The resolution characteristics of intensity modulated beam (IMB) profiles produced by milled compensators and by multileaf collimators (MLCs) are independently investigated with respect to the primary fluence. It is shown that both methods have different characteristics in the longitudinal and lateral direction and, as a consequence, the resolutions of the longitudinal and lateral delivered IMB profiles differ. For both methods, the restrictions are identified. For compensators, the maximum slopes in the machining process, which should not be exceeded, are quantified. For MLCs, emphasis is given to the direction perpendicular to leaf movement. A number of test modulations were created and the effect of different size MLCs on the intensity profile revealed that unacceptable errors can be introduced if the profiles are heavily modulated. The production of intensity modulated radiation therapy (IMRT) beams by both machined compensators or by MLCs is limited by physical constraints. Having identified these constraints, some steps should now be taken to accommodate them either in the objective function for the calculation of the beam profiles or in the delivery system.
A spatial modelling technique is presented to model inversely the attenuation behaviour of poly-energetic high-energy photon beams within a metal compensator for intensity-modulated radiotherapy. The algorithm aims to predict modulators (compensators), which produce an intended intensity-modulated beam after the beam has passed through the metal. The proposed method considers two spatial dimensions and is based on a least-squares approach. The nonlinear system is modelled by means of an initial 2-D spatial model, which is linear in the parameters, and which takes into account primary and scattered radiation. This model is then re ned and simpli ed resulting in a rotational-symmetrical 2-D spatial representation, which reduces the calculation time and makes the model more generally applicable. Compared with other techniques, it is not necessary to know the linear absorption coef cient of the compensator material.
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