Intensity modulation delivery techniques: "step & shoot" MLC auto-sequence versus 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 uniformity while maintaining dose to critical structures below a set tolerance defined by the user in terms of either a single dose value or a dose volume histogram curve for each critical structure. Two clinical cases were studied 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 to the target dose uniformity. In both cases, the target dose uniformity initially improved quickly as the IM level increased to 5, then started to approach saturation when the MLC technique was used. In the absence of the both space and intensity discreteness intrinsic to the MLC technique, the modulator technique produced greater tumor dose uniformity and normal structure sparing. The latter showed no systematic improvement with increasing IM level using the MLC technique. For the sinus tumor treatment of 2 Gy the treatment irradiation time of the modulator technique is no more than that of the conventional treatment. For the MLC technique the irradiation time increased 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 compromise between the dose optimization quality and treatment irradiation time. We showed that a realistic photon source model is necessary for dose computation accuracy in the MLC-IM treatments.