I would speak to your sixth form about your circumstances - if you want, they should allow you to repeat the year if you find you can't catch up.
Do you really want all of that extra stress whilst undergoing treatment and then recovering, though? You could take a year out and then begin your studies at a college (or possibly your school sixth form) in a more relaxed, focused environment. You could even use some of the spare time you have in that year to get ahead, if you wanted.
But first things first, speak to someone at your sixth form. They will be able to give you a much more accurate assessment of your options than anyone on TSR could.