SU-E-T-185: Clinically-Relevant Investigation of Flattening Filter Free Skin Dose
- Virginia Commonwealth University, Richmond, VA (United States)
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 cm{sup 2} 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.
- OSTI ID:
- 22545308
- Journal Information:
- Medical Physics, Vol. 42, Issue 6; Other Information: (c) 2015 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
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