Systems of 1 MeV or more must have adjustable or interchangeable beam-limiting devices. The devices must transmit no more than 2 percent of the useful beam at the normal treatment distance for the portion of the useful beam which is to be attenuated by the beam-limiting device. The neutron component of the useful beam is not to be included in this requirement.
Okla. Admin. Code § 252:410-11-52