@Richard Kramer
Other than ground and sim training, you can give training in flight when the trainee is qualified to be and acting as PIC. This could include things like flight reviews and refresher or WINGS training, but NOT hood work since you'd then need a current medical.
The way you could work around this is find a state-licensed physician willing to sign Basic Med for you, in which case you could do any training you could have done before cancer, assuming you can honestly say you currently feel well enough to fly safely. The problem here is that many (most?) cancer treatments involve meds that would interfere with your ability to fly safely and are on the “do not fly” list or otherwise leave you too debilitated. But that's a personal decision for you and the physician who signs the form.
Even without Basic Med, you can fly with someone else acting as PIC so you can stay current and proficient, including flight reviews and IPC's (even putting on the hood yourself while flying to maintain your instrument currency) so you're ready to get back to flying yourself and instructing once you beat the disease.
@Ronald Levy
Thanks for the response. As far as flying under the hood with a safety Pilot I would not be able to log anytime? Is that correct? So an IPC would be needed if I get cleared in the future. Can I log approved Sim time?
@Richard Kramer
You don't have to be the PIC or have a medical to log the time – just be the sole manipulator of the controls of an aircraft for which you are rated. You can log the time, instrument events, landings, everything. And it all counts for recent experience. See 14 CFR 61.51(e) for details.
And, of course, you can also log the instrument time and events for 61.57( c) recent instrument experience in that approved sim.