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. This was found to hold with a standard error within 5% across the full extent of the maze entrance by measuring in a grid pattern. The ISL should therefore be used with caution in the design of radiotherapy bunker mazes as it may not provide accurate estimates of the dose in surrounding areas. The results of the grid pattern measurements also indicate that the centre of the maze entrance is the best position to affix passive dosemeters for the purpose of environmental monitoring. This may not always be practical (e.g. if the bunker does not have a door), but factors have been found to correct readings taken elsewhere to the central reading. For instance, if the dosemeter is placed on the wall, the measured dose rate will be equal to 94% ± 8% of the value at the centre of the bunker.