Re: What should I tell my boss?
If you are leaving for just 6 months, I'm sure she won't assume it's life-threatening. You're not required to state your specific condition, and saying you're leaving for some medical reasons you have to take care of is just fine. She should accept that without questioning you further. If she does, you might just calmly say that you prefer not to discuss the details, but assure her that's not a permanent disability that will stop you from working altogether.
I hope this helps a bit. You might consider contacting the people with the ADA and reading up on this, perhaps ask their advice.
Let us know what you decide. I think the more low-key and matter-of-fact you are, the better.