Typically, medical treatment in the USA is undertaken on a B-1 ("Business" visa. Strange, huh?). But do not get confused. Consulates often mark visas as B-1/B-2 or even B-2 (visitors/tourist) for medical treatment. Once here, you can ask for extensions of stay if the circumstances so require. I remember extending the stay of a child and his parents for two years (in 6-monthly chunks), where the baby was being treated for ongoing medical problems.