2.2 Radiation Dose and Dose Rate Determinations
13
(RBE) and they range from 1 to 20. This is called the equivalent dose and it is
measured in units of Sieverts (Sv) internationally and rem in the United States.
Multiplying the absorbed dose by the RBE tells us how much biological damage—
the risk of developing cancer in the future as well as the risk of developing shortterm ailments such as bone marrow or lung damage—was caused by the radiation
exposure. For example, exposure to enough alpha radiation (which has an RBE of
20) to deposit 1 J/kg would produce an absorbed dose of 1 Gy and an equivalent dose
of 20 Sv.
There are times that ingested or inhaled radioactivity will travel preferentially
to a single organ—I-131, for example, is absorbed primarily by the thyroid while
uranium will concentrate in the kidneys and the bones. Different tissues also have
different sensitivities to radiation and are more or less liable to develop cancers; if
we are to determine the risk to a person from an intake of such a radionuclide we
must understand better how a particular organ is affected by the radiation and the
risk this poses to the person. For this reason, each of the body’s major organs and
organ systems have been assigned an organ weighting factor that is used to determine
the effective dose, which can be used to determine the risk to the person exposed as
though their entire body had been exposed. These organ weighting factors are shown
in Table 2.5.
For example, a radiation exposure of 1 Sv to the red bone marrow (which has an
organ weighting factor of 0.12) produces an effective dose of 0.12 Sv, or 120 mSv.
These terms can also be combined. A person who inhales enough Rn-222 to deposit
0.1 J per kg of lung tissue would receive a lung dose equivalent of 2 Sv (0.1 Gy × 20)
because the alpha radiation that Ra-226 emits has a relative biological effectiveness
(RBE) of 20; and an effective dose equivalent of 0.24 Sv (2 Sv to the lung × 0.12
organ weighting factor). The effective dose equivalent (abbreviated EDE) can help
us to determine the risk to the person exposed from the radiation to which they were
exposed, even if the radiation or radioactivity is only administered to a small part of
the body.
Table 2.5 Radiation
weighting factors for various
tissues [3]
Tissue
Weighting
factor
Tissue
Weighting
factor
Gonads
0.08
Esophagus
0.04
Red bone
marrow
0.12
Thyroid
0.04
Colon
0.12
Skin
0.01
Lung
0.12
Bone surface
0.01
Stomach
0.12
Salivary
glands
0.01
Breasts
0.12
Brain
0.01
Bladder
0.04
Rest of body
0.12
Liver
0.04
Total
1.00
13
(RBE) and they range from 1 to 20. This is called the equivalent dose and it is
measured in units of Sieverts (Sv) internationally and rem in the United States.
Multiplying the absorbed dose by the RBE tells us how much biological damage—
the risk of developing cancer in the future as well as the risk of developing shortterm ailments such as bone marrow or lung damage—was caused by the radiation
exposure. For example, exposure to enough alpha radiation (which has an RBE of
20) to deposit 1 J/kg would produce an absorbed dose of 1 Gy and an equivalent dose
of 20 Sv.
There are times that ingested or inhaled radioactivity will travel preferentially
to a single organ—I-131, for example, is absorbed primarily by the thyroid while
uranium will concentrate in the kidneys and the bones. Different tissues also have
different sensitivities to radiation and are more or less liable to develop cancers; if
we are to determine the risk to a person from an intake of such a radionuclide we
must understand better how a particular organ is affected by the radiation and the
risk this poses to the person. For this reason, each of the body’s major organs and
organ systems have been assigned an organ weighting factor that is used to determine
the effective dose, which can be used to determine the risk to the person exposed as
though their entire body had been exposed. These organ weighting factors are shown
in Table 2.5.
For example, a radiation exposure of 1 Sv to the red bone marrow (which has an
organ weighting factor of 0.12) produces an effective dose of 0.12 Sv, or 120 mSv.
These terms can also be combined. A person who inhales enough Rn-222 to deposit
0.1 J per kg of lung tissue would receive a lung dose equivalent of 2 Sv (0.1 Gy × 20)
because the alpha radiation that Ra-226 emits has a relative biological effectiveness
(RBE) of 20; and an effective dose equivalent of 0.24 Sv (2 Sv to the lung × 0.12
organ weighting factor). The effective dose equivalent (abbreviated EDE) can help
us to determine the risk to the person exposed from the radiation to which they were
exposed, even if the radiation or radioactivity is only administered to a small part of
the body.
Table 2.5 Radiation
weighting factors for various
tissues [3]
Tissue
Weighting
factor
Tissue
Weighting
factor
Gonads
0.08
Esophagus
0.04
Red bone
marrow
0.12
Thyroid
0.04
Colon
0.12
Skin
0.01
Lung
0.12
Bone surface
0.01
Stomach
0.12
Salivary
glands
0.01
Breasts
0.12
Brain
0.01
Bladder
0.04
Rest of body
0.12
Liver
0.04
Total
1.00