Dosimetry and Radiation Protection
effecting the dose, and for this reason a unit term-
ed the equivalent dose is also defined,calculated by
multiplying the absorbed radiation dose by a
radiation weighting factor
w
R
as defined by the
International Commission on Radiological Protec-
tion (ICRP 60 [1]). The SI unit for this quantity is
the Sievert (Sv), where this replaces the rem. The
radiation weighting factors are listed in Table 1,
and it can be seen that the factor for X-rays,
gamma rays and beta particles is 1, giving an
equivalent dose for these radiations numerically
equal to the absorbed dose.
When a radiopharmaceutical is administered to
a human subject the internal radiation dose to
the body may be estimated by observing its sub-
sequent behaviour. The resulting dose can be
shown to depend upon a large number of vari-
ables. These range from the physical properties of
the radionuclide itself and the radioactivity of the
tracer and the route of its administration, to the
subject’s weight, gender and age, and the biodistri-
bution of the labelled tracer (including the kinetics
of tracer uptake and its clearance from each target-
ted organ or system). The existence of any free or
unlabelled radionuclide or the presence of any phy-
siologically active agent or disease state can also
effect an influence upon the body’s handling of the
Introduction
In respect of radiation safety the sentinel node
technique has a great advantage in that it requires
the injection of relatively low levels of tracer
activity.As such the technique conveys little radia-
tion risk to the patient receiving the investigation
or to any of the staff groups involved in per-
forming the procedure, whether in the nuclear
medicine department, the operating theatre or
the histopathology laboratory. Establishing the
sentinel node procedure as a routinely perform-
ed diagnostic technique should entail only a
minimal revision of existing arrangements for
radiation protection. However, radiation must
always be used in the safest manner possible, and
to ensure this the operator needs to adhere both to
common sense radiation protection principles and
to the relevant legislative requirements.
This chapter is intended as an introduction to
the topics of radiation dosimetry and radiation
safety as applied to the sentinel node technique,
and the prospective user is strongly advised to
consult with both the hospital nuclear medicine
department and the locally appointed radiation
protection adviser (RPA) for further specific ad-
vice and information, particularly regarding local
radiation protection arrangements.
Principles of Radiation Dosimetry
The SI unit of absorbed radiation dose is the Gray
(Gy), defined as the deposition of 1 Joule of energy
per kilogram absorbing medium (e.g. body tis-
sue). The Gray replaces the rad, although the latter
unit is still often used. This quantity does not take
account of the relative biological damage to tissue
that is caused by the specific type of radiation
chapter
8
Table 1.
Radiation weighting factors for different types of
radiation
Ty pe of radiation
Radiation weighting
factor (w
T
)
X-rays,
g
-rays, electrons
1
Protons
5
Thermal neutrons
5
Fast neutrons
5 – 20 (dependent
on energy)
a
-Particles, fission fragments 20
