Purpose: When performing single-point dose verification in VMAT, it is necessary to avoid the regions with steep dose gradient. We propose a method to obtain the estimated value ( U) of uncertainty of the absolute dose measurement due to the phantom setup error by using dose gradient calculated from treatment planning system (TPS), for evaluating the appropriate measurement points.
Methods: The dose gradient was calculated from the planned dose values in the vicinity of the isocenter point using TPS. The phantom setup error was estimated. The U was calculated using the proposed formula after estimating the phantom setup error. Then, the dose gradient was calculated from the measured dose values in the vicinity of the isocenter point specified by TPS using the Tough water phantom with ionization chamber (IC), and U was calculated as in U.
Results: The correlation coefficient between U and U was 0.984, which indicates a high correlation. The average of the difference between U and U was -0.24%. We considered that this result was caused by the influence of volume averaging effect of IC.
Conclusion: The U obtained from this proposed method reflects the uncertainty of the absolute dose measurement due to the phantom setup error and is useful for evaluating the appropriate measurement points for absolute dose measurement.
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