Improvements on the Planning and Delivery of Intensity-Modulated Arc Therapy

In the past decade, intensity-modulated radiation therapy (IMRT) has taken a significant step towards dose conformality and has now become a standard radiotherapy technique in the clinic. In this era, a rotational IMRT technique called intensity-modulated arc therapy (IMAT) was also proposed to possibly further reduce normal tissue toxicity and compete with conventional IMRT. However, clinical implementation of IMAT had been stagnant primarily due to the lack of mature planning and delivery systems. In this study, various aspects of treatment planning and delivery of IMAT have been investigated and improved. The dosimetric accuracy and computational efficiency of IMAT planning has been greatly augmented by the use of Monte Carlo technique which is immune to the large number of discrete beams in approximating a continuous rotation as compared with traditional arc calculation methods. An efficient single-arc form of IMAT delivery has also been explored and extended in contrast to the original multi-arc IMAT. Here the clinical feasibility of single-arc IMAT was established by comparing to multi-arc IMAT and conventional IMRT. It was demonstrated that when using multiple arcs, the requirements on aperture shape connectivity incurred fewer constraints on the optimisation so that the plan quality became the best among the three methods studied although the dosimetric differences among them were generally small and considered clinically insignificant. Nevertheless, single-arc IMAT was able to provide a plan quality in between multi-arc IMAT and fixed-field IMRT with a significant delivery efficiency advantage. Single-arc IMAT may require dose-rate variation for delivery, which is only available with the new treatment machines. To expand the clinical utilisa-