@DD,
If you can, take leave during your chemo and radiation. That way, there won't be any chance that you'll get popped for a random drug test. Once you get better, if you can wait a month or so after you quit using the medical marijuana before you go back to work, you should be fine by then. If you can't afford to take off work for the entire time, depending on how often you get tested, you may be able to just chance it and hope you don't get called in for a random test. Also, laws on this may be different in different states, so maybe it would be worth it for you to pay for a consultation with an attorney who is familiar with employment laws in your state, just to make sure you are fully aware of your rights and the risks involved in whatever medicine you are considering taking...