Introduction: Stereotactic radiosurgery (SRS) is one of the mainstays of treatment for patients with brain metastases (BM). Radiographic response to SRS has been reported to be between 85-95% based upon measurement of maximum tumor diameter, but studies evaluating treated lesions with use of volumetric measurements have been lacking. We used volumetric measurements to evaluate the radiographic response of BMs after SRS, and evaluated a series of prognostic factors for response.
Methods: A group of patients with 5 or more BM that were being evaluated for another study were used for this analysis. 116 patients (763 BMs total) were treated, using the RTOG 9805 dosing protocol, between 1999 and 2010 with SRS at the Cleveland Clinic. Volumetric measurements were made on day of treatment and first follow-up MRIs with use of BrainLAB IPlan software. Volume changes greater than 20% were used as the threshold for tumor change. Statisical evaluations included univariate and multivariate analyses.
Results: Lung cancer was the most common pathology (44%,332/763) and 145 lesions (19%) were radio-resistant tumors (RCC, melanoma). Median tumor volume was 0.14cc (range: 0.01-36.1). Overall, 80%(615/763) of the lesions shrank after SRS, 14%(103/763) remained stable, and 6%(45/763) enlarged. Notably, 33%(251/763) of lesions completely resolved after SRS. Radio-resistant lesions responded less well than non-radio-resistant tumors (88% versus 95%; p=0.001). In addition, resolving tumors tended to be smaller (median volume: 0.05 cc versus 0.28 cc, p<0.0001), non-radio-resistant (37% versus 14%; p<0.0001), and had less conformal treatment plans (median conformality ratio: 2.15 versus 1.92; p<0.0001).
Conclusions: When measured volumetrically, BMs treated with SRS had a favorable response rate (shrinkage + stable) of 94% on initial post-SRS MRI. Primary tumor pathology was the most important factor in radiographic response of BM to SRS. In addition, smaller tumors, non-radio-resistant pathology, and those treated with less conformal plans had a greater chance to resolve completely.
Patient Care: It would be helpful by determining more precise responses to treatment after SRS in brain metastases and defining associated predicive factors
Learning Objectives: Precise volumetric measurement results to determine tumor volume changes after SRS and prognostic factors contributed to it.