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Program Information

Internal Emitter Dose Estimation

L Williams

L Williams1*, (1) City of Hope Medical Center, Duarte, CA

MO-A-217BCD-1 Monday 8:00:00 AM - 9:55:00 AM Room: 217BCD

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. à 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*à 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 Vanderbilt University. In this case, the corresponding animal or patient à value must be normalized using blood flow arguments. A second format is modification of a phantom’s S values for a particular patient using the latter’s geometry as found in CT or MRI scans. Corrections in such cases may be 2-fold or more because of patient organ size variability. These variations may be due to genetic reasons and/or disease. Two caveats to the use of the above dose formulation should be mentioned. One exception is that the geometry may vary during tissue irradiation; e.g., by tumor size decrease due to immediate radiation dose effects. In this case, the standard formula is replaced by its differential form: dD/dt = S(t)*A(t). Dose rate may also be an important biological factor in assessing tissue response. A second important biological consideration is that effects – such as tumor regression - may depend upon higher powers of D than the first. Thus, the tissue response may not be a linear function of D, but would exhibit a sigmoid shape. One would anticipate such responses due to saturation of a biological system.
Learning Objectives:
1. Knowing the general formula for internal emitter absorbed dose estimation.
2. Understanding the various methods used to measure activity, at depth, in source organs in a living animal or patient.
3. Realizing that two types of dose may be computed: one for a phantom and a second type for an individual patient. S values must be modified accordingly for these two computations.
4. Estimating uncertainties – including those in both à and S - involved in the dose estimation process.

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