The design, physical properties and clinical utility of an iris collimator for robotic radiosurgery

Abstract

Robotic radiosurgery using more than one circular collimator can improve treatment plan quality and reduce total monitor units (MU). The rationale for an iris collimator that allows the field size to be varied during treatment delivery is to enable the benefits of multiple-field-size treatments to be realized with no increase in treatment time due to collimator exchange or multiple traversals of the robotic manipulator by allowing each beam to be delivered with any desired field size during a single traversal. This paper describes the Iris variable aperture collimator (Accuray Incorporated, Sunnyvale, CA, USA), which incorporates 12 tungsten-copper alloy segments in two banks of six. The banks are rotated by 30 degrees with respect to each other, which limits the radiation leakage between the collimator segments and produces a 12-sided polygonal treatment beam. The beam is approximately circular, with a root-mean-square (rms) deviation in the 50% dose radius of <0.8% (corresponding to <0.25 mm at the 60 mm field size) and an rms variation in the 20-80% penumbra width of about 0.1 mm at the 5 mm field size increasing to about 0.5 mm at 60 mm. The maximum measured collimator leakage dose rate was 0.07%. A commissioning method is described by which the average dose profile can be obtained from four profile measurements at each depth based on the periodicity of the isodose line variations with azimuthal angle. The penumbra of averaged profiles increased with field size and was typically 0.2-0.6 mm larger than that of an equivalent fixed circular collimator. The aperture reproducibility is < or =0.1 mm at the lower bank, diverging to < or =0.2 mm at a nominal treatment distance of 800 mm from the beam focus. Output factors (OFs) and tissue-phantom-ratio data are identical to those used for fixed collimators, except the OFs for the two smallest field sizes (5 and 7.5 mm) are considerably lower for the Iris Collimator. If average collimator profiles are used, the assumption of circular symmetry results in dose calculation errors that are <1 mm or <1% for single beams across the full range of field sizes; errors for multiple non-coplanar beam treatment plans are expected to be smaller. Treatment plans were generated for 19 cases using the Iris Collimator (12 field sizes) and also using one and three fixed collimators. The results of the treatment planning study demonstrate that the use of multiple field sizes achieves multiple plan quality improvements, including reduction of total MU, increase of target volume coverage and improvements in conformality and homogeneity compared with using a single field size for a large proportion of the cases studied. The Iris Collimator offers the potential to greatly increase the clinical application of multiple field sizes for robotic radiosurgery.

CyberKnife® fixed cone and Iris™ defined small radiation fields: Assessment with a high‐resolution solid‐state detector array

“…These results, which were supported by equivalent SRS diode measurements, were consistent with those of a previous investigation in which FWHM and penumbra values for fixed cones and the Iris were found to be in substantial agreement, with a maximum discrepancy of 0.2 mm in penumbra width for the 5 mm diameter 36. By the vendor's technical specifications, the average penumbra for the Iris is expected to be larger by 0.2 to 0.6 mm than that for the equivalent fixed cone and to increase with field size, a consequence of the stepwise approximation of a divergent collimator shape because of the increase in transmission penumbra 3. To our knowledge, no other intercomparison between Iris and fixed cones collimator dose profiles exists in the literature.…”

“…The Iris collimator is designed to achieve an aperture reproducibility of 0.2 mm at 800 mm SDD,3 with the current recommendation (Accuray Physics Essentials Guide 2012, P/N 1023868‐ENG A) for QA suggesting monthly film measurements of all 12 field sizes. Nonequivalent DPs reproducibility after a reset of the Iris aperture were found to be accurate within 2% for all profiles, with a maximum discrepancy of 1.9% for the 5 mm diameter in‐plane profile (<0.1 mm) and of 1% for the 10 mm diameter in plane and cross‐plane profiles.…”

“…Following the approach recommended by the vendor,3 and as requested by the CyberKnife system TPS, for any given field size DPs were measured at different angles with respect to the in‐plane direction. For the fixed cones, the representative equivalent circular profile was then taken as the average of the profiles measured at 0° and 90°, while for the Iris it was taken as the average of the profiles measured at 0°, 15°, 90°, and 105°, to sample the underlying collimator asymmetry.…”

“…1). 1, 3 The latter, allowing for the radiation field size to be varied during treatment delivery, has the potential to decrease the peripheral dose compared to fixed collimators4 and to reduce treatment time 3. A CyberKnife system, the first of its kind in Australia, was recently installed at the Sir Charles Gairdner Hospital (SCGH), Nedlands, WA, with promising early clinical results 5…”

Clinical usefulness of MLCs in robotic radiosurgery systems for prostate SBRT

“…We expect that there are two likely causes of these results: the reproducibility of the leaf position of the MLC and the dependence of the MLC beam modeling accuracy on the TPS. Concerning the collimator position reproducibility, a variable collimator is designed to achieve an aperture reproducibility of ≤0.2 mm at a SAD of 800 mm 25. Meanwhile, Fürweger et al demonstrated using Bayouth tests that the average leaf position offset resulting from using a Cyberknife M6 with an InCise MLC was ≤0.5 mm 24.…”