Physics and Radiobiology of Nuclear Medicine

the same probability of cancer induction in that organ. For example, a dose
of 3 rem to the whole body causes some probability of cancer induction; a
dose of 100 rem to the thyroid causes the same numerical probability of
thyroid cancer induction. Then the WTfor thyroid is equal to 0.03.
The effective dose equivalent(HE) is defined as the sum of weighted dose
equivalents in all tissues and organs, and is calculated as

(14.25)

where WTis the tissue weighting factor for an organ and HTis the dose
equivalent (rem) to the organ.HEcan be explicitly written as

(14.26)

where (rad)T, ris the absorbed dose to tissue Tfrom radiation of type rand
Wris the radiation weighting factor discussed earlier.
The effective dose equivalent provides an overall risk estimate for an
individual exposed to radiation, which is computed from dose equivalent
to each organ that is weighted for tissue sensitivity. For assessment of risk
versus benefit, the effective dose equivalent is a more appropriate para-
meter than the whole-body dose, because it takes into consideration the dif-
ferent tissue sensitivities of the organ. The WTvalues are assigned such that
their sum equals one. These WTvalues from ICRP 26 have been adopted
by the NRC (10CFR20).
In 1990, ICRP adopted a different set of WTvalues and renamed the
effective dose equivalent as simply the effective dose (ED) (ICRP 60).
Table 14.5 summarizes the WTvalues recommended by ICRP for both EDE

HWWET

T

r

r

=×∑∑ ()radTr,

HWHETT

T

=∑

Effective Dose Equivalent and Effective Dose 221

Table14.5. Tissue weighting factors WT.

Tissue WTfor EDE* WTfor ED†

Gonads 0.25 0.20

Breast 0.15 0.05

Thyroid 0.03 0.05

Bone surfaces 0.03 0.01

Bone marrow (red) 0.12 0.12

Lung 0.12 0.12

Colon not given 0.12

Stomach not given 0.12

Bladder not given 0.05

Liver not given 0.05

Esophagus not given 0.05

Skin not given 0.01

Remainder 0.3 0.05

Total Body 1.0 1.0

*WTvalues from 10CFR20. NRC adopted these values from

ICRP 26 (1977).

†Adapted with permission from ICRP 60.1990 Recommen-

dations of International Commission of Radiological Protec-

tion.New York: Elsevier, 1991.