Cubical S values for use with SPECT, PET, and autoradiographic imaging data in performing small-scale dosimetry

A traditional assumption made in nuclear medicine dosimetry methodologies such as the MIRD schema is that the activity in the source organ is uniformly distributed. Localization techniques such as quantitative SPEC and PET imaging allow one to dispense with this assumption and look at realistic nonuniform activity distributions in selected organs or organ regions. Therapy applications further emphasize the need for direct treatment of nonuniformities. Many researchers have relied upon elaborate computational techniques such as dose kernels to assess dose distributions in these regions. In this work, a simplified approach is proposed which allows direct use of the MIRD schema in conjunction with imaging data to rapidly assess organ dose distributions with minimal computational effort. The EGS4 radiation transportcode has been used with a cubical array of tissue voxel elements for a centrally located source cube of {sup 32}P, {sup 131}I, {sup 89}Sr, {sup 90}Y, and {sup 99m}Tc. Three sets of voxel dimensions are considered: 6 mm for SPECT images, 3 mm for PET images, and 50 {mu}m for autoradiography. Radionuclide S values are subsequently tabulated as a single function of the source-to-target voxel separation distance. Isodose contours are shown for (1) a mouse renal cell carcinoma with {sup 131}I-labeledmore » antibody, (2) a human colon adenocarcinoma with {sup 131}I-labeled antibody, and (3) various tumors directly injected With {sup 32}P.« less