The Effect of Field Size on the Dose to the Patient in Diagnostic Radiology

Measurements are reported demonstrating quantitatively the effect of field size on the dose to the patient in diagnostic radiology. These measurements were made mainly at 80 kVp in a phantom 8 in. thick, but some account was taken of changing kilovoltage and phantom thickness. It is shown that the distance from the edge of the beam is the most important factor influencing the dose at a point outside the main beam. By comparison, changing kilovoltage, filtration or even field size as such, has only a small effect. The conclusion is therefore drawn that although field size should be kept to a minimum, more attention should be paid to the proximity of the field edge to vital regions such as the gonads.