Commissioning of photon beams of a flattening filter-free linear accelerator and the accuracy of beam modeling using an anisotropic analytical algorithm.
OBJECTIVE
To investigate dosimetric characteristics of a new linear accelerator designed to deliver flattened, as well as flattening filter-free (FFF), beams. To evaluate the accuracy of beam modeling under physical conditions using an anisotropic analytical algorithm.


METHODS
Dosimetric data including depth dose curves, profiles, surface dose, penumbra, out-of-field dose, output, total and scatter factors were examined for four beams (X6, X6FFF, X10, and X10FFF) of Varian's TrueBeam machine. Beams modeled by anisotropic analytical algorithm were compared with measured dataset.


RESULTS
FFF beams have lower mean energy (tissue-phantom ratio at the depths of 20 and 10 cm (TPR 20/10): X6, 0.667; X6FFF, 0.631; X10, 0.738; X10FFF, 0.692); maximum dose is located closer to the surface; and surface dose increases by 10%. FFF profiles have sharper but faster diverging penumbra. For small fields and shallow depths, dose outside a field is lower for FFF beams; however, the advantage fades with increasing phantom scatter. Output increases 2.26 times for X6FFF and 4.03 times for X10FFF and is less variable with field size; collimator exchange effect is reduced. A good agreement between modeled and measured data is observed. Criteria of 2% depth-dose and 2-mm distance-to-agreement are always met.


CONCLUSIONS
Reference dosimetric characteristics of TrueBeam photon bundles were obtained, and successful modeling of the beams was achieved.
Department of Radiation Oncology, University Hospital Zürich, Zürich, Switzerland. jan.hrbacek@usz.ch
Jan
J
Hrbacek
Stephanie
S
Lang
Stephan
S
Klöck
D000465:Algorithms; D016880:Anisotropy; D004867:Equipment Design; D005374:Filtration / Q000295:instrumentation; D008962:Models, Theoretical; D010315:Particle Accelerators / Q000295:instrumentation*; D017785:Photons / Q000627:therapeutic use; D011874:Radiometry / Q000379:methods*