(Saturday, 4/4/2020) [Mountain Time (GMT-6)]
Purpose: To assess the accuracy of dose calculation algorithms, Eclipse Analytic Anisotropic Algorithm (AAA) and Acuros XB (AXB) in 6 MV beams commonly used in head and neck radiotherapy in the vicinity of dental materials used in current dental practice
Methods: Five samples of thickness 2 mm, representative of dental implants; Titanium (Acnis), Cobalt-Chromium (MESA), Zirconium (Nacera), Pekk (Juvora) and Pekkton (Cendres+Metaus), were studied. The centers of samples were placed at isocenter, imbedded in 15x25x25 cm block of solid water at a depth of 51 mm to top surface. Dose profiles were measured using Gafchromic EBT3 film placed in contact with the samples, 1mm upstream, immediately above and below, 1, 4 and 9mm downstream normal to beam direction. Samples were irradiated with 6MV beam from a Truebeam linear accelerator. Radiation (450 MU) was delivered with a field size of 10×10 cm. The setup was modeled in Eclipse. AAA and AXB algorithms were used to calculate dose distributions at corresponding depths. Film measurements were verified using a thin (27 µm) window ionization chamber
Results: For metallic samples, dose was increased immediately upstream by 23-39% due to electron backscatter, and to downstream sides by 4-5%. Immediately downstream of metallic samples dose was reduced by 15%. For non-metallic samples, dose perturbation was negligible. The increase in dose due to backscatter was underestimated by AAA and AXB by 20-37%. The downstream dose reduction was underestimated by 4-7% and 15-17% by AAA and AXB respectively. The dose discrepancies decreased with increasing distance from sample, being within 9% for AAA and 2% for AXB with 1 mm upstream and 9 mm downstream.
Conclusion: Metallic dental restorations cause significant dose perturbations compared to nonmetallic polymer type implants. Both AAA and AXB algorithms were in significant error in calculating dose both immediately upstream and downstream of metallic implants.
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