INTENSITY-MODULATED RADIOTHERAPY BY MEANS OF STATIC TOMOTHERAPY - A PLANNING AND VERIFICATION STUDY

There is currently much research interest in developing, evaluating, a nd verifying intensity-modulation techniques. Of particular interest i s how well the delivery of intensity-modulated profiles can be simulat ed by planning algorithms, and how accurately these profiles can be de livered given the specification constraints of linear accelerators. In this paper we present a planning and verification study based on deli vering radiation in ''static-tomotherapy'' mode via the NOMOS MIMiC (M ultileaf intensity-modulation collimator), which sheds some light on t hese issues. An inverse-planning algorithm was used to compute intensi ty-modulated profiles for a 9-coplanar-field plan for a body phantom. The algorithm makes several approximations about the form of the eleme ntary 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 compari ng the predicted versus the delivered dose, these assumptions were mad e irrelevant by a final one-step forward dose calculation performed us ing the optimized intensity profiles. This forward dose calculation to ok into account the penumbral characteristics of the delivery system b y decomposing the intensity profiles into the set of delivery componen ts. Each component was assigned the appropriate penumbral functions th ereby 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 m m. At the 50% isodose level consistent spatial agreement was again fou nd to within 3 mm, the largest deviation being about 5 mm. The close c orrespondence between the predicted and measured dose distribution dem onstrates the potential of the MIMiC delivery system. Our results indi cate the level of dose conformation that is achievable in practice and the accuracy of the dose computation algorithm. However, this study o nly concerned delivery of radiation to a 2 cm thick slice, and the dos e distribution was only verified in the central plane of the phantom w here the him was placed. We therefore cannot comment as yet on what ha ppens to the dose distribution away from the central film-plane. (C) 1 997 American Association of Physicists in Medicine.