It is always a good idea for a green card holder to speak with a lawyer before leaving the U.S. for any great length of time, and you ought to speak with a lawyer about the specifics of your case to ensure that you have the proper documents assembled before trying to reenter now.
In general, absences of less than six months aren't much of an issue and are rarely questioned.
Between six months and a year spent out of the U.S., an inquiry will be made into ties maintained to the U.S. (property here, job to return to, family here, etc.) The better documentation presented, the more likely a determination will be made that you have maintained your ties to the U.S. and have not abandoned your green card. This is why a conversation with a lawyer is a good idea.
So you know, after more than a year there is basically a presumption that the green card has been abandoned. It is still in theory possible to overcome this with a strong showing of ties to the U.S., but the better course of action is to deal with the situation before leaving if you know you will be out of the U.S. a year or more. There is a procedure for obtaining a document from USCIS which serves essentially as advance permission to leave for up to two years without abandoning permanent residence. This too should be pursued through a lawyer.
Good luck - with both the reentry and your medical treatments!