Exhibit Hall | Forum 4
Purpose: Radiotherapy plans for patients with stabilizing spinal implants have increased dosimetric uncertainty, which could result in diminished treatment efficacy. This study aims to quantify the dosimetric impact of three types of spinal implants, and assess differences in dose as calculated with Monte Carlo(MC) versus collapsed cone(CC) algorithms.
Methods: A clinically commissioned MC treatment planning system(TPS) (RayStation,RaySearch Laboratories,Stockholm) was used to generate a 3D and VMAT treatment plan for a unique anthropomorphic thorax phantom(CIRS,Norfolk,VA). The phantom had four interchangeable vertebral sections: bone(B), titanium screws(TS), carbon-fiber screws with metal heads(CFM), and carbon-fiber screws with carbon-fiber heads(CF). Plans were optimized on the B phantom CT scan, then recalculated on the CT scan of the phantom with each hardware insert using 0.2% uncertainty. The same plans were also calculated using a collapsed cone(CC) algorithm commissioned in the same TPS. The changes in coverage(D95%) for a representative planning target volume(PTV) with hardware excluded was evaluated between the bone versus hardware inserts. Additionally, D95%PTV for MC vs. CC, was assessed.
Results: The MC 3D plan D95%PTV dropped by 0.4%, 6.1%, and 12.0% between bone and CF, CFM, and TS hardware, respectively; scaling monitor units for D95%PTV coverage in the TS plan resulted in large >120% hot spots in the spinal canal (133% point max). The same comparisons for the VMAT plan yielded 0.3%, 1.7%, and 4.25% drops in coverage, respectively. A 1.3% increase in the D95PTV was seen between the MC and CC for the TS 3D plan, suggesting near equivalence in native patient anatomy.
Conclusion: Selection of CF or CFM spinal implant hardware for patients expected to receive localized radiotherapy minimizes dosimetric implications using 3D planning techniques. VMAT increases plan robustness in the presence of TS hardware. CC performed comparably to MC when the dose calculation inside the hardware was excluded.