Potential human error in setting stereotactic coordinates for radiosurgery: implications for quality assurance.

PURPOSE The error frequency in setting stereotactic coordinates for gamma knife radiosurgery was investigated to determine what quality assurance safeguards are necessary. METHODS AND MATERIALS A prospective study of 200 consecutive isocenter settings for gamma knife radiosurgery was analyzed to identify the frequency of spontaneous errors in setting and checking stereotactic coordinates (corrected prior to treatment). An additional 25 coordinate errors were introduced at random among the next 200 consecutive isocenter settings to provide additional data on identification of errors. RESULTS Stereotactic coordinates required resetting in 12% (24/200) of the isocenters treated due to errors of 0.25-0.50 mm (8%) and 1-20 mm (4%). This comprised 2.2% (26/1200) of the individual coordinate settings. The frequency of these errors was significantly related to the specific directional coordinate set (p = 0.0004) and experience (p = 0.016). Errors were identified by 83.5% (91/109) of the observers checking the settings (60.0% of 0.25 mm errors, 94.6% of errors > or = 0.5 mm, p = 0.0000). Verification of stereotactic coordinates by two observers reduces the probability of an undetected error > or = 0.25 mm to 1/1,392 and to 1/154,712 for errors > or = 1 mm. CONCLUSION Errors in setting stereotactic coordinates are common (12% prior to checking) but are corrected with a high degree of confidence by a quality assurance policy requiring coordinate verification by a minimum of two observers.