Purpose: Optimization of modulated stereotactic radiosurgery (SRS) plans sometimes results in (i) leaf pair openings that do not irradiate the planning target volume (PTV) and (ii) individual leaf ends far (>2 mm) outside the PTV. Our goal is to understand the dosimetric impact of these rogue leaf openings.
Methods: Single fraction, modulated Varian Eclipse SRS plans from 10 patients were selected retrospectively. After projecting a 2 mm expansion of the PTV (PTV+2mm) onto the isocenter plane, leaf pair openings not irradiating the PTV+2mm were flagged as superfluous. Individual leaves were also flagged as nonconformist if there was a gap between the leaf end and the PTV+2mm projections. Two modified plans were generated with in-house software while respecting machine parameters: for both “clean” and “conformed” plans, superfluous leaf openings were pushed behind the jaws. Additionally, in “conformed” plans, the nonconformist leaf ends were pushed to the edge of the PTV+2mm to enforce conformality. The doses for the modified plans were recalculated, renormalized to the same PTV coverage, and DVH metrics were compared.
Results: Clean and conformed plans reduced total plan fluence by up to 12.9 and 14.7%, respectively. Despite this reduction in fluence, small changes in the maximum and minimum doses to 0.002cc of the PTV were observed (largest changes of 1.68 and -1.95%, respectively). Although conformed plans exhibited large relative changes in normal brain V2Gy, V5Gy, and V12Gy (up to -24.4, -15.8, and -16.4%, respectively) the maximum absolute change in V12Gy was -0.39 cc.
Conclusion: Based on analysis of 10 patient plans, modulated SRS plans contain rogue leaf openings that unnecessarily irradiate normal tissue. Post processing can remove these rogue leaf positions without compromising PTV coverage. The resulting modified plans reduce normal brain V2Gy, V5Gy, and V12Gy.
MLC, Stereotactic Radiosurgery, Optimization