Mean Absorbed Doses And Dose Distributions Of Nucleus, Cell And Micromestatate For Targeted Radiotherapy

In targeted radiotherapy, radiation energy is imparted at a low dose rate to tumors from internally distributed radionuclides. The most important quantity is the mean absorbed doses of the nucleus, the cell, the tumor and normal tissues to evaluate the efficacy and toxicity. Specialized dosimetric models and computational methods are needed.Nonuniform distributions of radionuclides will be encountered at subcellular, cellular, and multicellular levels. A new MIRD-style approach for calculating mean absorbed dose is provided. Two independent methods are developed in computation of energy deposition in cell or nucleus according to the given energy loss forms: polynomial presentation of particle traveled distance or function of particle residual range, respectively. Details of the mathematical calculations are analytically derived for different source-target combinations. Several hypothetical distributions of radionuclides in cell are considered: (1) uniform, (2) increasing linear, (3) decreasing linear, (4) exponential, dependence on the radial distance from the cell center, (5) located at the cell center, (6) cell surface. Cellular absorbed fractions are then calculated. The resulting absorbed fractions are employed to calculate cellular S-values and dose rate profiles for several alpha-, beta-, Auger-emitters commonly used in nuclear medicine procedures and for boron neutron capture therapy (BNCT).Two different target regions are considered: the entire cell and the cell nucleus. The cell and cell nucleus are taken to be concentric spheroids. The cell, cell nucleus, and interstitial spaces are considered to be unity density matter. Illustrative computations have been completed for a typical mammal cell with a radius of 5 μm and nucleus 4 μm in radius. The spherical multi-cellular cluster is assumed to be a collection of cells with simple cubic, body center cubic, face center cubic, and hexagonal close-packed arrangements, repectively. Each cell contains the same activity. The distance from the center of each cell to the target cell is calculated. The dose contributions to the target cell from surrounding cells are summed up.The mean absorbed dose to the target cell or nucleus in a cell cluster is the sum of the dose from radiations emitted within: (a) the target cell , (b) the surrounding cells, and (c) the extracellular space. The contribution of extracellular radionuclides has not been carried out because it can be calculated and added to the S-values according to conventional MIRD method. The contributions of photon radiations are ignored because they are negligible compared to the contributions of the particle radiations, or can be calculated and added to the S-values using MIRD formulae.Various target←source combinations including cell←cell, cell←cell membrane,...