Mr. Wheeler,
Sorry to hear about your diagnosis but glad to hear the treatments are progressing.
FWIW, I'm not a doctor (or a medical professional in any way), but I've sort-of been in your position and have learned more medical stuff than I ever cared to know. I'm currently 57 but in July of 2008 I had quadruple bypass surgery, then in April 2013 I was diagnosed with Stage 4 tounge/throat cancer. While I never relied on flying as a primary source of income, I am a CFII and keep it current in order to possibly teach again when I retire. I follow this forum in hopes of one day building my own plane. Without knowing the specifics of your medical situation, I'll just tell you about my experience and hopefully you'll get some good use out of it.
Bottom line is that in both of my situations (cardiac and cancer) I was able to get my special issuance back from the FAA in what I thought was a reasonable amount of time following reasonable requirements. I'm currently flying with a 3rd class without any restrictions (other than having to wear glasses, but I've had that one since 4th grade!).
Retaining your 1st class medical could be more involved but that will depend on your your recovery and whatever side effects you may have following treatment. For comparison, I have to have a stress-echo test annually for my cardiac issues and supply all results to the FAA for my annual medical (yes annual, even with the 3rd class). Cardiac testing would be more involved if I wanted to maintain a 2nd class or better but a 3rd class meets my purposes.
Regarding the cancer, all the FAA requires is a status report from all attending physicians and their prognosis submitted with my annual FAA medical. And details of any possibly limiting side-effect issues. Since you're only having chemo (not trying to minimize the chemo issues by saying "only", it all sucks), you don't have to be concerned with issues from surgery of radiation. For comparison, I had 3 chemo and 35 radiation treatments (no surgery). My permanent side effects from chemo are tingling and intermittent numbness of the feet, especially the toe areas. My permanent side effects from radiation are lack of taste (especially sweets) and significant dry mouth (most of the salivary glands got fried). The good news is that, while inconvenient and sometimes more annoying than others, all the issues can be lived with (and worked around) and, most importantly, are not problems as far as the FAA is concerned.
The best advice I can give regarding dealing with the FAA and your medical is:
1) Have an AME who is willing (and preferably experienced) to deal with the FAA on more complicated cases like yours. There will be more paper work involved so make sure he/she knows what their doing.
2) Have your paperwork in order. The FAA medical reviewers in Ok City will only have the paperwork submitted to them to make judgement calls. Make sure you provide a record of every doctor, medical visit, and test result you've had during your treatment (and I mean every!). And provide it in a logical, easy to understand sequence. There's no formal process here, just put yourself in the reviewers shoes and think how you would want the data presented.
One final thought. Like most everyone else, I like to make insulting remarks about government processes as much as anyone, but my experience with the FAA during my medical issues was very reasonable (in my opinion). Once my AME submitted my paperwork to Ok City for review, I had my 3rd class Special Issuance medical in 4 to 5 weeks.
Good luck with your recovery, and please let me know if there's anything else I can help you with.
Steve Daudelin