I took medical retirement, because as a postman I had pain and fatigue walking (which was actually attributed to another cause than it really was, but whatever...) and after a couple of months of sick I was given it.
HOWEVER, my manager did say it would be in my interest to go for it from the start even though he knew I wanted to work, as he said I could end up being moved sideways and eventually just pushed out of whatever job I was given. At least this way I got a medical retirement package as opposed to nothing.
That sounds like I was falsely going down that road, but that isn't strictly true. I didn't want to particularly do anything else they would have to offer, and if I did, it would involve travelling and expense I couldn't afford. Besides, walking was naking me very tired and I figured it wouldn't do me any good.
I was also sent ATOS (privately as opposed to a benefit claimant) where I was assessed for what I could do and what I was experiencing, and then later to a company doctor who rubber stamped me for medical retirement. Because I wanted it, I did sort of exaggerate my problems, though in hindsight, it was a good job I did.
I'm actually in a similar position to you except it isn't cancer (I have a benign brain tumour) and as with you, I have good days and bad days as to what I can do, and also as with you, I don't know how long. I'm not sure as yet if mine is operable or treatable, but if not it will slowly affect various functions of my body.
Going back to you, I am surprised at the 18 months, as this does seem a long time to still be employed. You don't say if you are in a private pension scheme, but if you are, I BELIEVE you can claim early pension if you are medically retired (private that is, not state).
I would seriously suggest playing (though it sounds as if you are not playing, if you get my drift) up the worst aspects of this and try not to be a matyr to it. As you say there will be hospital visits and treatment to contend with, and you really need to think of you not them.
If they haven't already suggested it, I would bring it up - it's not like a broken leg, where you know after x months you will be fine.
All the best anyhow.