Delivery of intensity-modulated radiation therapy with a multileaf collimator: comparison of step-and-shoot and dynamic leaf motion methods

Intensity-modulated radiation therapy (IMRT) can be delivered with a conventional multileaf collimator (MLC), either by the step-and-shoot (SAS) or the dynamic method (DMLC). The advantage of DMLC is its ability to closely deliver the desired intensity profile produced by inverse planning. The SAS method requires the desired profile to be approximated by discrete levels of intensity. The leaf sequence is then designed to deliver such discrete profile. Inevitably, there is loss of accuracy in converting from the original profile to the discrete one. However, the SAS method resembles multi-segment conventional treatment, and therefore is commonly used. The number of segments in the SAS method depends on the number of intensity levels in the discrete profile. A large number of levels makes the SAS method equivalent to the DMLC method but it requires many segments of short beam-on-times, which may present a stability problem for some linear accelerators. A small number of segments degrades the resolution, and thereby affects the dose distribution. In this work, the authors investigate the effects of the number of levels In the SAS method on the delivered dose by comparing with the DMLC method. The effects are quantified in terms of dose parameters. A head-and-neck case and a prostate case were investigated. For the prostate case, a 5-level SAS delivery produces fairly close results relative to the DMLC delivery. For the head-and-neck case, a 5-level SAS delivery is insufficient, even a 10-level SAS delivery is still inferior to the DMLC delivery.