Novel correction methods as alternatives for the six-dimensional correction in CyberKnife treatment

Abstract

Purpose

During CyberKnife treatment, the 6D correction method is used to correct patient positional errors, including rotational ones. We developed novel correction methods for translating rotational errors into 3D, with the aim of making their correction safer than with 6D correction and as accurate as possible.

Materials and methods

These novel correction methods were named the gravity correction and the beam correction method. With the gravity correction method, the beam coordinates after rotation are corrected to match the tumor gravity point with 3D translational components translated by the affine transformation matrix. For beam correction, the beam coordinates are corrected to match the translated tumor target coordinates for each treatment beam. The effectiveness and impact of these methods were demonstrated by means of dose volume histogram (DVH) shift evaluation. For analysis of the treatment data of 10 patients, the treatment beam was rotated in three patterns of rotational degree and corrected with the two methods. The amount of tumor gravity point shift in the rotation was also calculated, and the deterioration of the tumor DVH was studied.

Results

In the case of ±1°, ±3°, and ±1° rotation for the X, Y, Z axes, the tumor gravity point of all 10 patients moved around 2.4 mm on average. Tumor DVH was deteriorated worse as the distance between the tumor gravity point and the rotational origin became more distant. With the planned D90, which represents the dose above which 90% of the tumor volume is irradiated set at 100%, the postrotational average D90 dose deteriorated to 96.12% after (±1°, ±3°, and ±1°) rotation. The dose was improved to 99.9% (SD ± 0.41) after the gravity correction, or to 99.87% (SD ± 0.55) after the beam correction.

Conclusion

The correction methods developed by us can correct tumor DVH findings to the same degree as with 6D correction and are safer because the movement required for correcting the linac is not rotational but translational only.