Physics and Engineering of Radiation Detection

622 Chapter 11. Dosimetry and Radiation Protection

where the subscriptunresstands for unrestricted and implies that theδ-electrons
are to be considered while evaluating the quantity.

11.2.HLETandRBE..........................

Earlier in the chapter we used the term linear energy transfer orLETon a few
occasions. Due to its importance in dosimetry it is worth spending a little time
describing this quantity. LET is a measure of the energy absorbed in a medium
when charged particles pass through it and is closely related to stopping power
(−dE/dx) of charged particles as presented in chapter 2. The difference between
the two is thatLETis supposed to exclude delta rays that carry away energy from
the dosimeter. The equivalence ofLETand stopping power is therefore guaranteed
for very large targets in which the secondary electrons also get fully absorbed. For
smaller targets, such as biological cells, the target volume is quite small and therefore
a significant fraction of the total energy lost by charged particles manages to leave
the target without getting absorbed. This escaped energy does not cause any damage
to the target and should therefore be excluded from the dose calculations. That is
why, instead of the stopping power, generallyLETis used in dosimetry. Later on,
during the discussion on microdosimetry, we will see thatLETis also replaced by
a more useful statistical quantity called lineal energy.
Now, let us turn our attention to a very important parameter extensively used
in dosimetry, that is the relative biological effectiveness orRBE. To understand
this quantity we first note that the main purpose of dosimetry is to quantify the
effectiveness of radiation on biological organisms. The problem, however, is that
the quantityabsorbed dosedoes not faithfully represent the damage a particular
radiation field is capable of causing. The difference actually lies in the fact that
different kinds of radiation can cause different types and amounts of damage. To
account for such difference, the quantity relative biological effectiveness orRBEhas
been defined as

RBE=

Dref

Dtest

, (11.2.42)

whereDref is the x-ray dose andDtestis the dose from the test radiation that
produces the same biological effect. Unfortunately it is not at all trivial to determine
RBEanalytically as can be appreciated from the following list of parameters on
which it depends.

Dose

Dose rate

Type of biological entity

Linear energy transfer

Before 1990, the averageRBE for different biological effects were approximated
by an average quality factor. Since the quality factor has now been replaced by
weighting factor, we will not discuss it here. The weighting factors will be described
in the next subsection.