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A new treatment technique, intensity modulated arc therapy (IMAT), has been implemented in the authors' clinical practice. Instead of delivering intensity-modulated beams with fixed gentry angles, IMAT delivers optimized dose distributions using rotational beams. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly. Treatment plans are developed with a combination of forward and inverse planning. Arcs are approximated as multiple shaped fields spaced every 10 degrees around the patient. Multiple coplanar or non-coplanar arcs are used in combination with wedges. The beam weights can be optimized outside of the commercial planning system. A leaf-sequencer is used to convert the fields from the final plan into MLC leaf sequences. Comparisons were made with conventional and inversely optimized treatment plans. Dosimetric accuracy of the entire process was verified with phantoms before IMAT was used clinically. Patient-specific verifications were performed using both the absolute and relative dose measurements in a humanoid phantom. The authors have demonstrated the clinical feasibility and accuracy of IMAT treatments on patients with head and neck cancers. Generally, no more than 3 arcs are needed to achieve a conformal dose distribution. The complexity of the IMAT is not significantly greater than existing are therapy from the point of view of the operator. The treatment time using IMAT is similar to conventional treatments. Clinical examples of IMAT treatments will be presented to illustrate the dosimetric advantage of rotational delivery.

Intensity-modulated arc therapy, or MAT, is one of the methods for delivering intensity-modulated radiation treatments to achieve high dose conformity. Instead of delivering intensity-modulated beams with fixed gantry angles [1], IMAT delivers optimized dose distributions by rotating the radiation beam around the patient. During delivery, the field shape, which is formed by a multileaf collimator (MLC), changes constantly as determined by the required intensity distributions at different beam angles. Intensity distributions at all angles around the patient are achieved with multiple overlapping arcs of different field apertures and gantry speed. Therefore, IMAT is also different from tomotherapy[2,3], which use intensity modulated fan beams rotating around the patient to delivery the treatment slice-by-slice. As with tomotherapy, IMAT combines intensity modulation and rotational delivery. Detailed description of the technique was reported by Yu in a previous article [4].