Biomedical Engineering Reference
In-Depth Information
are needed to apply to other kinds of radiation and to other materials, particularly
tissue. The primary physical quantity used in dosimetry is the absorbed dose. It is
defined as the energy absorbed per unit mass from any kind of ionizing radiation
in any target. The unit of absorbed dose, J kg
-1
, is called the gray (Gy). The older
unit, the rad, is defined as 100 erg g
-1
. It follows that
10
7
erg
10
3
g
1J
kg
=
10
4
erg
g
(12.3)
1Gy
≡
=
=
100 rad.
The absorbed dose is often referred to simply as the dose. It is treated as a point
function, having a value at every position in an irradiated object.
One can compute the absorbed dose in air when the exposure is 1 R. Photons
produce secondary electrons in air, for which the average energy needed to make
an ion pair is
W
=
34
JC
-1
(Sect. 10.1). Using a more precise
W
value,
1)
34
eV ip
-1
=
one finds
10
-4
C
kg
2.58
×
33.97 J
C
10
-3
Jkg
-1
.
1R
=
×
=
8.76
×
(12.4)
Thus, an exposure of 1 R gives a dose in air of
8.76 × 10
-3
Gy (
=
0.876 rad).
Calculations also show that a radiation exposure of 1 R would produce a dose of
9.5 × 10
-3
Gy (
=
0.95 rad) in soft tissue. This unit is called the rep (“roentgen-
equivalent-physical”) and was used in early radiation-protection work as a measure
of the change produced in living tissue by radiation. The rep is no longer employed.
Dose Equivalent
It has long been recognized that the absorbed dose needed to achieve a given level
of biological damage (e.g., 50% cell killing) is often different for different kinds
of radiation. As discussed in the next chapter, radiation with a high linear energy
transfer (LET) (Sect. 7.3) is generally more damaging to a biological system per unit
dose than radiation with a low LET (for example, cf. Fig. 13.16).
To allow for the different biological effectiveness of different kinds of radiation,
the International Commission on Radiological Protection (ICRP), National Coun-
cil on Radiation Protection and Measurements (NCRP), and ICRU (Chap. 1) intro-
duced the concept of dose equivalent for radiation-protection purposes. The dose
equivalent
H
is defined as the product of the absorbed dose
D
and a dimensionless
quality factor
Q
, which depends on LET:
H
=
QD
.
(12.5)
In principle, other multiplicative modifying factors can be included along with
Q
to allow for additional considerations (e.g., dose fractionation), but these are not
ordinarily used. Until the 1990 recommendations made in ICRP Publication 60,
the dependence of
Q
on LET was defined as given in Table 12.1. Since then, the
1
See p. 29 in Attix reference, Section 12.11.
