How can a radiologist tell whether lesions on the brain are from a demyelinating process (ex. ms) or brain metastasis from an unknown cancer??

Usually evident: Radiologists are trained to interpret various lesion patterns on films, and each disorder has characteristic features. The classical MS lesion is periventricular, and perpendicular to the ventricle (Dawson's fingers). The typical metastasis is isolated toward the grey-white junction & likely surrounded by edema. A tough call is tumefactive MS which can be confusing.