ABSTRACT
Dynamic delivery techniques such as dynamic wedges and intensity-modulated radiation therapy (IMRT) employing dynamic multileaf collimators (MLCs), tomotherapy, and intensity-modulated arc therapy (IMAT) are being used on a routine basis in many radiotherapy clinics. In a dynamic beam delivery, the particle fluence is modulated by a beam modifier whose position within the beam is varied as a function of time. In some techniques, not only the fluence is modified but also the incident direction and energy of the beam may be changed, for example, in an arc therapy delivery or a scanned proton beam. In the absence of Monte Carlo (MC) simulations, the influence of the beam modifiers on the spatial and energy distribution of incident particles is often approximated by the treatment planning system with poorly defined dosimetric consequences, especially when multiple field segments or beam angles are delivered. Accurate dose calculation methods are required to characterize the dynamic beam modifiers, to test the accuracy of the treatment planning system dose distribution, and to perform independent monitor unit calculations as well as to reconstruct the patient dose delivery. If properly validated, a dynamic MC model of the beam can serve as a commissioning tool to replace extensive complicated measurements, especially if measurement resolution or accuracy is questionable. This topic comprises the first part of this chapter. The methods for simulation of dynamic beam delivery devices will be discussed followed by examples of specific applications to dynamic radiotherapy techniques.
