TH‐C‐T‐617‐04: Measurements and Monte Carlo Simulations of Dose Perturbations Due to Metallic Implants in Proton Radiotherapy

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Purpose: To quantify the dose perturbation behind implanted metallic appliances and fiducials in proton radiotherapy. Method and Materials: Dose perturbations from stainless steel spheres of 6.4–15.9 mm diameter were investigated in proton therapy beams. Passive lateral spreading was combined with collimation to produce 17.8‐cm diameter fields. Penetration ranges were varied between 40 and 160 mm. Dose profiles were measured in planes perpendicular to the proton beam axis, at distances of 0–150 mm behind the implants, using radiographic film exposed to 0.4–0.5 mGy. The experiments were modeled in detail with Monte Carlo simulations that included hadron and electron transport physics. 120×106 proton histories were simulated, corresponding to 8% or less statistical uncertainty. Results: Edge effects are most prominent at the highest proton beam energies and produce concentric perturbations of +/−10–40% for the implants considered here. Thick implants (w.r.t. the beam range) may stop the protons, leaving a zero dose shadow behind it. For thin and intermediate implants, the range loss in the implant produces a cone of dose enhancement of up to 20% directly behind the implant due to in increase in the stopping power values. In an air cavity, shadows reduce the dose by up to 60% at 15 cm distance downstream from the implant. The Monte Carlo simulations agree with the measurements to better than 8% in all cases and typically to within 4%. Additional simulations will be presented for implants of gold and tantalum, ranging in size from 0.8 to 3 mm. Conclusion: We have identified several representative cases in which implants produce significant dose perturbations. In such cases, a treatment plan should include suitable simulations or measurements to ensure that the implants do not result in cold spots in the tumor or hot spots in normal tissue. © 2005, American Association of Physicists in Medicine. All rights reserved.

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Medical Physics

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