Intensity modulated arc therapy (IMAT) is an intensity modulated radiation therapy delivery technique originally proposed as an alternative to tomotherapy. IMAT uses a series of overlapping arcs to deliver optimized intensity patterns from each beam direction. The full potential of IMAT has gone largely unrealized due in part to a lack of robust and commercially available inverse planning tools. To address this, we have implemented an IMAT arc-sequencing algorithm that translates optimized intensity maps into deliverable IMAT plans. The sequencing algorithm uses simulated annealing to simultaneously optimize the aperture shapes and weights throughout each arc. The sequencer enforces the delivery constraints while minimizing the discrepancies between the optimized and sequenced intensity maps. The performance of the algorithm has been tested for ten patient cases (3 prostate, 3 brain, 2 head-and-neck, 1 lung, and 1 pancreas). Seven coplanar IMAT plans were created using an average of 4.6 arcs and 685 monitor units. Additionally, three noncoplanar plans were created using an average of 16 arcs and 498 monitor units. The results demonstrate that the arc sequencer can provide efficient and highly conformal IMAT plans. An average sequencing time of approximately 20 min was observed.
The recent development in linear accelerator control systems, named volumetric-modulated arc therapy (VMAT), has generated significant interest in arc-based intensity-modulated radiation therapy (IMRT). The VMAT delivery technique features simultaneous changes in dose rate, gantry angle and gantry rotation speed as well as multi-leaf collimator (MLC) leaf positions while radiation is on. In this paper, we describe a generalized VMAT planning tool that is designed to take full advantage of the capabilities of the new linac control systems. The algorithm incorporates all of the MLC delivery constraints such as restrictions on MLC leaf interdigitation and the MLC leaf velocity constraints. A key feature of the algorithm is that it is able to plan for both single- and multiple-arc deliveries. Compared to conventional step-and-shoot IMRT plans, our VMAT plans created using this tool can achieve similar or better plan quality with less MU and better delivery efficiency. The accuracy of the obtained VMAT plans is also demonstrated through plan verifications performed on an Elekta Synergy linear accelerator equipped with a conventional MLC of 1 cm leaf width using a PreciseBeam VMAT linac control system.
Using direct aperture optimization, we have developed an inverse planning approach that is capable of producing efficient intensity modulated radiotherapy (IMRT) treatment plans that can be delivered without a multileaf collimator. This "jaws-only" approach to IMRT uses a series of rectangular field shapes to achieve a high degree of intensity modulation from each beam direction. Direct aperture optimization is used to directly optimize the jaw positions and the relative weights assigned to each aperture. Because the constraints imposed by the jaws are incorporated into the optimization, the need for leaf sequencing is eliminated. Results are shown for five patient cases covering three treatment sites: pancreas, breast, and prostate. For these cases, between 15 and 20 jaws-only apertures were required per beam direction in order to obtain conformal IMRT treatment plans. Each plan was delivered to a phantom, and absolute and relative dose measurements were recorded. The typical treatment time to deliver these plans was 18 min. The jaws-only approach provides an additional IMRT delivery option for clinics without a multileaf collimator.
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