Background and purpose: An optimization algorithm has been developed to determine the best beam-arrangement for a small number of intensity-modulated radiotherapy (IMRT) fields. The algorithm is designed to avoid, if possible, beam-orientations that pass through organs-at-risk (OARs) with low radiation tolerance.
Materials and methods: An independent, fast IMRT algorithm based on the Bortfeld algorithm was developed to determine the profile of the intensity-modulated beams (IMBs) for each beam-arrangement and a fast-simulated-annealing algorithm finds the 'optimal' beam-arrangement. The final beam-arrangement was transferred to the CORVUS (NOMOS Corporation) treatment planning system, and the IMBs were re-optimized for comparison with a standard nine-field, equi-spaced arrangement. The algorithm has been initially tested on a single example patient, with a parotid gland carcinoma.
Results: The nine-field, IMRT plan for an example patient with a parotid gland tumour significantly reduced the dose to the cochlea compared with the conformal radiotherapy plan. In addition, the planning-target-volume (PTV) homogeneity was improved, but the plan produced a higher dose to the contralateral parotid (73% of the OAR received more than 6 Gy). The beam-orientation optimization algorithm produced a three-field plan that greatly reduced the dose to the contralateral parotid (maximum dose of 2 Gy), whilst maintaining the PTV dose homogeneity and the reduced cochlear dose of the nine-field plan. Some changes in the dose to the other OARs, namely the brain and the oral cavity, were seen, but were deemed not to be clinically significant.
Conclusions: In conclusion, IMB-orientation optimization for head and neck treatment sites can produce improvements in treatment plans with only a few fields.