Measurements of head-scatter factors with cylindrical build-up caps and columnar miniphantoms

Head-scatter factors, S-h, also referred to as output factors, rue measured in-air with an ion chamber and a semiconductor diode fitted with cylindric al build-up caps and columnar miniphantoms fabricated from materials of dif ferent atomic number. S-h increases with field size less rapidly when cylin drical build-up caps are constructed from high atomic number materials. Thi s is a consequence of a net scatter of contamination electrons away from th e detector. Ion chambers and diodes give identical results when the same ty pe of build-up caps are used. Contamination electrons can be avoided by the use of columnar miniphantoms that have sufficient wall thickness in the ra dial direction. This radial wall thickness is characterized in this work fo r 6, 10, and 18 MV x-ray beams. S-h increases with held size less rapidly w hen columnar miniphantoms are constructed from high atomic number materials . This is due to the decrease in the average energy of photons at large fie ld sizes. It is concluded that to obtain S-h for dosimetry in water, cylind rical build-up caps and columnar miniphantoms should be constructed from ma terial with an atomic number close to that of water. (C) 1999 American Asso ciation of Physicists in Medicine. [S0094-2405(99)0014-2].