Scattered fractions of dose from 6,10,18, and 25 MV linear accelerator x rays in radiotherapy facilities

Data for computation of primary and secondary shielding barriers in radiotherapy facilities are presented in the National Council on Radiation Protection and Measurements Report 49, which covers primary barriers for accelerating voltages up to 10 MV and secondary barriers up to 6 MV. Inconsistencies in reported scattered fraction values found in the literature for megavoltage accelerators are due, in part, to differences in measurement techniques. A consistent approach was used to measure the scattered fractions of 6, 10, 18, and 25 MV photon dose. A reference ion chamber was placed on the symmetry axis of a cylindrical water phantom, approximating the human torso, 25.6 cm diameter, at the gantry isocenter. Measurements were made with a second ion chamber at 2m from the isocenter at angles ranging from 10 degrees through 150 degrees from the beam central axis with a field size of 20 cm x 20 cm at the isocenter. The dose measured at each angle is reported as a fraction of the reference chamber dose at the center of the phantom and as calculated to a reference chamber depth of d(max), also at the isocenter. The results of these measurements were compared with Monte Carlo calculations and show reasonable agreement. A comparison with previously published data was not as good due to the differences in measurement techniques. However, after some adjustments to correct for the differences, the comparison is very good. Scattered fractions beyond 30 degrees are highest for the lowest energy beam investigated. This indicates that the scattered fraction values reported in NCRP 49 for 6 MV are sufficient for use in conservative shielding calculations for the higher energy x rays beyond 30 degrees. For angles smaller than 30 degrees, the new data are more appropriate and show the scatter fractions are larger for higher MV x rays.