TITLE:
Dosimetric Effect of Cobalt-Chrome Stabilization Hardware in Paraspinal Radiation Treatments
AUTHORS:
Grace Tang, Thomas LoSasso, Seng-Boh Lim, Yoshiya Yamada, Mark Bilsky, Dale Michael Lovelock
KEYWORDS:
Cobalt-Chrome, Spinal Hardware, Paraspinal Radiation Therapy
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.11 No.3,
August
31,
2022
ABSTRACT: High
density materials are assigned with an apparent density of 3.2 g/cm3 in 12-bit CT images due to saturation. This is often ignored in planning for
spine tumors with titanium (density 4.40 g/cm3) spinal hardware.
However, new cobalt-chrome hardware has a density of 8.11 g/cm3,
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 cm2 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.