A PHANTOM BASED METHOD FOR DERIVING TYPICAL PATIENT DOSES FROM MEASUREMENTS OF DOSE-AREA PRODUCT ON POPULATIONS OF PATIENTS

One of the chief sources of uncertainty in the comparison of patient d osimetry data is the influence of patient size on dose. Dose has been shown to relate closely to the equivalent diameter of the patient. Thi s concept has been used to derive a prospective, phantom based method for determining size correction factors for measurements of dose-area product. The derivation of the size correction factor has been demonst rated mathematically, and the appropriate factor determined for a numb er of different X-ray sets. The use of phantom measurements enables th e effect of patient size to be isolated from other factors influencing patient dose. The derived factors agree well with those determined re trospectively from patient dose survey data. Size correction factors h ave been applied to the results of a large scale patient dose survey, and this approach has been compared with the method of selecting patie nts according to their weight. For large samples of data, mean dose-ar ea product values are independent of the analysis method used. The chi ef advantage of using size correction factors is that it allows all pa tient data to be included in a survey, whereas patient selection has b een shown to exclude approximately half of all patients. Reduction of the size of the data set may lead to mean dose-area product values tha t are less reliable indicators of typical practice. The use of size co rrection factors will be of particular benefit in the analysis of paed iatric dosimetry data, where a wide range of sizes exist, even within accepted age bands.