Wow, I don't know what to say. There is a lot of information out about this condition and it all states there is a high return rate and all suggest doing an mri before treating. How could a doctor decide how defined it is w/o the tests?
I would find a different doctor, two, or three to at least consult. Of course that is just my opinion. I know it is difficult and can be costly, but worth it for your child. Right now we are going over 1700 miles for treatments for my child and that happens every 10-12 weeks.
Don't give up, educate yourself as much as possible, and ask questions. Find a doctor with whom you are very comfortable with. One clue is the nurse who told you they do not usually grow back, when the info says that is very common for them to come back if present in infancy.
Possibly some of the information I provided will help you in your quest to find someone who can give you all the information so you can make the best decisions.
Keep me posted.