Extent of perilesional edema differentiates radionecrosis from tumor recurrence following stereotactic radiosurgery for brain metastases

Differentiation of tumor recurrence from radionecrosis is a critical step in the follow-up management of patients treated with stereotactic radiosurgery (SRS) for brain metastases. A method that can reliably differentiate tumor recurrence from radiation necrosis using standard MR sequences would be of significant value. We analyzed the records of 49 patients with 52 brain metastases treated with SRS who subsequently underwent surgical resection of the same lesion. Forty-seven of the lesions had preoperative MRI available for review (90), including T1 postcontrast, T2, and fluid attenuated inversion recovery sequences. Pre-SRS and preoperative lesion and edema volumes were manually contoured and measured in a blinded fashion using radiation treatment planning software. A neuropathologist analyzed samples for the presence of tumor and/or radiation necrosis. Longer time between SRS and resection (P .001) and a larger edema/lesion volume ratio (high T2/T1c, P .002) were found to be predictive of radionecrosis as opposed to tumor recurrence. Using a cutoff value of 10 for the edema/lesion volume ratio, we were able to predict the presence of tumor with a positive predictive value of 92, which increased to 100 when looking only at patients who underwent resection 18 months following SRS. On follow-up imaging, lesions with a high edema/lesion volume ratio and lesions that progress later after SRS are more likely to contain radionecrosis. These indices may help guide clinical decision making in the context of evolving lesions after SRS for brain metastases and thereby avoid unnecessary interventions.