Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

Medical Internal Radiation Dose Methodology
RADIATION DOSIMETRY 241
To account for the partial absorption of gamma-ray energy in organs and tissues,
the Medical Internal Radiation Dose (MIRD) Committee of the Society of Nuclear
Medicine developed a formal system for calculating the dose to a “target”
organ or tissue (T) from a “source” organ (S) (Fig. 6-10) containing a uniformly
distributed radioisotope. S and T may be either the same organ or two different
organs bearing any of the possible relationships to each other shown in Figure
6-10. The MIRD system separates the dose calculation into two basic components:
the physical factors dealing with the radiation and the fraction of energy
radiated by the deposited activity that is absorbed by the tissue, and the biological
factors that are derived from physiologically based biokinetic models of the
radionuclide. The fraction of the radiated energy that is absorbed by the target
tissue is calculated by the application of Monte Carlo methods to the interactions
and fate of photons following their emission from the deposited radionuclide.
Monte Carlo methods are useful in the solution of problems where events such as
the interaction of photons with matter are governed by probabilistic rather than deterministic
laws. In Monte Carlo solutions, individual simulated photons (or other
corpuscular radiation) are “followed” in a computer from one interaction to the
next. The radioisotope is assumed to be uniformly distributed throughout a given
volume of tissue. Since radioactive transformation is a random process occurring at a
mean rate that is characteristic of the given radioisotope, we can start the process by
randomly initiating a radioactive transformation. For any of these transformations,
we know the energy of the emitted radiation, its starting point, and its initial direction.
We also know the probability of each possible type of interaction within the
organ and the energy transferred during each interaction. A situation is simulated
by starting with a very large number of such nuclear transformations, following the
history of each particle as it traverses the target tissue, and summing the total amount
of energy that the particles dissipate within the target tissue. For a concentration of
Figure 6-10. Possible relationships between source organ and target organ.