In this work, a novel three-dimensional superposition algorithm for photon dose calculation is presented. The dose calculation is performed as a superposition of pencil beams, which are modified based on tissue electron densities. The pencil beams have been derived from Monte Carlo simulations, and are separated into lateral and depth-directed components. The lateral component is modeled using exponential functions, which allows accurate modeling of lateral scatter in heterogeneous tissues. The depth-directed component represents the total energy deposited on each plane, which is spread out using the lateral scatter functions. Finally, convolution in the depth direction is applied to account for tissue interface effects. The method can be used with the previously introduced multiple-source model for clinical settings. The method was compared against Monte Carlo simulations in several phantoms including lung- and bone-type heterogeneities. Comparisons were made for several field sizes for 6 and 18 MV energies. The deviations were generally within (2%, 2 mm) of the field central axis d(max). Significantly larger deviations (up to 8%) were found only for the smallest field in the lung slab phantom for 18 MV. The presented method was found to be accurate in a wide range of conditions making it suitable for clinical planning purposes.
Purpose: To describe in detail the recently improved extra‐focal photon source component of an accurate analytical source model for the radiation output from a linear accelerator. Method and Materials: In the Anisotropic Analytical Algorithm used in the Eclipse™ Integrated Treatment Planning System (Varian Medical Systems Inc.) the radiation output from a linear accelerator is modeled using a multiple‐source model with separate sub‐sources for primary photon radiation, extra‐focal photon radiation and electron contamination. Closely related to this, analytical models for accessories such as hard wedges and dynamic wedges are included. We describe in detail the extra‐focal photon source component. A recent improvement to the extra‐focal source leading to enhanced accuracy in cases of small MLC apertures that are common during IMRT treatments is discussed. Another related improvement significantly improves the modeling of the Varian Enhanced Dynamic Wedge (EDW). Results: The new extra‐focal source model significantly improves the modeling of small MLC apertures that occur during dynamic IMRT treatments. In our test case to water phantom the maximum error (disregarding the buildup region) in depth dose curves has been reduced to 1% of dose maximum. Modeling of the Varian Enhanced Dynamic Wedge has been significantly improved also: in our example dataset the worst case deviation of local value at CAX has been reduced by more than 50%. Conclusion: The analytical source and accessory models used in the Anisotropic Analytical Algorithm in combination with a scheme to optimize the free parameters of the model has been already shown to be clinically acceptable. In this work we discuss two improvements related to the extra‐focal source model which further improve its accuracy. Conflict of Interest: The authors are employed by Varian Medical Systems Finland Oy.
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