Variation of radiation dose with distance from radiotherapy linac bunker maze entrances.

Instantaneous dose rate (IDR) measurements were made at the entrances of a number of linac bunkers for a range of beam energies and gantry angles in a large radiotherapy department. The results demonstrate that the IDR does not decrease with distance from the maze entrances according to the inverse square law (ISL), which is commonly used in the design of such bunkers. No simple relationship exists, but a useful guiding principle was found that the IDR at 1 m back from the maze entrance is approximately 50% of that at the entrance itself.