Fig. 1

Axial post-contrast T1WI in a patient with an esophageal carcinoma metastasis (a, dotted circle) to the left cingulate gyrus, treated with 20 Gy single-fraction SRS. The lesion enlarged after SRS, with extension to the adjacent corpus callosum. On the 12-month post-SRS MRI (b), a small focus of enhancement developed in the right cingulate gyrus (arrow), discontinuous with the dominant lesion, which allowed a more confident diagnosis of RN. Note the “open ring” enhancement to the left of the falx cerebri; in contrast, if this was contiguous metastatic disease, abnormal enhancement of the falx cerebri would be expected. RN was supported by subsequent follow-up imaging, which demonstrated a change in the shape of the overall abnormality, with some regression of the dominant left-sided component, but further mild enlargement on the right. The appearances stabilized thereafter, and remain stable at most recent follow-up more than 12 months later. The patient also had a right frontal metastasis (b, arrowhead) treated with SRS, but this developed appearances suggestive of PD, prompting resection (which confirmed PD)