Modeling of flattening filter free photon beams with analytical and Monte Carlo TPS

Abstract

Recent conventional medical linear particle accelerators (linacs) come with the capacity to deliver flattening filter free (FFF) beams, allowing higher dose rates. Intensity modulated radiation therapy (IMRT) and small-fields stereotactic techniques no longer seem to require flattened beams. To implement such beams, it is important to correctly measure and model the dose distribution they deliver. The objective of this work was to model the recent VERSA-HD (Elekta®) FFF beams using RayStation® TPS, as done externally on Monaco TPS. There exists no publication on the modeling of this machine in FFF mode. Also, no published work relates to the use of RayStation® to model unflattened beams. Flattened X6 and unflattened 6FFF and 10FFF beams have been modeled. Lateral and depth dose profiles as well as output factors were measured using ionization chambers and diodes detectors. Models were created in RayStation® and Monaco® treatment planning systems (TPS), offering analytically- and Monte Carlo-algorithms, respectively. Surface dose, out-of-field dose and collimator transmission were also measured to improve models. All steps and parameters used during the modeling process are presented, along with measured and calculated dose profiles. Agreement between measurements and models was assessed by one-dimensional gamma analysis. Stereotactic and VMAT (volumetric modulated arc therapy) treatment plans were controlled by measuring isocenter absolute dose and dose distribution with the ArcCheck device. The impacts of the model parameters on each region of the depth and lateral dose profiles were explained. FFF beams models exhibited different energy spectra, smaller source sizes and lower electron contamination compared to flattened beams, in both TPS. Monaco® and RayStation® exhibit good agreement with depth and lateral dose profile measurements, 1% and 1 mm criteria being respected in the high dose regions. Stereotactic and VMAT treatment plans measured led to good agreement with calculation.