A simple method to control aspects of fluence modulation in IMRT planning

Many inverse-planning algorithms and commercial systems generate intensity-modulated beam profiles that have considerable structure. This is the desirable outcome of the quest for high dose-space conformality. However, when these profiles are realized experimentally using the dynamic multileaf collimator (DMLC) method of delivery the monitor-unit efficiency can be quite small, with unwanted consequences. Also the interpretation of these fields leads to the generation of small field segments, again with undesirable consequences. In this note it is shown that the features of beam-space can be user-controlled to minimize these problems. There is a tradeoff between obtaining desirable features in beam-space and high conformality in dose-space.