Patient-specific scaling of reference S-values for cross-organ radionuclide S-values: what is appropriate?

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The Medical Internal Radiation Dose Committee (MIRD) formalism assumes reference mass values for the organs (source and target) and the total body. MIRD publication 11 provides guidance on how patient-specific scaling of reference radionuclide S-values are to be performed for the electron component of the emission spectrum. However, guidance on patient-specific scaling of the photon contributions to the S-value is given only for those cases where the source and target organs are either far apart or are the same. The photon component of the S-value is derived from photon-Specific Absorbed Fractions (SAFs). These are obtained by Monte Carlo calculation of photon transport. The objective of this work is to verify the MIRD 11 guidance and to examine the relationship between photon SAFs and source/target organ mass when the conditions listed above do not apply. Furthermore, the scaling for photon cross-dose to distributed organs is at present not defined due to lack of data for models other than the reference model. The validity of mass scaling for cross irradiation from near and distant photons sources, especially for Red Bone Marrow (RBM) as a target tissue is also investigated. This is achieved by comparing Monte Carlo-derived SAFs for different source organs to RBM across the GSF voxel phantom series. The results show that, for photon energies greater than 100 keV, the SAF of most source organs to RBM need not be corrected for target mass (error < 5%). In contrast to the results obtained for well-defined source organs, the SAF for RBM irradiating RBM gives a deviation of up to 16% across the different GSF voxel phantoms.