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Physics

Session: Physics Abstract Presentations | QUALITY ASSURANCE AND TREATMENT DELIVERY

Proton Stereotactic Body Radiotherapy for Spine Metastasis Dosimetry Evaluation with CFR-PEEK Phantom

Saturday, March 22, 2025
9:01 AM – 9:11 AM MST
Location: Sonoran I/II

Objectives: : The current published experience of proton stereotactic body radiotherapy (SBRT) for spine metastases is limited; though dosimetric analyses demonstrate improved target coverage and organ at risk sparing with this approach compared to photon SBRT. To better understand feasibility of proton SBRT for spine metastases, we performed film measurement of proton- and photon-based SBRT platforms with comparison.

Methods: : A phantom containing three interchangeable spinal inserts representing native spine and two different spinal stabilizations consisting of titanium only and carbon fiber reinforced polyetheretherketone (CFR-PEEK) and titanium hybrid was acquired. Two planning approaches of a target volume consisting of the entire vertebral segment circumference were compared: a proton multi-field optimized plan with spinal cord sparing and a robotic-based photon SBRT plan. Comparative plans were generated using 24 GyE in 2 fractions. Raystation 12A (Stockholm, Sweden) was used to generate and optimize the intensity modulated proton therapy (IMPT) plans with a 1.1 RBE constant and robust optimization parameters of ± 3.5% range and 2-mm setup uncertainties. Accuray Precision (Sunnyvale, CA) with Monte Carlo (2% uncertainty) was used to generate the robotic-based SBRT plans. Plans were measured using gafchromic film and analyzed using Matlab (Natick, MA). Gamma analysis using 2 mm/2% was completed.

Results: : Gamma passing rates for the native bone spinal insert using 2 mm/2% were 95.89% vs 97.17% for robotic SBRT vs IMPT SBRT, respectively. Gamma passing rates for the CFR-PEEK/titanium hybrid spinal insert using 2 mm/2% were 93.89% vs 91.91% for robotic SBRT vs IMPT SBRT, respectively. Gamma passing rates for the titanium only spinal insert using 2 mm/2% were 94.58% vs 79.73% for robotic SBRT vs IMPT SBRT, respectively. This prompted a further investigation of larger distance-to-agreement and higher maximum dose differences parameters for gamma passing rates for the titanium only spinal insert with IMPT SBRT. Gamma passing rates using 2mm/3%, 3mm/2%, 3mm/3% were 84.19%, 91.63%, and 93.45%, respectively.

Conclusion(s):: Based on the gamma passing rates, proton SBRT plans for native spine and those stabilized with CFR-PEEK/titanium hybrid hardware show numerically similar deliver uncertainty compared to those treated with photon SBRT. Proton SBRT plans of spine segments stabilized with titanium only showed the lowest gamma passing rates. Caution is advised when treating patients with titanium inserts using spine SBRT proton therapy.