SU‐E‐T‐881: Simulation of a Multileaf Collimator vs. Iris Variable Aperture Collimation for Prostate Robotic Radiosurgery and IMRT

Purpose: To study potential advantages and disadvantages of a multileaf collimator (MLC) relative to iris variable aperture collimation for prostate cancer treatment with a CyberKnife robotic radiosurgery system. Methods: Six treatment plans for three prostate cases are generated using target conformal segments with a hypothetical MLC vs. circular beams with an iris variable aperture collimator. Conformality, homogeneity, number of MUs and segments are used as plan quality indicators. Quasi Newton Gradient descent optimization is applied as optimization strategy. MU reduction during the optimization process is achieved with two‐dimensional median fluence smoothing. The final sequencing is performed at sequencing levels 2 and 3 to reduce the number of segments. Iris baseline plans were generated using the vendorˈs TPS. Treatment plans A and B are conventionally fractionated IMRT with homogeneous PTV coverage. Plans C and D are IMRT with homogeneous PTV coverage (−/+ integrated boosts). Plans E and F are hyperfractionated SBRT with homogeneous PTV coverage (−/+ integrated boosts, and stricter OAR dose constraints). Results: With identical number of CyberKnife nodes for an iris and MLC treatment plan, an improvement in dose conformality is achieved with a MLC for all treatment plans and dose homogeneity is only slightly degraded. In all cases, the total number of MUs with multileaf collimated fields is reduced in comparison to circular collimated iris variable aperture fields (avg. reduction 28%). The number of segments for a comparable MLC treatment plan with an iris plan is also reduced for 4 cases and slightly increased for 2 of them (avg. reduction 17%). Conclusions: This investigation shows that a MLC could provide several benefits for intensity modulated prostate radiosurgery with a CyberKnife robotic radiosurgy system. Multileaf collimated beams for radiosurgery allow a highly conformal radiation. Their target conformal segments in addition reduce the required total number of MUs and segments.