns. Methods: a) Experiment: An EBT‐II GaF‐chromic film was positioned in a stack or water equivalent material (RW3). A treatment, gated using the Varian RPM system, was delivered using two matched 6MV half beams. One anterior (upper) and one lateral (lower). The gating phantom allowed movement perpendicular to the match line. Subsequently, the amplitude of the breathing motion was reduced with 2mm. The films for both irradiations were analyzed yielding absolute dose. b) Mathematical Model: A mathematical model based on Cauchy distributions was developed to model the penumbra of a half beam blocked field and the errors in matching them. The mean parameters of the function are a shape parameter gamma, quantifying the slope, and a positioning parameter. Results: The experiment showed a region of under dosage of 17%. The mathematical model applied to a 6MV half beam yielded a good fit with a gamma of 0.387(±0.006). The model resulted in a calculated penumbra of 1.06cm while the measured penumbra was 1.02cm. Entering the data from the gated treatment in the model yielded an error level of 16.09%. The shape parameter needed to reduce the error of this particular treatment to a level of 3% yielded a penumbra of at least 5.83cm. Conclusions: We have shown that it is possible to correct for matching problems by increasing the penumbra and optimize the increase as a function of the errors detected. The errors introduced here are exceedingly large, due to the limitations with which the amplitude variable could be adjusted in our phantom. In a follow up we expect to analyze the clinically more relevant breathing pattern variations of our current breast‐only cohort. © 2011, American Association of Physicists in Medicine. All rights reserved.

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