Detection of Occult Brain Metastasis with High Resolution MRI Using Double Vs. Single Dose Gadolinium

To characterize the detection of additional brain metastases with high resolution magnetic resonance imaging (MRI) at the time of stereotactic radiosurgery (SRS) using single dose and double dose gadolinium contrast. A single institution retrospective review of 134 consecutive radiosurgical procedures over 2 years in patients with ≤5 brain metastases diagnosed using MRI was performed. On the day of SRS all patients underwent axial MRI on a 1.5 Tesla magnet using a 3 dimensional spoiled gradient recalled acquisition sequence using 2mm cuts with no gap. Contrast was 20ml of 0.5mol/L gadopentetate dimeglumine in the first 68 consecutive patients and 40ml of 0.5 mol/L in the subsequent 66 consecutive patients. An increased number of metastases were seen at the time of SRS compared to the original diagnostic scan (mean 2.84, s.e.m. 0.24 vs. mean 1.92, s.e.m. 0.11 (p < 0.001)). Additional metastases were detected in 36.8% of all cases, in 25.0% of cases with a single metastasis on diagnostic scan, and 49.2% of cases with 2-5 metastases on diagnostic scan. The number of known tumors was predictive of finding additional tumors at the time of SRS (OR=1.5 [95% CI, 1.2-2.1], p = 0.003). This remained significant on multivariate analysis. Sex, age, prior fractionated radiation therapy, control of extra-cranial disease, and time between diagnostic and treatment scans (median 23 days) were not significant. Detection of new tumors with double dose vs. single dose gadolinium (40.9% vs.30.9% , respectively) did not reach significance (p = 0.22). In subgroup analysis, gadolinium dose was significant in detecting new tumors in melanoma patients (p = 0.028), who were also less likely to have undergone prior fractionated radiation compared to other histologies 11.1% vs. 52.6% (p = 0.001). High resolution, contrast enhanced MRI at the time of SRS detects a higher number of metastases than the original diagnostic MRI. The chance of detecting new metastases on the high resolution scan increases with an increasing number of index metastases. For each additional index metastasis, there was a 1.5x greater likelihood of finding new metastases on the high resolution scan. Effective detection of metastases facilitates thorough treatment with SRS.