The effects of out-of-plane rotations on two dimensional portal image registration in conformal radiotherapy of the prostate.

PURPOSE

Rotations of the patient out of the image plane can significantly degrade the accuracy of two-dimensional (2D) image registration. This study determines the magnitude of the geometric errors introduced by 2D image registration as a result of out-of-plane rotations, and analyzes the dosimetric effects of these errors.

METHODS AND MATERIALS

The magnitude of the errors introduced by 2D registration were determined by comparing orthogonal view portal images of a rotated phantom to simulator reference images of the same phantom without rotation. Dosimetric effects were calculated for three-dimensional (3D) conformal prostate treatments by applying the registration errors to patient treatment plans. The calculations were performed using a modified version of the dose calculation software used in our Cancer Center for 3D treatment planning based on computed tomography (CT). A method to detect out-of-plane rotations, specific to pelvic treatments, is introduced that uses the relative displacement of the centers of gravity of the acetabula in lateral images.

RESULTS

The inherent uncertainty in the registration algorithm was 0.6 +/- 0.5 mm in translation and 0.7 +/- 0.8 degree in rotation within the image plane. For a 2 degrees out-of-plane rotation, the errors increase to 2.3 +/- 1.0 mm and 1.2 +/- 1.1 degrees. In some clinically realizable treatment scenarios it was observed that the errors introduced by the registration procedure could result in an overdosing of the rectal wall. The method to detect out-of-plane rotations was found to have an accuracy of better than 1 degree for rotations of less than 10 degrees.

CONCLUSIONS

The errors introduced to the patient position by 2D image registration have dosimetrically significant consequences for out-of-plane rotations of 2 degrees or more. However, when used in conjunction with the method to detect out-of-plane rotations, 2D registration software was found to cause insignificant dose errors and, thus, become a more reliable and accurate clinical tool.