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Override of High-Density Implants and Validation of Their Dose Perturbation in Pencil Beam Scanning Proton Therapy

W Zheng*, X Li, J Vadas, D Yan, D Moore, K Les, C Stevens, P Kabolizadeh, X Ding, William Beaumont Hospital, Royal Oak, MI

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: Significant overestimation of the proton dose beyond the high-density implants could happen if material override is not appropriately handled. Herein, we proposed a workflow to override the material for high-density implants and validate their perturbation in PBS.


Methods: Several materials including a hip implant system (ceremic, Ti6Al4V) ( DePuy Ceramax™), a cement bone (Surgical Simplex® P), a PEEK spine spacer (polyetheretherketone) (ACIS®) and a Vitallium rod in a spine system (ES2®, Stryker) were investigated in this study. The relative stopping power (RSP to water) of the implants was calculated using software package SRIM (www.srim.org) with stoichiometry and factional mass density of the elements. Dose was then calculated with material override using Monte Carlo simulation and compared with the film measurements. Method I overrides the material based on water, setting effective mass density equal to RSP. Method II is to create a new override material with similar mean nuclear scattering cross-section, and mean excitation energy and an effective mass density matched to the calculated RSP interpolated in the TPS pre-calculated materials.


Results: The material override with nuclear scattering cross-section and RSP matching resulted in negligible discrepancy between the calculated dose and film measurements. In contrast, override simply based on RSP to water could lead to up to 18% higher calculated dose beyond the implant than the film measurements for Vitallium, 10% for ceramic, and 6% for Ti6Al4V implants. While for the materials dominantly made of organic elements, the dose discrepancy is negligible between two override methods.


Conclusion: We proposed a clinical workflow to override the high-density implants and validate their dose perturbation for the materials that are not provided in the proton planning system. Such an approach could provide a more accurate dose calculation in the presence of high-density implants that may be associated with local recurrence (Staab 2011).

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