Scatter fractions from linear accelerators with x-ray energies from 6 to 24 MV

Computation of shielding requirements for a linear accelerator must take in to account the amount of radiation scattered from the patient to areas outs ide the primary beam. Currently, the most frequently used data are from NCR P 49 that only includes data for x-ray energies up to 6 MV and angles from 30 degrees to 135 degrees. In this work we have determined by Monte Carlo s imulation the scattered fractions of dose for a wide range of energies and angles of clinical significance including 6, 10, 18, and 24 MV and scatteri ng angles from 10 degrees to 150 degrees. Calculations were made for a 400 cm(2) circular field size impinging onto a spherical phantom. Scattered fra ctions of dose were determined at I m from the phantom. Angles from 10 degr ees to 30 degrees are of concern for higher energies where the scatter is p rimarily in the forward direction. An error in scatter fraction may result in too little secondary shielding near the junction with the primary barrie r. The Monte Carlo code ITS (Version 3.0) developed at Sandia National Labo ratory and NIST was used to simulate scatter from the patient to the barrie r. Of significance was the variation of calculated scattered dose with dept h of measurement within the barrier indicating that accurate values may be difficult to obtain. Mean energies of scatter x-ray spectra are presented. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)010 08-1].