Physics and Engineering of Radiation Detection

608 Chapter 11. Dosimetry and Radiation Protection

wherewTis the tissue weighting factor andHTis the equivalent dose in the tissue
T. The rational behind weighting the equivalent dose withwTis that, as explained
earlier, each tissue or organ responds differently to radiation. As with equivalent
dose, the effective dose is also measured in units of sievert. ThewT for different
tissues and organs are given in Table 11.2.2. The reader should note that these
numbers are based on the ICRP recommendations at the time of writing this book.
For the most up to date factors the reader is encouraged to refer to the most recent
ICRP publications.

Table 11.2.2: Tissue weighting factorswTaccording to 1990 recommendations of
ICRP(26).

Tissue or Organ wT

Gonads 0.20

Bone marrow (red), Colon, Lung, Stomach 0.12

Bladder, Breast, Liver, Oesophagus, Thyroid 0.05

Skin, Bone surface 0.01

Remainder 0.05

The effective dose as computed from the above relation is good for any single
tissue or organ and only one type of radiation. In case of mixed radiation exposure
to more than one tissues and organs, the total effective dose can simply by obtained
by adding the respective contributions together, that is

Etotal=

∑

i

wT,iHT,i, (11.2.5)

where the subscriptirefers to each organ.HT,iis the total equivalent dose for each
organias calculated from equation 11.2.3.

Example:

During a CT scan of the stomach, that had to be repeated several times, a

patient receives a total absorbed dose of 0.3Gy. Compute the total effective

dose received by the patient.

Solution:

Since CT scan is performed with x-rays therefore the radiation weighting factor

wR= 1. The equivalent dose received by the patient’s stomach is

HT,R = wR·DT,R

= (1)(0.3)

=0. 3 Sv.