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Dosimetry for Dynamic IMRT Photon Beams

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X Liang

X Liang1*, T Zhu1, (1) Univ Pennsylvania, PHILADELPHIA, PA

SU-C-213AB-4 Sunday 1:30:00 PM - 2:15:00 PM Room: 213AB

Purpose: To develop a model accounting for differences of dose per monitor unit between dynamic and stop-and-shoot IMRT.

Methods: For IMRT photon beams, treatments can be divided into stop-and-shoot mode and dynamic mode based on the movement of the multileaf collimator (MLC) during the photon delivery. However, dose measurements from a Varian photon beam based on these two different types of treatments are varied for at least 8%. We propose a model to compensate for this discrepancy, based on the physical parameters of both the MLC shape and moving pattern. In this model, A MLC minimum width is developed to explain the cause of the discrepancy, and dose calculation is implemented by convolving the source kernel with the multi-leaf profile function with different moving patterns. Thus, dose calculation under stop-and-shoot mode and dynamic mode are both obtained using this convolution method.

Results: Analytical models have been obtained for stop-and-shoot and dynamic modes for IMRT. The theory predicts that doses from dynamic mode are higher than dose from the stop-and-shoot mode, which reflects the experimental results. MLC minimum widths for different photon beam energies are also obtained.

Conclusions: This work models the MLC shape and movement pattern for IMRT dose calculation, and explains the dose difference between two modes. The theory, as well as the results in this work provides a useful tool for photon beam IMRT dose calculation.

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