410 IAC 5-6.1-41 "Field size" defined

Sec. 41. As used in this rule, "field size" means the dimensions along the major axes of an area in a plane perpendicular to the specified direction of the beam of incident radiation at the normal treatment distance and defined by the intersection of the major axes and the fifty percent (50%) isodose line, for diagnostic applications. For therapeutic x-ray applications, "field size" means the two (2) longest perpendicular lengths, in combination, of a figure defined by the fifty percent (50%) isodose line at a cross section of the x-ray beam, measured in the plane perpendicular to the central axis of the x-ray beam at the normal treatment distance. In either case, field size is determined when material is placed in the beam so that maximum dose is achieved at the normal treatment distance.