Model for the calculation of output for elongated fields at nonstandard distances for a 25-MV betatron and for radiocobalt teletherapy units.

Output-area factors are usually provided only for square fields at one standard distance. To calculate dose for mantle, extended field, total nodal, subtotal body, and total body irradiation, output-area factors are required for a continuous range of shapes and distances. Dose rate measurements have been made for different elongation ratios at various distances with an Allis-Chalmers 25-MV betatron, an AECL Theratron-80 60Co unit, a Picker C-9 60Co unit, and a Picker C-10000 60Co unit. A model is presented that permits the extension of dose rates and area factors for square fields at one distance to elongated fields at any distance. An illustrative calculation for one of the units is given.