Volume specific response criteria for brain metastases following salvage stereotactic radiosurgery and associated predictors of response

Background. We aimed to derive three-dimensional volume-based (V-3D) response criteria that approximate those based on Response Evaluation Criteria in Solid Tumours (RECIST) in patients with brain metastases (BM) treated with salvage stereotactic radiosurgery (SRS). Material and methods. Seventy patients with 178 BM were treated with SRS. Each BM was characterised at baseline and at each follow-up MRI according to its widest diameter and V3D using ITK-SNAP image segmentation software. Results. The median tumour diameter was 1.2 cm (range, 0.2-4.5 cm) and V-3D was 0.73 cm(3) (range, 0.01-22.7 cm(3)). The V-3D percent changes that best matched RECIST response criteria were: an increase of >= 71.5% for progressive disease, a >= 58.5% decrease for partial response and a < 58.5% decrease or increase of < 71.5% for stable disease (k = 0.85). A baseline diameter > 3.0 cm (p = 0.006) and a V-3D > 6.0 cm(3) (p = 0.043) predicted for local failure, and a baseline cumulative V-3D of > 3.0 cm(3) (p = 0.02) was adversely prognostic for survival. Conclusions. We define 3D volume specific criteria to base response upon for brain metastases treated with salvage SRS. Tumours with a V-3D of greater than 6 cm(3) are at a higher risk of local failure.