My mri found numerous periventricular and subcortical lesions, and the impression listed was "extensive ms lesions." could it be anything but ms?

Radiologically : Isolated syndrome is term for your findings, if you have had no prior ms symptoms. Lesions might be c/w head trauma, migraine, and even drug reaction (methotrexate), lupus, lyme disease, lymphoma. Likely ms if both older and newer lesions, and also involvement of spinal cord and/or gray matter. May be useful to get spinal fluid. Do not worry, this is treatable and controllable if ms.