Intensity modulation delivery techniques: "Step & shoot" MLC auto-sequenceversus the use of a modulator

Two intensity modulation radiotherapy (IMRT) delivery systems, the "step & shoot" multileaf collimator (MLC) auto-sequence and the use of an intensity modulator, are compared with emphasis on the dose optimization quality and the treatment irradiation time. The intensity modulation (IM) was created by a dose gradient optimization algorithm which maximizes the target dose u niformity while maintaining dose to critical structures below a set toleran ce defined by the user in terms of either a single dose value or a dose vol ume histogram curve for each critical structure. Two clinical cases were st udied with and without dose optimization: a three-field sinus treatment and a six-field nasopharyngeal treatment. The optimization goal of the latter case included the sparing of several nearby normal structures in addition t o the target dose uniformity. In both cases, the target dose uniformity ini tially improved quickly as the IM level increased to 5, then started to app roach saturation when the MLC technique was used. In the absence of the bot h space and intensity discreteness intrinsic to the MLC technique, the modu lator technique produced greater tumor dose uniformality and normal structu re sparing. The latter showed no systematic improvement with increasing IM level using the MLC technique. For the sinus tumor treatment of 2 Gy the tr eatment irradiation time of the modulator technique is no more than that of the conventional treatment. For the MLC technique the irradiation time inc reased rapidly from 4.4 min to 12.4 min as the IM level increased from 2 to 10. Both clinical cases suggested that an IM level of 5 offered a good com promise between the dose optimization quality and treatment irradiation tim e. We showed that a realistic photon source model is necessary for dose com putation accuracy in the MLC-IM treatments. (C) 2000 American Association o f Physicists in Medicine.