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  4. The Dosimetric Impact of Uncertainties in HU Assignment of Spinal Implants for Photon and Proton RT: Carbon Vs. Titanium Screw Systems

The Dosimetric Impact of Uncertainties in HU Assignment of Spinal Implants for Photon and Proton RT: Carbon Vs. Titanium Screw Systems

BS Müller1*, YM Ryang2, M Oechsner1, M Düsberg1, B Meyer2, SE Combs1,3, JJ Wilkens1, (1) Department of Radiation Oncology, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany, (2) Department of Neurosurgery, Klinikum rechts der Isar, Technical University of Munich, Munich, Germany, (3) Institute of Innovative Radiotherapy, Helmholtz Zentrum München, Neuherberg, Germany

Presentations

(Sunday, 7/29/2018) 3:00 PM - 6:00 PM

Room: Exhibit Hall

Purpose: Over the last years carbon (C) pedicle screw systems were introduced for spinal stabilization, promising improved accuracy of radiotherapy (RT) treatment planning compared to titanium (Ti) screws. We simulated uncertainties in assigned CT-values of both implant types for intensity modulated proton therapy (IMPT) and volumetric arc photon therapy (VMAT), and evaluated the dosimetric impact.

Methods: Retrospective planning was performed on CT data of 10 patients, five with titanium, five with carbon systems, who had previously undergone spinal fixation surgery and postoperative RT. Carbon systems (Icotec, Altstätten, Switzerland) comprised non-metallic screws, a titanium coating in the pedicle area and titanium rods. For both screw systems metallic components were contoured and assigned varying stopping powers (SP) (protons) and electron densities (Ï?) (photons). One initial IMPT and VMAT plan was calculated with a nominal SP or Ï? (dose to planning target volume (PTV): 10x3 Gy), and recalculated with increased and decreased SP or Ï? by ±6%.

Results: CT-artifact and metal volumes were smaller for carbon systems. VMAT dose distributions were hardly affected by variations of Ï?, i.e. deviations of evaluated dose criteria were less than 1%. Recalculated IMPT plans resulted in more heterogeneous target coverage, represented by an increasing standard deviation in the PTV by +7.6±2.3% (Ti) and +3.4±1.2% (C). Larger SP values lead to lower target minimum doses, decreased SPs to higher dose maxima. Dosimetric consequences were greater for titanium systems than for carbon.

Conclusion: While for VMAT plans the dosimetric impact of HU variations was negligible for both screw types, IMPT plans demonstrated the dosimetric benefit of carbon screws through a lowered risk of PTV underdosage compared to titanium. Generally, reducing metal components in implants will improve RT planning - not only in terms of dose calculation accuracy but also through a more accurate delineation process due to less artifacts.

Funding Support, Disclosures, and Conflict of Interest: Y. Ryang and B. Meyer are consultants for Icotec.

Keywords

Radiation Therapy, Protons, Image Artifacts

Taxonomy

TH- External Beam- Particle therapy: Proton therapy - treatment planning/virtual clinical studies

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