Analytic Determination of Size of Water Cylinder Absorbing Equal Radiation Dose as a System of Concentric Cylinders of Arbitrary Composition: Application for Pediatric CT
D Bakalyar1*, (1) Henry Ford Health System, Detroit, MISU-D-217BCD-5 Sunday 2:15:00 PM - 3:00:00 PM Room: 217BCD
Purpose: To develop analytic formulae for calculating absorbed dose to a system of concentric cylinders and to use this to determine the size of water cylinder which would absorb the same dose in a matching field. Appropriate scaling of radiation output to patient size is of particular interest in pediatric CT (TG204).
Methods: For a given CT scanner output, absorbed dose increases as patient size decreases. Analytic functions can calculate the dose delivered to systems of concentric cylinders of varying size and composition. In particular these functions provide useful insight when considering examinations in body regions with large variations in attenuation, such as the chest (which can be modeled as concentric cylinders of heart, lung, rib and soft tissue). Iterative calculations rapidly determine the size water cylinder required to absorb the same dose in matching radiation fields. More general findings put limits on some of the methods for determining size specific dose estimates (SSDE).
Results: For the simple chest model, in the absence of scatter or bow tie filter considerations, the equivalent water phantom would have a diameter of just over half the chest equivalent concentric cylinder phantom. A more general result is that in the limit of very small size, the absorbed dose in a fixed irradiating field depends only on mass attenuation coefficient.
Conclusions: The concentric cylinder model developed can be used to study the scaling of radiation dose with patient size. In particular, the water phantom absorbing the same dose as the chest region has only about half the diameter. An important conclusion is that in the limit of small size, absorbed dose depends on mass attenuation coefficient only and that in this limit an equivalent water phantom does not exist.