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Dosimetric Impact of Cobalt-Chrome Stabilization Hardware in Paraspinal Radiation Therapy

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G Tang

G Tang1*, T LoSasso1 , Z Saleh1 , J Mechalakos1 , S Lim1 , I Laufer2 , M Bilsky2 , J Yamada3 , D Lovelock1 , (1) Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY, (2) Department of Neurosurgery, Memorial Sloan Kettering Cancer Center, New York, NY, (3) Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY


SU-E-T-328 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose:Due to saturation, high density materials result in an apparent density of 3.2 g/cm³ in CT images. The true density of traditional titanium stabilization rods (~4.4 g/cm³) is typically ignored in treatment planning. This may not be acceptable for new cobalt-chrome rods with a density of 8.5 g/cm³. This study reports the dosimetric impact of cobalt-chrome rods in paraspinal radiotherapy.

Methods:For titanium and cobalt-chrome rods, two planning studies were done for both IMRT and VMAT in Varian Eclipse using AAA. 1) The effect of planning without assigning the true rod density was assessed by comparing plans generated with the apparent density and recalculated with the true density for titanium and cobalt-chrome. 2) To test if TPS can compensate for high density rods during optimization. Furthermore, TPS calculation accuracy was verified using MapCheck for a single 20 x 10 cm² field. The MapCheck was incrementally shifted to achieve measurement resolution of 1 mm.

Results:PTV coverage was ~0.3% and ~4.7% lower in plans that were recalculated with the true rod density of titanium and cobalt-chrome, respectively. PTV coverage can be maintained if the correct density is used in optimization. Measurements showed that TPS overestimated the dose locally by up to 11% for cobalt-chrome rods and up to 4% for titanium rods if the density is incorrect. With density corrected, maximum local differences of 6% and 3% were seen for cobalt-chrome and titanium rods, respectively. At 2 cm beneath a rod, electrons scattered from the side of the rod increased the lateral dose and diminished as depth increases. TPS was not able to account for this effect properly even with the true rod density assigned.

Conclusion:Neglecting the true density of cobalt-chrome rods can cause under coverage to the PTV. Assigning the correct density during treatment planning can minimize unexpected decrease in PTV dose.

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