Answer to Question #11780 Submitted to "Ask the Experts"

I have checked many sites and cannot find any information on radiation scatter per unit distance from the source. Do you know what the dose would be at a given distance away from the source? For example, if someone were a few centimeters (cm) to the side of the source, how much would the dose be reduced?

Scatter radiation is generated in the patient when an x-ray beam strikes the patient. The maximum scatter radiation occurs perpendicular to the x-ray beam. In other words, if the x-ray unit is above the patient, the highest scatter radiation at a set distance from the patient will be out to the side of the patient. The amount of scatter radiation at a given location can be determined by performing a measurement with a calibrated survey instrument. A general rule of thumb is that the amount of scatter radiation at 1 meter (m) from the side of the patient will be 0.1% of the intensity of the primary x-ray beam.

If you are interested in knowing the scatter radiation dose at another point along that plane, you would correct the dose at a known point to the point of interest using something called the inverse-square law. This rule simply states that as you double your distance from a source, the radiation dose will drop by one-fourth.

Scatter radiation dose is much easier to measure than it is to calculate at any given point. If a radiation worker is concerned about the amount of scatter radiation at any point in an x-ray room, the best solution is to discuss those concerns with the radiation safety officer or the medical physicist at the facility. This discussion would include possible measurements by the physicist and following proper radiation safety procedures for the worker (protection by time, distance, and shielding). For a patient, the radiation dose from scatter is a minimal fraction of the dose received from the primary x-ray beam.

Kennith "Duke" Lovins, CHP