Table B.25: Red bone marrow in the whole body - Helmholtz ...
Table B.25: Red bone marrow in the whole body - Helmholtz ...
Table B.25: Red bone marrow in the whole body - Helmholtz ...
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5<br />
3. Calculational method<br />
___________________________________________________________<br />
The organ dose conversion factors were calculated us<strong>in</strong>g a computer code<br />
simulat<strong>in</strong>g photon transport <strong>in</strong> various media. For this procedure, three<br />
ma<strong>in</strong> components are necessary: a model of <strong>the</strong> radiation source, a technique<br />
to simulate <strong>the</strong> radiation <strong>in</strong>teractions and energy depositions and a<br />
model of <strong>the</strong> human <strong>body</strong>.<br />
Accord<strong>in</strong>g to <strong>the</strong> irradiation conditions mentioned above, <strong>the</strong> radiation<br />
source was modelled as a po<strong>in</strong>t source rotat<strong>in</strong>g around <strong>the</strong> longitud<strong>in</strong>al axis<br />
of <strong>the</strong> <strong>body</strong>; <strong>the</strong> beam size at <strong>the</strong> axis of rotation was 1 cm <strong>in</strong> height and<br />
50 cm <strong>in</strong> width for <strong>the</strong> symmetrical beam (for asymmetrical beams, <strong>the</strong><br />
width was reduced on one side). To simulate <strong>the</strong> x ray spectra as <strong>the</strong>y<br />
emerge from <strong>the</strong> x ray tube, a semi-empirical method was used consider<strong>in</strong>g<br />
<strong>the</strong> target material, <strong>in</strong>herent as well as added filtration, air path, target<br />
angle and tube voltage /8/.<br />
3.1. Monte Carlo method<br />
The radiation transport <strong>in</strong> <strong>the</strong> phantoms was calculated us<strong>in</strong>g a Monte<br />
Carlo code follow<strong>in</strong>g <strong>in</strong>dividual photon histories. For each s<strong>in</strong>gle particle<br />
history, <strong>the</strong> parameters <strong>in</strong>fluenc<strong>in</strong>g its actual course are selected randomly<br />
from <strong>the</strong>ir probability distributions. The radiation <strong>in</strong>teraction processes<br />
considered were photoelectric absorption and Compton scatter<strong>in</strong>g. The<br />
cross section data for <strong>the</strong> radiation <strong>in</strong>teractions were taken from <strong>the</strong> Oak<br />
Ridge National Laboratory /9/. The energy was regarded as be<strong>in</strong>g deposited<br />
at <strong>the</strong> po<strong>in</strong>t of photon <strong>in</strong>teraction; secondary particles were not followed<br />
("Kerma approximation"). A more detailed description of <strong>the</strong> photon<br />
transport code used is given elsewhere /10/.<br />
The absorbed doses were obta<strong>in</strong>ed by divid<strong>in</strong>g <strong>the</strong> total amount of energy<br />
deposited <strong>in</strong> an organ by <strong>the</strong> mass of this organ. It is to be emphasised that