Importance of MR technique for stereotactic radiosurgery.

We investigated how frequently the imaging procedure we use immediately prior to radiosurgery--triple-dose gadolinium-enhanced MR performed with the patient immobilized in a nonrelocatable head frame and 1-mm-thick MPRAGE (magnetization-prepared rapid gradient echo) images (SRS3xGado)-identifies previously unrecognized cerebral metastases in patients initially imaged by conventional MR with single-dose gadolinium (1xGado). Between July 1998 and July 2000, the diagnoses established for 47 patients who underwent radio-surgical procedures for treatment of cerebral metastases at The Gamma Knife Center of New York University were based initially on the 1xGado protocol. In July 1998, we began using SRS3xGado as our routine imaging protocol in preparation for targeting lesions for radio-surgery, using triple-dose gadolinium and acquisition of contiguous 1-mm Tl-weighted axial images. Because our SRS3xGado scans sometimes unexpectedly revealed additional metastases, we sought to learn how frequently the initial 1xGado scans would underestimate the number of metastases. We therefore reviewed the number of brain metastases identified on the SRS3xGado studies and compared the results to the number found by the 1xGado protocol, which had initially identified the brain metastases. Additional metastases, ranging from 1 to 23 lesions per patient, were identified on the SRS3xGado scan in 23 of 47 patients (49%). In 57% of the 23 patients, only one additional lesion was identified. The mean time interval between the 1xGado and the SRS3xGado scans was 20.6 days (range, 4-83 days), and the number of additional lesions detected and the time interval between two scans were negatively correlated (-0.11). The number of lesions detected on the SRS3xGado was associated only with the number of lesions on the 1xGado and not with any other patient or tumor pretreatment characteristics such as age, gender, largest tumor volume on the 1xGado, or number of days between the 1xGado and the SRS3xGado or prior surgery. The identification of additional lesions with SRS3xGado MR may have implications for patients who are treated with stereotactic radiosurgery alone (without whole-brain irradiation) with single-dose gadolinium imaging, in that unidentified lesions may go untreated. As a result of these findings we continue to use and advocate SRS3xGado scans for radiosurgery.