Smoothing intensity modulated beam profiles to facilitate delivery

Intensity-modulated radiation therapy (IMRT) is rapidly gaining ground as a means of delivering dose distributions that conform more closely to the target and provide greater organ-sparing. The optimization algorithms that determine the IM beam profiles are computationally complex and intensive. Moreover, for reasons of efficiency, these algorithms often contain simplifications, such as the use of partial scattering. Consequently, the resulting IM beam profiles are inherently uncertain and noisy, often exhibiting narrow and sharp peaks and valleys. Such profiles are difficult to generate, requiring long treatment times, while affecting the final dose distribution only marginally. In this work the authors discuss techniques of smoothing the beam profiles. The simplest approach is to smooth at the end of each iteration. Alternatively, penalty terms for the "unsmoothness" of the beam profiles can be included within the objective function used to drive the optimization process. The functional form of these terms is similar to those used in the first approach, thus essentially applying the same filter. The latter method achieves better dose distributions in situations where a low-tolerance critical organ is in the middle of the field, with much sharper dose gradients at the interfaces between the critical organ and the target. In addition, it is more efficient, requiring fewer iterations, because the process of smoothing does not undo the process of optimization, as the case is in the former method.