Leakage through gaps around shielding windows in radiotherapy treatment rooms

Radiation streaming down gaps between shielding windows and walls of radiotherapy treatment rooms has been investigated. The calculations were based on a cobalt-60 source and were performed with the computer code RANKERN. The dose rate near the exit of the gap on the cold side of the window was calculated as the sum of contributions from direct line of sight and radiation scattered off the faces of the gap, using albedo data. The RANKERN code employs point kernel equations which are solved by a stochastic integration method. It is shown that although for a single element window small gaps could lead to significant leakage of radiation, for a double or triple element window, even a gap of ten millimetres would result in a dose rate no greater than that through the surrounding concrete.