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A Monte Carlo Evaluation of Flattening-Filter-Free MV Photon Dose Distributions in the Presence of High-Z Metals

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B Ispir

B Ispir1*, N Sarigul2 , K Yenice3 , H Schlattl4 , Z Yegingil5 , (1) Acibadem Adana Hospital, Adana, ,(2) University of Hacettepe, Ankara, Beytepe, (3) University of Chicago, Chicago, IL, (4) Helmholtz Zentrum Munchen, Munich, Neuherberg, (5) University of Cukurova, Adana,


SU-I-GPD-T-356 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Accurate assessment of photon depth dose variation in tissue in the presence of metal implants is important for accurate treatment of prosthesis patients. Our goal is to study effects of high-Z metals on MV-photon dose distributions with Monte Carlo simulations and measurements for a better understanding of standard dose calculation algorithm predictions in clinical cases with metal implants.

Methods: The 6MV FFF beam from a TrueBeam machine was modeled using the Beamnrc code and validated against the measured data in water with DOSXYZnrc simulations. A 0.5 cm-thick steel and a 0.5 cm-thick aluminum heterogeneity slabs were separately introduced into a solid-water phantom at the depth of 5 cm from the phantom surface and MC simulations were run using the heterogeneity phantom with appropriate material properties. Dosimetric measurements were performed using a PTW pinpoint ion chamber and OSLD (InLight-Nanodots) chips to measure percentage depth doses for various field sizes: 2x2- 10x10. Normalized MC dose data in the heterogeneity slab phantom were compared with ion chamber and OSLD measurements as well as treatment planning calculations using the AAA and Acuros algorithms.

Results: Introduction of steel and aluminum slabs resulted in significant dose alteration at and beyond the metal-water heterogeneity interfaces. MC simulations agreed well within 1-2% of ion chamber measurements both before and after the metal heterogeneity. OSLD results were within 3% of MC results. Dramatic dose increase at the water-metal interface due to backscattered electrons and dose rebuild-up after the metal-water interface were well predicted with the Acuros XB algorithm in agreement with MC simulations.

Conclusion: Acuros algorithm, accounting for radiation transport, has been shown to predict dose variations in challenging situations when metal implants are used. When MC or sophisticated algorithms like Acuros are not available, OSLDs can be used to validate dose under test conditions within measurement uncertainties.

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