Characterization of a commercial multileaf collimator used for intensity modulated radiation therapy

The characteristics of a commercial multileaf collimator (MLC) to deliver s tatic and dynamic multileaf collimation (SMLC and DMLC, respectively) were investigated to determine their influence on intensity modulated radiation therapy (IMRT) treatment planning and quality assurance. The influence of M LC leaf positioning accuracy on sequentially abutted SMLC fields was measur ed by creating abutting fields with selected gaps and overlaps. These data were also used to measure static leaf positioning precision. The characteri stics of high leaf-velocity DMLC delivery were measured with constant veloc ity leaf sequences starting with an open field and closing a single leaf ba nk. A range of 1-72 monitor units (MU) was used providing a range of leaf v elocities. The field abutment measurements yielded dose errors (as a percen tage of the open field max dose) of 16.7 +/-0.7% mm(-1) and 12.8 +/-0.7% mm (-1) for 6 MV and 18 MV photon beams, respectively. The MLC leaf positionin g precision was 0.080 +/-0.018 mm (single standard deviation) highlighting the excellent delivery hardware tolerances for the tested beam delivery geo metry. The high leaf-velocity DMLC measurements showed delivery artifacts w hen the leaf sequence and selected monitor units caused the linear accelera tor to move the leaves at their maximum velocity while modulating the accel erator dose rate to deliver the desired leaf and MU sequence (termed leaf-v elocity limited delivery). According to the vendor, a unique feature to the ir linear accelerator and MLC is that the dose rate is reduced to provide: the correct cm MU-l leaf velocity when the delivery is leaf-velocity limite d. However, it was found that the system delivered roughly 1 MU per pulse w hen the delivery was leaf-velocity limited causing dose profiles to exhibit discrete steps rather than a smooth dose gradient. The root mean square di fference between the steps and desired linear gradient was less than 3% whe n more than 4 MU were used. The average dose per MU was greater and less th an desired for closing and opening Leaf patterns, respectively, when the de livery was leaf-velocity limited. The results indicated that the dose deliv ery artifacts should be minor for most clinical cases, but limit the assump tion of dose linearity when significantly reducing the delivered dose for d osimeter characterization studies or QA measurements. (C) 2001 American Ass ociation of Physicists in Medicine.