MO-A-217BCD-01: Internal Emitter Dose Estimation

Medical Physics
L E Williams

Abstract

Tissue absorbed dose (D) is a computed result for internal emitters. For fixed geometries, D is calculated by a matrix (S) multiplication of the integrated activity vector (Ã). The last quantity is usually measured by nuclear imaging of activity in various source organs and performing a temporal integration. Ã is the same as the total number of source decays. Dose is computed for a number of target organs - some of which will be the same as the source organs. The D = S*Ã relationship is general in that the same formula may also be used for voxels within organs or even down to the cellular level. Finding the activity (A) in source tissues may be done by a number of methods of which 6 are described. The most common clinical technique is the geometric mean (GM) image of an organ. Uncertainties in the GM method are ≈ +/- 30%. If one can do quantitative SPECT, PET or CAMI imaging, the variation is reduced to around +/- 6%. These last three techniques, however, require fusion of anatomic (e.g. CT) and nuclear images. The S matrix is generated via Monte Carlo methods and may be used in two formats. The most common is a set of phantom-derived values for regulatory or scientific considerations. An example is the OLINDA program from Vand...Continue Reading

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