POTENTIAL HUMAN ERROR IN SETTING STEREOTAXIC COORDINATES FOR RADIOSURGERY - IMPLICATIONS FOR QUALITY ASSURANCE

Purpose: The error frequency in setting stereotactic coordinates for g amma knife radiosurgery was investigated to determine what quality ass urance safeguards are necessary. Methods and Materials: A prospective study of 200 consecutive isocenter settings for gamma knife radiosurge ry was analyzed to identify the frequency of spontaneous errors in set ting and checking stereotactic coordinates (corrected prior to treatme nt). An additional 25 coordinate errors were introduced at random amon g the next 200 consecutive isocenter settings to provide additional da ta on identification of errors. Results: Stereotactic coordinates requ ired 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 w as 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 err ors, 94.6% of errors greater-than-or-equal-to 0.5 mm, p = 0.0000). Ver ification of stereotactic coordinates by two observers reduces the pro bability of an undetected error greater-than-or-equal-to 0.25 mm to 1/ 1,392 and to 1/154,712 for errors greater-than-or-equal-to 1 mm. Concl usion: Errors in setting stereotactic coordinates are common (12% prio r to checking) but are corrected with a high degree of confidence by a quality assurance policy requiring coordinate verification by a minim um of two observers.