Feasibility and advantages of using flattening filter-free mode for radiosurgery of multiple brain lesions

The 6-MV flattening filter-free mode (6F) of the Varian TrueBeam (Varian Medical Systems, Palo Alto, CA) enables faster dose delivery and shortens treatment time, which are especially beneficial for stereotactic radiosurgery. This study is to evaluate the feasibility and advantages of using 6F in stereotactic radiosurgery treatment of multiple brain lesions in comparison with regular 6-MV mode (6X).Ten patients having 2-12 brain metastases treated by intensity modulated stereotactic radiosurgery were selected for this study. For each patient, 2 RapidArc (RA; Varian Medical Systems) plans were generated: one using the 6F mode with a dose rate of 1400 monitor units (MU)/minute and another using the regular 6X mode of 600 MU/minute for a Varian TrueBeam linac. For each patient, both plans employed the same beam arrangement and optimization process.The dosimetric parameters of homogeneity, conformity, and gradient indices were calculated and found to be comparable in the 6F and 6X plans for each patient. The mean dose to the normal brain and maximal doses to brainstem, chiasm, eyes, and optical nerves were also comparable in both RA plans using either 6F or 6X. The total number of MUs in the RA plans using 6F was 10%-20% more than that in the RA plan using 6X, but the beam-on-time was much less if 6F was used for planning and dose delivery (50% less).The fast delivery of the 6F beam is not only beneficial in stereotactic radiosurgery of a single brain lesion, but also for treating multiple brain lesions (2-12 lesions in this study group). Due to the beam falloff away from the central axis for large field sizes, more MUs are needed for 6F beams as compared with 6X. However, for the 6F mode with 1400 MU/minute, the delivery times are still much shorter compared with the 6X mode, thus greatly shortening the treatment time.