Dosimetric Effect of Cobalt-Chrome Stabilization Hardware in Paraspinal Radiation Treatments

Abstract

High density materials are assigned with an apparent density of 3.2 g/cm 3 in 12-bit CT images due to saturation. This is often ignored in planning for spine tumors with titanium (density 4.40 g/cm 3 ) spinal hardware. However, new cobalt-chrome hardware has a density of 8.11 g/cm 3 , which would increase dosimetric uncertainty if the true density is not utilized in planning. This effect was evaluated in this study. Calculation accuracy was examined using MapCHECK2 with a single 20 × 10 cm 2 field with a titanium and a cobalt-chrome rod in a solid water phantom for 6X, 6FFF and 15X, at 2 cm and 6 cm beneath the rods. Measurement was compared to the calculation with density override (DO) with the true density and to the calculation with no-density override (NDO). Additionally, the dosimetric effect in clinical treatment plans was investigated for six IMRT and VMAT paraspinal cases. Plan quality was compared with the original NDO calculation and the DO recalculation. Compared to measurements, the treatment planning system (TPS) overestimated the dose locally by up to 13.2% for cobalt-chrome and 4.8% for titanium with NDO calculations. DO calculations improved the differences to 8.4% and 4.0%, respectively. Scatter from the rod increased the lateral dose and diminished as depth increased but was not properly accounted for by the TPS even with the correct density assigned. For the clinical plans, PTV coverage was lowered by an average of ~1.0% (range: 0.5% -2.0%) and ~0.3% (range: 0.2% -0.7%) in DO recalculations for cobalt-chrome and titanium, respectively. In conclusion, neglecting the true density of cobalt-chrome hardware during planning may result in an unexpected decrease in target coverage.

The dosimetric impact of titanium implants in spinal SBRT using four commercial treatment planning algorithms

“…More TPSs are supporting 16 bits CT, which will include the corresponding HU number for titanium alloy. This study and other studies 7 , 8 , 13 showed the common practice of not overriding titanium alloy during spine SBRT planning was clinically acceptable. However, if the implant was cobalt‐chrome, Tang et al.…”

“…This study and other studies 7,8,13 showed the common practice of not overriding titanium alloy during spine SBRT planning was clinically acceptable. However, if the implant was cobalt‐chrome, Tang et al 8 . showed that manually density override was recommended even with 16 bits CT images and dose difference with and without DO was larger than those observed in titanium alloy.…”

“…They found that plans without DO overestimated the dose by a small amount (<1%) affecting 4% of the target volume for seven cases, which was believed to be minimal. In the study of six IMRT and VMAT paraspinal cases by Tang et al, 8 the PTV coverage was lowered by an average of ∼0.3% in DO for titanium compared with plans calculated without DO. Müller et al 14 reported that uncertainties in HU assignment of titanium alloy for spine patients showed no relevant difference in dosimetric quality for VMAT plans.…”

“…[3][4][5][6] Using Pinnacle dose calculation algorithm for a SBRT plan using step-shoot intensity-modulated radiation therapy (IMRT), Wang et al 7 found a ∼0.5 Gy dose decrease for seven cases with prescription dose of 24 Gy when manually overriding the physical density with the correct number. Using the Analytical Anisotropic Algorithm (AAA) algorithm from Eclipse planning system for one step-shoot IMRT and five volumetric modulated arc therapy (VMAT) plans, Tang et al 8 found the differences in V95% to the PTV between density override (DO) and no-DO calculations were within 1% for both titanium and cobalt-chrome implants, while the differences to maximum dose in the spinal cord were 0.02-0.18 Gy for titanium implants and 0.06-0.25 Gy for cobaltchrome implants. In the present work, we selected 20 patients with spinal prostheses treated with VMAT SBRT to study dosimetric effect with and without DO.…”

“…Using the Analytical Anisotropic Algorithm (AAA) algorithm from Eclipse planning system for one step‐shoot IMRT and five volumetric modulated arc therapy (VMAT) plans, Tang et al. 8 found the differences in V95% to the PTV between density override (DO) and no‐DO calculations were within 1% for both titanium and cobalt‐chrome implants, while the differences to maximum dose in the spinal cord were 0.02–0.18 Gy for titanium implants and 0.06–0.25 Gy for cobalt‐chrome implants. In the present work, we selected 20 patients with spinal prostheses treated with VMAT SBRT to study dosimetric effect with and without DO.…”