Introduction
Accurate dose calculation is essential to providing quality care to patients who receive radiation treatments for life threatening tumors and other diseases. Of the various dose calculation methods available, the Monte Carlo method is widely recognized as providing the ultimate accuracy in radiation dose calculation. It is often referred to as the "gold standard" in dose calculation.

The ultra-fast Monte Carlo Dose Calculation feature implemented in the MultiPlan® Treatment Planning System provides not only the most accurate radiation dose calculation available, but does so with little to no difference in calculation time as compared to other methods. Initial performance tests show that the Monte Carlo Dose Calculation is fast enough to be used for routine treatment planning, generally taking just a few minutes. This ultra-fast calculation time is accomplished by the use of industry recognized variance reduction techniques, as well as by maximizing the use of the multiple processors available in the MultiPlan® System computer.

The most significant clinical advantage of using the Monte Carlo Dose Calculation method will be demonstrated in treatment delivered to regions of varying tissue densities, such as treatment in or near the lung.

Commissioning of the Monte Carlo Dose Calculation feature is easily performed in the MultiPlan® System. A ‘virtual source’ model is generated from customer measured data during commissioning of the algorithm, which is then used to simulate the radiation emitted from the linear accelerator. The model is used to determine the properties of an individual photon within the treatment beam. This photon is tracked, voxel by voxel, through the patient. At each voxel the probability of every possible interaction is assessed and compared against a random number. In this fashion, the stochastic nature of the physical interaction processes is simulated. Whenever an interaction occurs, the energy deposited at that point is recorded, and the properties of any secondary particles or photons generated during the interaction are recorded for later processing. The result is that dose delivered to a patient from this single photon, and all of its secondary particles and photons, is accurately calculated. The calculation is then repeated – for several million photons.

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