624 Chapter 11. Dosimetry and Radiation Protection
Example:
Calculate the equivalent radius of a rectangular beam of sides 0.5mmand
1.2mm.Solution:
The radius of the equivalent circle can be found by equating equations 11.2.43
and 11.2.45.
√
πr =2 yz
y+z
⇒r =
2 yz
√
π(y+z)=(2)(0.5)(1.2)
√
π(0.5+1.2)
=0. 4 mm.11.2.J InternalDose
Now that we are on the topic of dose and dose calculations, we will discuss a very
important class of dosimetry, namely calculation and measurement of internal doses.
By internal dose we mean the dose received by internal organs of a living thing
(actually we are almost always concerned with doses received by humans) by a
source or sources inside the body. These sources may or may not be uniformly
distributed throughout the organ under consideration. However, for most practical
purposes we can assume their uniform distribution. The reason is that the sources
digested or inhaled follow the usual metabolic path and then reside in an organ, such
as liver. The slow intake of the source assumes uniform distribution throughout the
organ tissues. Now, the question is how we know which source ends up where.
For example, one would assume that an inhaled source may reside inside the lung
for an extended period of time. This is certainly true but the source can also get
distributed to other organs through blood and diffusion. In fact, this is something
that has been discovered through experimentation and the medical physicists can
fairly accurately tell where in body to expect the most significant amount of any
particular source. For example, the radium and strontium almost always end up in
bones. Cesium and tritium get distributed throughout the body, while iodine finds
its way into the thyroid glands.
With advancement in microdosimetry, it has become possible to measure dose
inside the human body due either to external or internal sources. In certain situ-
ations, it is not really required that the dose be actually measured. For example,
in medical diagnostics where radiations sources are introduced into the body, one
is interested in determining theexpecteddose. It is certainly impractical and also
unnecessary to actually measure the dose with equipment.
In the next two subsections we will discuss the doses received by tissues due to
charged particles and thermal neutrons. A number of diagnostics and therapeutic
radioactive sources emit such particles and therefore estimation of their respective
doses is routine practice in nuclear medicine.