Dose Course 2018_Flipping book

Example 1: Source modelling in Elekta Oncentra 

Manual: Physics and Algorithms v4.3

Source modelling is part of the calculation of direct photon energy fluence (the energy fluence of non-scattered photons reaching the patient which calculated on a 1mm matrix resolution).

Beam source of finite size

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• Source intensity distribution model: 2D Gaussian

• Source intensity distribution parameters: Determined individually for each machine and beam energy using measured data (fitting measured profiles and small field output factors) • Source distribution is discretised on a 2D grid; amplitudes on each position renormalized to sum up to unity • The resolution of this grid depends on its size, the resolution of the energy fluence matrix and the distance to the collimators; derived at runtime • Geometric penumbra modelled by accounting for shape of collimator/MLC edge and its position with respect to the source • For collapsed code dose calculation: The effect of the finite source is considered by convolving the projected source distribution with a constant energy fluence bounded by the actual field shape. The method takes into account the position of the collimating element from the beam axis accounting of the distance of the collimating device from the source.

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