SU-E-T-632: Stereotactic Radiosurgery for Multiple Brain Metastases Using Flattening Filter-Free Beam and Comparison Study with the Regular 6MV

Abstract

Purpose: To investigate the benefits of using TrueBeam 6MV flattening filter-free (6F) mode in treating multiple intracranial metastases. To compare MU and dosimetric characteristics of the RapidArc (RA) plans using 6F and the regular 6 MV beam (6X). Methods: We have selected 10 patients with 2–12 brain metastases treated by IMRS. They also had RA plans done on Varian-23IX under ARIA-V8.6 in previous study. All these RA plans have been re-planned for TrueBeam which requires ARIA-V8.9. The prescribed dose is 2000 cGy and normalized to 98% of PTV. Two RA plans have been generated for each patient: 6X (600 MU/min) and 6F (1400 MU/min). The gantry arc-spans, collimator angles, and field sizes are the same in both plans for each patient. For fair comparison, the optimization objectives, priorities, even the timing of priority changes are kept the same at best in both planning process due to the adaptive nature of RapidArc optimization. Results: For dosimetric evaluation and comparison, the homogeneity, conformity, and gradient indices are calculated for both plans. It is found all the dosimetric indices in 6F plans are comparable with the corresponding 6X plans in each patient. The maximal dose to brainstem, chiasm, eyes, and optical nerves and the mean dose to normal brain are calculated. In the comparison study of critical organ doses, it is found that they are comparable or slightly better in 6F plans compared to 6X. Total MUs in 6F plans are about 10–20% more than MUs in 6X plan of each patient, with a group average of 16% in 6F plans. Conclusion: The dosimetric characteristics of 6F plans are found to be comparable with 6X plans in radiosurgery treatment of multiple brain lesions. Counting only 10– 20% more MUs if using 6F, the treatment delivery is much faster and thus greatly shortens treatment time.