Mild supratentorial white matter disease--what is this?

Maybe of no signif.: White matter lesions may be caused from a host of circumstances, such as migraine, prior head or neck trauma, hardening of the arteries, inflammation like multiple sclerosis, prior small strokes, effect of some medications such as methotrexate, and is not specific. The location of your concern maybe actually a chronic, lifelong issue. Consult your doctor.