Dose outside useful beam in water and realistic phantoms exposed to chest radioagraphic procedures

It is frequently assumed that radiation measurements in anthropomorphic phantoms yield more accurate dose estimates in humans than can be obtained from water phantoms. As a consequence a realistic phantom consisting of tissue equivalent material, skeleton and air cavities was exposed to X-rays in chest radiographic procedures and dose measurements were made outside the useful beam. A similar study was performed by another group using a water phantom. Data from these two independent studies were compared at corresponding points. The relative doses at various depths and distances from the edge of the useful beam were plotted together for the trunk and skull regions. In the trunk region the agreement between realistic and water phantom data was quite good for those points compared (up to 10 cm depth and 45 cm from the edge of the useful beam). In the skull region values for the realistic phantom were as much as 100% higher than those for the water phantom, This was accounted for by the anatomical asymmetry of the body. Results of this study show that water phantom data is comparable to realistic phantom data when applied to the region of the trunk. When applied to the skull region the dose estimates must be modified by appropriate correction factors.