# Effective dose, E

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The equivalent dose is a measure of the harm from radiation to a particular tissue. A dose of 1 mSv, for example, to the liver will give rise to the same cancer risk regardless of the type of radiation concerned.

However, different tissues show different sensitivities to radiation. The thyroid is less sensitive than other tissues. In addition, following intake, some radionuclides will buildup in particular organs and irradiate them preferentially. Iodine isotopes, for example, concentrate in the thyroid whereas plutonium concentrates in the liver and bone. In order to take these effects into account, equivalent doses in different tissues must be weighted. The resulting effective dose is obtained using $E=\Sigma_T (w_T*H_T)$

where $H_T$ is the equivalent dose in tissue or organ T and $w_T$ is the tissue weighting factor. A summary of tissue weighting factors is given in the table below.

  Tissue wT ΣwT Bone marrow (red), Colon, Lung, Stomach, Breast, Remainder tissues* 0.12 0.72 Gonads 0.08 0.08 Bladder, Oesophagus, Liver, Thyroid 0.04 0.16 Bone Surface, Brain, Salivary glands, Skin 0.01 0.04 Total 1.00 * Remainder tissues: Adrenals, Extrathoracic (ET) region, Gall bladder, Heart, Kidneys, Lymphatic nodes, Muscle, Oral mucosa, Pancreas, Prostate(♂), Samll intestine, Spleen, Thymus, Uterus/cervix(♀).

Further Information:

The ICRP weighting factors

Dosimetry & Shielding

Calculation of the Effective Dose:

Example 1

Suppose just the thyroid is irradiated by beta radiation and received an average absorbed dose of 400 mGy. What is the effective dose (E)?

Equivalent dose (HT) = 400 mGy x 1 = 400 mSv

Effective dose (E) = 400 mSv x 0.05 = 20 mSv

This dose would give rise to the same detriment as if all the organs in the body had been ex-posed to a total of 20 mGy.

Example 2

In the course of a medical therapy, a patient receives an intake of I-131 to the thyroid. The thyroid receives a dose of HT = 80 mSv. What is the whole body effective dose?

The thyroid tissue weighting factor (from the table) is wT = 0.05. If there are no additional organs irradiated, the effective dose is given by

E = wT . HT = 0.05 x 80 mSv = 4 mSv.

Hence the risk of cancer arising in the thyroid due to the 80 mSv irradiation, is exactly the same total cancer risk following a homogeneous whole body irradiation of 4 mSv. By a homogeneous whole body irradiation, every organ receives a dose of 4 mSv. Since the sum of the tissue weighting factor is 1, the whole body dose or the effective dose is 4 mSv.

Example 3

A patient receives an intake of radiation which results in the lungs receiving a dose of 2 mSv and the thyroid a dose of 1mSv. What is the whole body effective dose?

The lungs tissue weighting factor (from the table) is wT = 0.12. The thyroid tissue weighting factor (from the table) is wT = 0.05. If there are no additional organs irradiated, the effective dose is given by

E = wT . HT = (0.12 x 2) + (0.05 x 1) mSv = 0.29 mSv.

Hence the risk of cancer arising from the lungs receiving 2 mSv and the thyroid receiving 1mSv, is exactly the same total cancer risk following a homogeneous whole body irradiation of 0.29 mSv. By a homogeneous whole body irradiation, every organ receives a dose of 0.29 mSv. Since the sum of the tissue weighting factor is 1, the whole body dose or the effective dose is 0.29 mSv.

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