Assessment of patient-independent intrinsic error for a noninvasive frame for fractionated stereotactic radiotherapy

The purpose of our study was to examine the extent of patient-independent intrinsic error associated with multiple, repeat remounting of the Laitinen Stereoadapter. The Laitinen frame was repeatedly mounted on a solid water phantom and imaged using computed tomography (CT). The phantom contained five targets located in the center, anterior, right, left, and posterior orientations. The images were processed, fused, and analyzed on the Pinnacle 3-D treatment planning system. The coordinate values (in the x, y, and z directions) for each target were determined for each mounting, and an absolute mean deviation was calculated for 11 repetitions. The mean deviation in the x, y, and z direction for the central and right target, and in the x and y direction for the posterior and anterior target was less than 2.0 mm. However, the mean error in the z direction of the anterior and posterior targets was 1.79 +/- 1.02 mm and 2.20 +/- 1.32 mm, respectively. Rotational misalignment during repeat frame fixation contributed to the observed deviations and in particular affected the antero-posterior plane. With the exception of two occasions where an obvious mounting error occurred, a significant portion of error from remounting the Laitinen Stereoadapter is associated with the operator and the imaging process. The observation of an angular displacement around the axis through the earplugs suggests that a certain degree of rotational misalignment in daily remounting is possible. Targets in the antero-posterior plane are most susceptible to localization error as a consequence of rotational misalignment. In summary, the overall error is within the limits of current imaging technology but not within submillimeter accuracy. Clinical application should take these errors into consideration when designing field margins.