@Fishmom,
@gsfowler and
@chaplaincharlie are giving you good advice. The 8-year rule is more complicated for Reservists than for regular Active Duty members. If you're Active Component, it's easy. Just count the time since you enlisted/commissioned, and if it adds up to 8 years, you're golden.
For Reservists, you need to meet three criteria.
1. You have at least 8 years of total active service,
2. You were on Title 10 orders specifying a length of greater than 30 days at the time of your injury, and
3. You weren't released from those orders before the 30 days was up.
That's from AFI 36-2910, incidentally. You can find it here:
http://static.e-publishing.af.mil/production/1/af_a1/publication/afi36-2910/afi36-2910.pdf. The section is 1.10.2.2.2, on page 11.
So, in order to take the 8-year shortcut, you'll need to show that you were on orders for at least 31 days when your cancer became unfitting.
If you don't qualify for the 8-year, then you'll probably want that letter from your oncologist. You're not dead in the water - you just need to go through the same process as everyone else and prove that your condition was incurred or aggravated while you were entitled to basic pay.
@FTP's recommendation of Jason Perry is a good one, too. There aren't a lot of us who do this work regularly. Jason's great. If you decide to go with someone else, make sure to ask them how many of these cases they've done in the last year or two, and how many of those were for Reservists. You don't want someone who only takes a DES case every six months, or who doesn't know the rules for Reserves.