Flattening filter free beam energy selection and its impact in multitarget intracranial stereotactic radiosurgery treatments.

Extensive work has been done on the characteristics, dosimetry, and efficacy of flattening filter free (FFF) beams in radiosurgery. However, no study has addressed the dosimetric impact of FFF beam energy selection on treatment plan quality. This study aims to present a systematic dosimetric comparison of plan quality between 10 FFF vs 6 FFF beams in intracranial stereotactic radiosurgery (SRS) treatments using volumetric modulated arc therapy (VMAT). The dosimetric evaluation is based on radiation therapy oncology group (RTOG) dose conformity (CIRTOG) and gradient (GIRTOG) indices, and irradiated normal brain tissue volume. Thirty-five VMAT-based intracranial SRS treatments to multiple brain metastases using a 2.5 mm multileaf collimator (MLC) and 10 MV FFF beam were replanned with a 6 MV FFF and same MLC. The replans incorporated the same arc arrangement, planning target volume (PTV) and organs at risk structures, PTV coverage and prescription isodose normalization. The 6 MV FFF had a sharper dose fall-off compared to the 10 MV FFF (GIRTOG-10 FFF = 4.70, GIRTOG-6 FFF = 4.56; p < 0.05) and comparable conformity index (CIRTOG-10 FFF = 1.11, CIRTOG-6 FFF = 1.10; p = 0.9). On average, the irradiated normal brain tissue volume was 11% lower with 6 MV FFF compared to 10 MV FFF (p < 0.05). However, this difference was diminished for large target volumes and increased number of targets treated. The main dosimetric improvement of a 6 MV FFF over a 10 MV FFF beam is the sharper dose fall-off which directly correlates with less normal brain tissue volume irradiation.

The use of RapidArc volumetric-modulated arc therapy to deliver stereotactic radiosurgery and stereotactic body radiotherapy to intracranial and extracranial targets.

Twenty-three targets in 16 patients treated with stereotactic radiosurgery (SRS) or stereotactic body radiotherapy (SBRT) were analyzed in terms of dosimetric homogeneity, target conformity, organ-at-risk (OAR) sparing, monitor unit (MU) usage, and beam-on time per fraction using RapidArc volumetric-modulated arc therapy (VMAT) vs. multifield sliding-window intensity-modulated radiation therapy (IMRT). Patients underwent computed tomography simulation with site-specific immobilization. Magnetic resonance imaging fusion and optical tracking were incorporated as clinically indicated. Treatment planning was performed using Eclipse v8.6 to generate sliding-window IMRT and 1-arc and 2-arc RapidArc plans. Dosimetric parameters used for target analysis were RTOG conformity index (CI(RTOG)), homogeneity index (HI(RTOG)), inverse Paddick Conformity Index (PCI), D(mean) and D5-D95. OAR sparing was analyzed in terms of D(max) and D(mean). Treatment delivery was evaluated based on measured beam-on times delivered on a Varian Trilogy linear accelerator and recorded MU values. Dosimetric conformity, homogeneity, and OAR sparing were comparable between IMRT, 1-arc RapidArc and 2-arc RapidArc plans. Mean beam-on times ± SD for IMRT and 1-arc and 2-arc treatments were 10.5 ± 7.3, 2.6 ± 1.6, and 3.0 ± 1.1 minutes, respectively. Mean MUs were 3041, 1774, and 1676 for IMRT, 1-, and 2-arc plans, respectively. Although dosimetric conformity, homogeneity, and OAR sparing were similar between these techniques, SRS and SBRT fractions treated with RapidArc were delivered with substantially less beam-on time and fewer MUs than IMRT. The rapid delivery of SRS and SBRT with RapidArc improved workflow on the linac with these otherwise time-consuming treatments and limited the potential for intrafraction organ and patient motion, which can cause significant dosimetric errors. These clinically important advantages make image-guided RapidArc useful in the delivery of SRS and SBRT to intracranial and extracranial targets.