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

Authors


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.

Ancillary