SU-E-T-641: Flattening Filter Free Dynamic Conformal Arcs for Lung Radiotherapy

To investigate the use of dynamic conformal arcs (DCA) with the flattening filter free (FFF) beams for lung stereotactic body radiotherapy (SBRT) on the TrueBeam STx (Varian, Inc., Palo Alto, CA).

Eight SBRT patients receiving 10-18 Gy per fraction were planned in Eclipse 10 (Varian Medical Systems, Palo Alto, CA) for the TrueBeam STx using DCA's with 6 MV FFF beams. The isocenter was placed such that the point fell near midline on each patient allowing full arc rotation without patient collision.Several planning methods were employed depending on the location of the tumor. For peripheral tumors a single arc was used. For peripheral tumors with chestwall interference, additional oblique fields were used to help pull dose off of the chestwall. For tumors located in the central region or close to the spine, multiple DCA's were used. One standard open DCA was used to deliver the majority of the dose, then one or two additional DCA's with organs blocked automatically using the'Fit and Shield' feature of Eclipse were used to help shape the dose. The arcs were then weighted to create the desired dose distribution.

Beam on times for the 8 patients averaged 2.00 minutes with a minimum time of 1.22 minutes and a maximum time of 3.3 minutes total. With multiple arcs or additional static fields, treatment time is lengthened by the time it takes to mode up between arcs (∼1 minute) and/or move between fields. The PTV coverage of the 100% isodose line was >= 95% in all cases. The conformality index ranged 1.12 - 4.5, with an average of 2.5.

FFF DCA is a fast, efficient way to plan and delivery SBRT treatments. The short treatment times directly lead to reduced patient motion and discomfort, which may yield improved targeting and outcomes.