In general, F-1 students must enroll full time. This page from Homeland Security explains the exceptions, particularly that you can remain in F-1 status without enrolling full time in case of properly documented illness or medical condition, for up to 12 months. This is based on federal regulation 8 CFR 214.2(f)(6)(iii)(B). Unfortunately there isn't a provision for extending this beyond 12 months. So you would have to enroll full time by Fall 2018 to maintain your F-1 status.
If you aren't expected to recover by Fall 2018, you should discuss this problem with your Designated School Official and probably with a US immigration attorney, to try to find some other option. Depending on your circumstances, it might or might not be possible to:
- enroll full time if they can make some accommodation that would allow for your treatment
- transfer to a different F-1 school (where you would still have to enroll full time)
- change to a different immigration status based on some other activity (such as educational research, cultural exchange, or employment) or family tie
If you aren't able to enroll full time as F-1 or change to a different immigration status, you would be facing the choice to leave the US or stay without valid status.