Analysis of physical parameters and determination of inflection point for Flattening Filter Free beams in medical linear accelerator

Abstract

Background

Medical Linear accelerators manufactured without flattening filters are increasing popular in recent days. The removal of flattening filter results in increased dose rate, reduced mean energy, reduction in head leakage and lateral scattering, which have shown advantageous when used for special treatment procedures.

Aim

This study aims to analyze physical parameters of FFF beams and to determine the inflection point for standardizing the beam flatness and penumbra.

Materials and methods

The beam profiles and depth dose patterns were measured using Radiation Field Analyzer (RFA) with 0.13[[ce:hsp sp="0.25"/]]cc cylindrical ion chamber. The beam energy characteristics, head scatter factor (Sc) were obtained for 6FFF and 10FFF beams and compared with 6[[ce:hsp sp="0.25"/]]MV and 10[[ce:hsp sp="0.25"/]]MV photons, respectively. The symmetry and stability of unflattened regions were also analyzed. In addition, the study proposes a simple physical concept for obtaining inflection point for FFF beams and results were compared using the Akima spline interpolation method. The inflection point was used to determine the field size and penumbra of FFF beams.

Results

The Sc varied from 0.922 to 1.044 for 6FFF and from 0.913 to 1.044 for 10FFF with field sizes from 3[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]3[[ce:hsp sp="0.25"/]]cm to 40[[ce:hsp sp="0.25"/]]cm[[ce:hsp sp="0.25"/]]×[[ce:hsp sp="0.25"/]]40[[ce:hsp sp="0.25"/]]cm which is much less than FF beams. The obtained value of field size and penumbra for both simple physical concept and Akima spline interpolation methods is within the ±1.0[[ce:hsp sp="0.25"/]]mm for the field size and ±2[[ce:hsp sp="0.25"/]]mm penumbra. The results indicate that FFF beams reduce Sc compared with FF beams due to the absence of a flattening filter.

Conclusion

The proposed simple method to find field size and penumbra using inflection point can be accepted as it is closely approximated to mathematical results. Stability of these parameters was ascertained by repeated measurements and the study indicates good stability for FFF beam similar to that of FF beams.