Intensity-modulated radiotherapy by means of static tomotherapy: a planning and verification study

There is currently much research interest in developing, evaluating, and verifying intensity-modulation techniques. Of particular interest is how well the delivery of intensity-modulated profiles can be simulated by planning algorithms, and how accurately these profiles can be delivered given the specification constraints of linear accelerators. In this paper we present a planning and verification study based on delivering radiation in "static-tomotherapy" mode via the NOMOS MIMiC (Multileaf intensity-modulation collimator), which sheds some light on these issues. An inverse-planning algorithm was used to compute intensity-modulated profiles for a 9-coplanar-field plan for a body phantom. The algorithm makes several approximations about the form of the elementary fluence profile through bixels during delivery. Specifically, it is independent of the state of adjacent bixels (i.e., open or closed) and obeys the superposition principle. From the standpoint of comparing the predicted versus the delivered dose, these assumptions were made irrelevant by a final one-step forward dose calculation performed using the optimized intensity profiles. This forward dose calculation took into account the penumbral characteristics of the delivery system by decomposing the intensity profiles into the set of delivery components. Each component was assigned the appropriate penumbral functions thereby ensuring that the calculated dose distribution closely predicted the delivered dose distribution. The nine intensity modulated fields were delivered to a perspex phantom with the same geometry, containing a verification film. In general good agreement was found between the predicted and the measured delivered dose distributions. All the main features of the predicted dose distribution are seen in the delivered. The 90% isodoses were consistently in spatial agreement to within 3 mm. At the 50% isodose level consistent spatial agreement was again found to within 3 mm, the largest deviation being about 5 mm. The close correspondence between the predicted and measured dose distribution demonstrates the potential of the MIMiC delivery system. Our results indicate the level of dose conformation that is achievable in practice and the accuracy of the dose computation algorithm. However, this study only concerned delivery of radiation to a 2 cm thick slice, and the dose distribution was only verified in the central plane of the phantom where the film was placed. We therefore cannot comment as yet on what happens to the dose distribution away from the central film-plane.