The effect of source to image distance on radiation risk to the patient.

The Monte Carlo package PCXMC was used to determine the effect of source to image distance (SID) on effective dose to the patient in diagnostic radiology. A range of common examinations was used, with a range of distances from 75 cm to 200 cm. The results showed that the radiation risk (effective dose) is virtually independent of the SID. In cases where a radiation sensitive organ is close to the edge of the x-ray field, the effective dose increases slightly with distance. In other cases there was either no dependence, or a small reduction in risk with increasing SID. There is therefore no justification in terms of radiation risk for a change in the standard SID of 1 m that is currently used for most projections.