The Impact of Beam Selection: 6MV Flattening Filter Free (FFF) Vs. 6 MV Flattening Filter (FF) on the Dosimetric Quality and Beam on Time of Lung and Liver Stereotactic Radiotheraoy (SBRT) Treatment Plans
Abstract
Purpose
to evaluate the impact of beam selection: 6MV FFF beams vs 6MV FF on the dosimetric quality of lung and liver SBRT treatment plans, beam on time (BOT) and monitor units (MU).
Methods
Twenty clinical treatment plans (10 lung and 10 liver SBRT) were developed using 6FFF beams in the Eclipse 16.1 TPS and used to treat patients were retrospectively re planned using 6MV(FF) beams and the same optimization priorities as in the original plans. The 6MV FF plans were normalized to maintain the same PTV coverage as in the 6MVFFF plans. All plans (20 clinical 6MV FFF and 20 re planned 6MV FF) were delivered in dry runs using the Elekta versa HD linear accelerator and the actual BOT and dose rates were recorded. The FFF & FF plans were compared in terms of dosimetric parameters (CI, HI, hot spot) and doses to organs at risk (OAR) (for lung plans: lung mean dose and lung V20%, maximum dose to esophagus, heart, spinal cord, chest wall D 50cc and for liver plans: liver-ITV mean dose, liver V0cGy, Rt kidney mean dose, maximum dose to skin, spinal cord and body-PTV ). MUs were also compared.
Results
The CI and HI, hot spot and doses to OARS for both 6MV FFF & 6 MV FF beams were comparable, MU' differences varied from one lung SBRT plan to another whilst they were always higher in 6MV FF Liver plans. BOT was significantly shorter for 6MV FFF plans (the ratio of delivery time for 6FF to 6FFF ranged between 2.24 to 3.72).
Conclusion
6MV FF SBRT plans were dosimetrically more or less like 6 MV FFF plans. Beams delivery time was significantly shorter for 6MV FFF plans. Further investigation is needed into the correlation between PTV volume/shape and beam delivery time.