Static and dynamic beam intensity modulation in radiotherapy using a multileaf collimator

Investigations on computer optimization of radiotherapy treatment planning (inverse planning) have demonstrated that dose distributions can often be conformed tightly to a target volume by customizing the beam intensity profiles within the treatment field. The aim of this study was the development and clinical implementation of static and dynamic intensity modulated treatment techniques using a multileaf collimator. Using static beam intensity modulation, a technique for penumbra enhancement at the superior and inferior field edges of axial coplanar treatment plans was developed. Due to penumbra enhancement, the length of all treatment fields could be reduced by typically 1.5 cm, while still achieving an adequate dose in the planning target volume. As a result, the dose delivery to critical structures could often be reduced with respect to our (previous) standard treatment without intensity modulation. A dosimetrical study showed that application of this technique in lung treatments could also compensate for the increased lateral secondary electron transport in lung tissue. For calculation of the required leaf motions to generate optimized intensity modulated beam profiles by means of dynamic multileaf collimation an algorithm was developed that fully avoids tongue-and-groove underdosage effects. Dose measurements showed that, using these leaf motions, the accuracy and stability of intensity modulated profiles was generally within 2%. For individual patients, a fast and accurate method for pretreatment verification of each optimized beam was implemented, based on absolute dose measurements with an electronic portal imaging device. Static and dynamic beam intensity modulation is presently applied routinely in our institution for treatment of head and neck cancer patients and prostate cancer patients.