SU-E-T-523: Modeling Beam Data for Flattening Filter Free (FFF) Photon Beams

Area of Science:

• Medical Physics
• Radiation Oncology

Background:

• Flattening-filter free (FFF) photon beams have non-uniform profiles, rendering conventional dose calculation algorithms invalid.
• Accurate dose calculation is crucial for intensity-modulated radiation therapy (IMRT) planning and quality assurance.

Purpose of the Study:

• To develop and validate an empirical algorithm for accurate dose calculation in FFF photon beams.
• To provide a reliable method for secondary monitor unit (MU) checks in IMRT using FFF beams.

Main Methods:

• A kernel-based algorithm was employed, quantifying phantom scatter characteristics using parameters a0, w0, and d0.
• The model was adapted for FFF beams by fitting the primary off-axis ratio (POAR) using a linear function (1 - br).

Main Results:

• The proposed model accurately fitted fractional depth doses (FDD) and phantom scatter factors (Sp) for FFF beams (6 and 10 MV) within 1.8% accuracy.
• Incorporating the POAR shape (non-zero 'b' value) was essential, as neglecting it led to significant central-axis dose calculation errors (up to 4%).
• Calculated phantom scatter parameters showed consistency with Monte Carlo simulations.

Conclusions:

• The shape of the primary off-axis ratio (POAR) significantly impacts central-axis dose calculations for FFF beams.
• The conventional equivalent square law is not applicable for dose calculations with FFF photon beams, necessitating advanced algorithms.