Evaluation of clinical and volumetric outcomes following adaptive gamma knife radiosurgery for brain metastases

Background: Hypofractionated stereotactic radiosurgery is an effective technique to treat larger brain metastases and post-surgical cavities. Adaptive Gamma Knife radiosurgery (aGKRS), involving repeat magnetic resonance imaging (MRI) and replanning based on modified lesion contours, has emerged as a solution to account for inter-fraction tumor dynamics. Objective: To evaluate the impact of adaptive planning on treatment metrics, radiation dose to structures at risk (SARs), and clinical and radiologic outcomes in patients with brain metastases. Methods: Over an 8-year period (2016–2023), 31 patients were treated with aGKRS to 48 brain metastases. Lesions were re-contoured and adaptive plans created using updated MRIs acquired prior to the second radiation fraction. Treatment metrics, including target coverage, Paddick Conformity Index (PCI), Gradient Index (GI), and doses to SARs, were analyzed. Clinical outcomes and radiologic response were assessed. Results: Lesion volumes changed significantly between radiation fractions (mean change– 28.25%). aGKRS resulted in significant improvements in target coverage, from 91.9% on non-adaptive plans to 97.02% following adaptive replanning. The PCI improved by 35.3% across plans in this series. Adaptive plans also reduced the dose to SARs by an average of 4.7%. Radiologic response was excellent– median volume reduction of treated lesions was 98.9%. Clinically, 72% of patients either remained stable or improved in neurologic status; 32% improved in performance status by at least one grade. Conclusion: Lesion dynamics during hypofractionated treatments pose a significant challenge to the delivery of conformal and safe SRS treatments. aGKRS significantly improves dosimetric parameters, reduces radiation to SARs, and enhances clinical and radiologic outcomes for brain metastases.