SU‐E‐T‐185: Clinically‐Relevant Investigation of Flattening Filter Free Skin Dose

Abstract

Purpose:Flattening‐filter‐free (FFF) beams are increasingly used for small‐field treatments due to inherent advantages like higher MU efficiency and reduced treatment time and scatter dose. Removal of the flattening‐filter increases the electron contamination and low‐energy x‐rays. As such, surface‐dose characteristics are different from traditional flattened (FF) beams. The goal of this work is to investigate surface dose of 6/10 MV FFF and FF beams under conditions representative of emerging complex techniques like small‐field stereotactic treatments which use small fields formed with multi‐leaf‐collimators (MLCs) at closer SSDs.Methods:A parallel‐plate PTW Markus‐chamber (N23343) placed in custom air‐ and water‐equivalent phantoms was used to measure surface‐dose at 2/3/4/6/8/10/20/30 cm2 field sizes, at 80/90/100 cm source‐to‐surface distances, and at fields defined by jaws and MLCs. The effect of dose rate (600 and 1400/2400 MU/min) was also investigated at 100 cm SSD. Measurements were performed on TrueBeam linac (Varian Medical Systems, Palo Alto, CA) for 6X/6XFFF/10X/10XFFF beam energies.Results:No dose‐rate dependence was seen for FFF skin‐dose. Air‐phantom measurements were, on average, 5±3% larger than for water‐phantom measurements. With SSD increase from 80 to 100 cm, skin‐dose decreased by an average of 3.9±2.5%. FFF beams were found to be more sensitive to SSD changes in comparison to FF beams. The difference in skin dose between MLC‐ and jaw‐fields was less variable with field size for FFF compared to FF beams. 10 MV beams showed greater difference in FFF‐to‐FF ratio, 50% (jaws) and 22% (MLC), between the largest and smallest field sizes compared to 6 MV beams, 30% (jaws) and 9% (MLC).Conclusion:Under clinically‐relevant conditions, surface dose for FFF beams was higher at small field size (<10 cm), lower at largest field size (30 cm), more sensitive to SSD changes, and had less variation with field size compared to dose for FF beams.