Precision modeling of the IBA Universal Nozzle double scattering mode at the University Proton Therapy Dresden for Monte Carlo simulation

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Monte Carlo (MC) simulations are indispensable for many research and advanced clinical questions in proton therapy (PT). However, the necessary site-specific modeling of a double scattering (DS) PT system is extensive and challenging and requires a clear strategy. This work describes a comprehensive method for precise and accurate modeling of a DS nozzle that minimizes additional measurement effort. A detailed model of the IBA universal nozzle is created within the TOPAS simulation framework. This model is subsequently fine-tuned using a step by step procedure to match the same dose profiles used for the commissioning of the treatment planning system. In the proposed bottom-up approach, the geometry of beam-shaping elements is first adjusted to measured quantities and then the beam and model properties are optimized using iterative methods. The resulting dose distributions are validated with a set of independent measurement data to estimate the achieved quality. The resulting simulated dose distributions agree well with the data and show residual range differences of maximum 0.6 mm. The shape of the SOBP plateau regions is accurately reproduced with a spread of the residuals below 1% (i.e., near to the statistical limit) over a large part of the machine settings. The simulated lateral dose profiles, although not directly included in the optimization, match the shape of the validation data better than 0.14 mm. The minimal measurement effort and high-precision proton field modeling make this method attractive, in particular, for retrospective beam modeling needed in clinical outcome or relative biological effectiveness studies after DS treatment.