Room: Exhibit Hall | Forum 1
Purpose: The separation in the brain of multiple metastatic lesions results in highly modulated VMAT plans to deliver small dose distributions off axis while sparing the sensitive structures. The Varian HD-120 MLC was designed with two leaf sizes; 2.5 mm and 5.0 mm. The off-axis delivery may utilize control points that require the larger leaf size. This investigation includes comparison of single isocenter dose distributions examining the inner versus outer wider leaves.
Methods: Modeling of 6 MV x-ray FFF beam from a TrueBeam STX was performed on RayStation in accordance with manufacturers recommendations. Test plans were created with multiple lesions in a CyberKnife end-to-end(E2E) head phantom (CKPhantom). This phantom allowed films to be placed in the phantom center on two separate planes. E2E testing was performed on plans with and without off-axis delivery. VMAT plans were created for the 4-lesion plan with central axis isocenter delivery and off-axis isocenter delivery. Gamma index analysis was performed using radio-chromic film..
Results: The film results of the two 4-lesion plan showed good agreement to the central axis and off axis delivery. The gamma pass rates at 2%/2mm for the central axis axial plane were 98.56% and 99.49% for the sagittal plane. The gamma pass rates at 2%/2mm for the off-axis axial plane were 94.1% and 96.77% for the sagittal plane.
Conclusion: Differences in the gamma pass rates for the two identical plans suggest that the planning system can account for the inability to deliver the same desired dose to the phantom given the differing leaf widths. The other major difference was the dose to normal tissue between the two plans. The off-axis delivery with wider leaves showed a significant increase in normal tissue brain dose. These findings support the practice of creating a 2-isocenter plan when lesion extents are more than 6.0cm apart.