Our daughter had her first at 18 months, full body. We were told by the Dr. no problem with the exposure. She then had just a hand 12 months later and will continue with the hand every 12 months as long as everything is ok. I am sorry I don't know of any place where this is written down. The new booklet by John Hopkins just says it is important to monitor growth (bone and height) but is not specific as to when and how frequent. Possibly an E-Mail to their authors might help?