11.2. Quantities Related to Dosimetry 619
11.2.FMeasuringKermaandExposure
We saw earlier that air Kerma is directly related to the exposure through the relation
(see equations 11.2.27 and 11.2.29)
Kair=Wair
e(1−g)X. (11.2.33)
wheregis a factor that characterizes the loss of energy through radiative processes.
This equation implies that measuring Kerma is essentially equivalent to measuring
exposure. This is one of the reasons that even though exposure is termed as an
obsolete quantity, it is still widely mentioned in the literature and used in measure-
ments.
Exposure and Kerma can be measured by different kinds of detectors including
semiconductor devices. However ionization chambers have been, and still are, exten-
sively used for the purpose. Later on in the chapter when we discuss the ionization
chamber dosimetry we will look at two kinds of ionization chambers, namelyfree-
in-airandcavity, which are most commonly used to determine exposure and related
quantities.
11.2.GCavityTheories..........................
Absorbed dose is a very useful quantity in terms of determining the strength of radi-
ation interactions in a material. For example, one might be interested in determining
the dose received by a patient during radiation therapy to evaluate the effectiveness
of the method. Measuring the dose, however, is not as easy as it may sound. The
reason is that to measure dose one has to use a detector and every detector has a
detection medium, which might not be the same as the surrounding material. Hence
the dose measured from the instrument would be different from what it should be.
For example, if one uses a carbon dioxide filled ionization chamber to measure the
dose, the results will not be directly applicable to the air surrounding the chamber.
This is where cavity theories come into play since they relate the dose measured by
the detector to the dose in the surrounding medium. In the following we will discuss
two of the most important cavity theories.
G.1 Bragg-Gray Cavity Theory
A Bragg-Gray cavity refers to such a small detection volume that it does not influ-
ence the particle fluence when used in a medium. Theoretically this would mean
constructing a detector having a volume that shrinks to zero. Of course this is
practically not possible but in most cases one can say that it holds up to a good
approximation.
According to Bragg-Gray cavity theory, the ratio of the dose absorbed in a cavity
(that is, a detector) to the dose absorbed in its surrounding mediumDmedis given
by
Dmed
Dcav
=
(
L/ρ ̄)
( med
L/ρ ̄ )
cav, (11.2.34)
where the factorL/ρ ̄ represents the spectrum averaged unrestricted mass collision
stopping powers of the cavity and the medium. The term unrestricted implies that