Calculation of rotational setup error using the real-time tracking radiation therapy (RTRT) system and its application to the treatment of spinal schwannoma

Abstract

Purpose : The efficacy of a prototypic fluoroscopic real-time tracking radiation therapy (RTRT) system using three gold markers (2 mm in diameter) for estimating translational error, rotational setup error, and the dose to normal structures was tested in 5 patients with spinal schwannoma and a phantom. Methods and Materials : Translational error was calculated by comparing the actual position of the marker closest to the tumor to its planned position, and the rotational setup error was calculated using the three markers around the target. Theoretically, the actual coordinates can be adjusted to the planning coordinates by sequential rotation of γ degrees around the z axis, β degrees around the y axis, and α degrees around the x axis, in this order. We measured the accuracy of the rotational calculation using a phantom. Five patients with spinal schwannoma located at a minimum of 1–5 mm from the spinal cord were treated with RTRT. Three markers were inserted percutaneously into the paravertebral deep muscle in 3 patients and surgically into two consecutive vertebral bones in two other patients. Results : In the phantom study, the discrepancies between the actual and calculated rotational error were −0.1 ± 0.5°. The random error of rotation was 5.9, 4.6, and 3.1° for α, β, and γ, respectively. The systematic error was 7.1, 6.6, and 3.0° for α, β, and γ, respectively. The mean rotational setup error (0.2 ± 2.2, −1.3 ± 2.9, and −1.3 ± 1.7° for α, β, and γ, respectively) in 2 patients for whom surgical marker implantation was used was significantly smaller than that in 3 patients for whom percutaneous insertion was used (6.0 ± 8.2, 2.7 ± 5.9, and −2.1 ± 4.6° for α, β, and γ). Random translational setup error was significantly reduced by the RTRT setup ( p < 0.0001). Systematic setup error was significantly reduced by the RTRT setup only in patients who received surgical implantation of the marker ( p < 0.0001). The maximum dose to the spinal cord was estimated to be 40.6–50.3 Gy after consideration of the rotational setup error, vs. a planned maximum dose of 22.4–51.6 Gy. Conclusion : The RTRT system employing three internal fiducial markers is useful to reduce translational setup error and to estimate the dose to the normal structures in consideration of the rotational setup error. Surgical implantation of the marker to the vertebral bone was shown to be sufficiently rigid for the calculation of the rotational setup error. Fractionated radiotherapy for spinal schwannoma using the RTRT system may well be an alternative or supplement to surgical treatment.