Although clinical decisions regarding the use of stereotactic radiosurgery (SRS) are frequently based on the number of metastases identified, there is no consensus methodology for determining the number of metastatic lesions.

     This study presents a retrospective analysis of expected versus identified number of metastases in patients treated with GammaKnife using a uniform imaging technique.

     100 patients high-resolution imaging studies were evaluated.

     A retrospective, IRB approved review of 100 consecutive newly diagnosed patients treated with stereotactic radiosurgey for brain metastases using GammaKnife (GK) Perfexion from October 2009 to May 2011 was performed. Non small cell lung cancers comprised 40% of patients followed by breast, renal cell and melanoma ; small cell cancers were excluded.

      All GK all patients underwent rigid head fixation followed by 3-dimensional stereotactic magnetic resonance imaging (MRI) using double-dose gadolinium contrast with fast spoiled-gradient sequence (SPGR) performed using 2-mm thick axial cuts with no spacing.

     This was a retrospective review.

     For patients with the expectation of single metastases (50% of all patients), 25.5% were found to have one or more additional metastases. For patients with 2 – 3 expected metastasis, 48.6% had additional lesions, and among patients with more than 3 metastases expected, 93% had additional lesions at the time of GK imaging. Overall additional metastases were detected in 43% of cases. 

     The probability of finding additional metastases was highly correlated with the number on the baseline exam (p< 0.0001 for 1 vs. 2-3 or 1 vs. >3 lesions).