An interlaced IMRT technique for elongated tumor volumes.

Treatment of large target volumes with intensity modulated radiotherapy (IMRT) can be restricted by the maximum field size of the multileaf collimator (MLC). In this work, a straightforward technique for MLC-based IMRT is presented, which is generally applicable and does not depend on the capabilities of the linear accelerator's IMRT delivery system. A dual isocenter technique was developed that maximizes beam overlap. The beams at the first isocenter are arranged such that they interlace with the beams at the second isocenter. All beams contribute to the overlap region, whereas only some contribute to the superior and some to the inferior part of the target. The interlaced technique (9 beams) was compared with an alternative more complex approach (14 beams) for a head-and-neck case with simultaneous integrated boost and 3 different dose levels. The plans were compared in terms of complexity, dosimetry, and the effect of inaccurate translation between the isocenters. The interlaced and the more complex IMRT technique resulted in nearly identical dose distributions without clinically relevant differences. The total number of monitor units (MUs) was comparable with more MUs per segment for the interlaced technique. For the interlaced technique, the number of segments <or=5 MU was reduced by 43%. Simulation of isocenter setup errors of +/-1, +/-2, and +/-3 mm revealed maximum dose point errors of 1.8%, 3.8%, and 5.4% in the target volume for the interlaced technique. The interlaced IMRT technique resulted in an equivalent plan to the more complex technique without compromising the dose distribution. The technique is less complex and is robust against inaccurate isocenter translations of up to +/-1 mm. Due to the versatility of the technique, it can easily be applied to other anatomical regions and is well suited for clinical routine usage.