PART 3 - الفيزياء الطبية



RADIATION AND DOSE MEASUREMENTS

In radiation protection we need to be able to determine the potential hazard from radiation. To do this we need to measure the quantities which describe the radiation field (known as radiometric quantities) and we also need to measure the effects produced by the radiation dose (known as dosimetric quantities).

1 Energy

The energy of ionizing radiation is measured in terms of electron volts (eV), where one electron volt is the amount of energy gained by an electron when it is accelerated through a potential difference of one volt. In terms of joules:

1 eV = 1.6 x 10-19 J

The electronvolt is a very small unit of energy, even in atomic terms, so in practice the energy of radiation is usually given in kiloelectronvolts (keV) or megaelectronvolts (MeV).

2 Exposure (X)

Historically, x-rays and gamma rays were quantified by the amount of ionization they produced in air (i.e. the exposure). The special unit of exposure (symbol X) was originally called the roentgen (R), named after the discoverer of x-rays, Wilhelm Roentgen. The SI unit of exposure is the coulomb per kilogram (C kg-1) and this is related to the roentgen as follows:

1 R = 2.58 x 10-4 C kg-1 of air

The roentgen is no longer used in radiation protection but there are many instruments which give readings in roentgen (R) or roentgen per hour (R h-1).

3 Kerma (K)

The term kerma refers to the kinetic energy released per unit mass of absorber and is basically a measure of the kinetic energy of charged particles produced in an absorbing medium by uncharged radiations (i.e. photons and neutrons). When the absorbing medium is air, the term air kerma is used.

The unit of kerma is the joule per kilogram and it is given the special name gray (Gy):

1 Gy = 1 J kg-1

For x- and gamma-ray energies up to 1 MeV, the interactions which take place between the radiation and the particles of air are nearly the same for both exposure measurement and air kerma measurement. At x- and gamma ray energies above 1 MeV interactions occur which lead to different measurements of air kerma compared with exposure. For example, cobalt-60 has two gamma radiations with energies 1.173 MeV and 1.333 MeV and the measurement of air kerma produced by these radiations will be about 0.3% higher than if only exposure is considered.

4 Summary of radiometric quantities

Table 1 summarises the radiometric quantities.

Table 1

Summary of Radiometric Quantities

|Quantity |Symbol |SI Unit |SI Unit Name |Conversion |

|Energy | |Joule |electronvolt |1 eV = 1.6 x 10-19 J |

| | |J |(eV) | |

|Exposure |X |Coulomb per kilogram | |1 R= 2.58 x 10-4 C kg-1 |

| | |C kg-1 | | |

|Kerma |K |Joule per kilogram |gray |1 Gy = 1 J kg-1 |

| | |J kg-1 |(Gy) | |

2 Dosimetric Quantities

The term dose is a very general term which applies to the amount of energy deposited when radiation passes through a material. It is often used very loosely and, depending on the context, may actually mean absorbed dose, equivalent dose, effective dose or even exposure or kerma. The following sections explain the basic dosimetric quantities used in radiation protection.

1 Absorbed dose (D)

Absorbed dose is a measure of the energy deposited in any medium by any type of radiation. It is given the symbol D.

The SI unit of absorbed dose, like that of kerma, is the joule per kilogram (J kg-1) and is given the name of gray (Gy). However, when we are talking about absorbed dose, it is very important to specify the type of material in which the energy is being deposited, for example 1.3 mGy absorbed dose to water.

The original unit of absorbed dose was the radiation absorbed dose (rad) and one gray is equal to 100 rad.

1 Gy = 100 rad or 1 mrad = 10 μGy

In radiation protection, we often need to convert between old (non-SI) and new (SI) units for dose and dose rates. The following example shows how this conversion is performed for absorbed dose.

EXAMPLE

Question

Convert the following absorbed doses and dose rates between old and new units:

a) 0.4 mrad to Gy.

b) 7.5 (Gy h-1 to rad h-1.

Answer

a) 1 mrad = 10 (Gy

So 0.4 mrad = 0.4 x 10 (Gy = 4 (Gy

Hence 0.4 mrad is equal to 4 (Gy or 4 x 10-6 Gy

b) 1 Gy h-1 = 100 rad h-1

or 1 (Gy h-1 = 0.1 mrad h-1

So 7.5 (Gy h-1 = 7.5 x 0.1 mrad h-1 = 7.5 x 10-4 rad h-1 = 0.75 mrad h-1

Hence 7.5 (Gy h-1 is equal to 0.75 mrad h-1 or 7.5 x 10-4 rad h-1

2 Equivalent Dose (H)

Absorbed dose tells us how much energy is deposited in an absorbing material but it does not tell us how much damage may be done to tissue, nor does it indicate the level of the potential hazard. For example, the level of damage produced by an absorbed dose to tissue of 0.5 Gy would be very much greater if the energy were deposited by alpha radiation or by neutrons than it would be if the energy were deposited by gamma radiation. Hence, a quantity known as equivalent dose is used as a measure of the biological effect of a particular type of radiation on organs or tissues. It is calculated by multiplying the absorbed dose to an organ or tissue (measured in gray) by a dimensionless factor called the radiation weighting factor (wR). Radiation weighting factors as recommended in ICRP 60 are given in Table 2.

Table 2

Radiation Weighting Factors

|Type and Energy Range |Radiation Weighting Factor (wR)|

|Alpha particles, all energies |20 |

|Beta particles, all energies |1 |

|Gamma and x-rays, all energies |1 |

|Neutrons: | |

| 100 keV to 2 MeV |20 |

| > 2 MeV to 20 MeV |10 |

| > 20 MeV |5 |

Equivalent dose (symbol H) is defined as shown in Equation 1 for a particular type of radiation interacting with a particular organ or tissue:

[pic] [1]

where [pic] is the equivalent dose to an organ or tissue T delivered by radiation type R

[pic] is the absorbed dose to an organ or tissue T delivered by radiation type R

[pic] is the radiation weighing factor for radiation type R

The SI unit for equivalent dose is also the joule per kilogram but it is given the special name of sievert (Sv) to distinguish it from absorbed dose.

1 Sv = 1 J kg-1

Example 2 shows how the equivalent dose may be determined from absorbed dose.

If an organ is irradiated by more than one type of radiation, it is necessary to sum the equivalent doses to account for the different types of radiation. Equation 2 shows how the total dose to a particular organ from a variety of types of radiation may be determined:

[pic] [2]

where [pic] is the total equivalent dose to an organ or tissue T delivered by all radiation types

[pic]indicates the sum for each radiation type R

[pic] is the absorbed dose to an organ or tissue T delivered by radiation type R

[pic] is the radiation weighing factor for radiation type R

The original unit of equivalent dose was the rem where one sievert is equal to 100 rem.

1 Sv = 100 rem

or 1 mrem = 10 μSv

Example 4 shows how we can convert between these old and new units:

3 Effective Dose (E)

Some tissues and body organs are more sensitive to radiation than others and an equivalent dose in one organ may be more hazardous than the same equivalent dose in another organ. The ICRP recommends tissue weighting factors (wT) which are applied to specific body organs. These dimensionless factors take into consideration the different radiosensitivities of the different organs and tissues. Table 3 lists the tissue weighting factors as recommended in ICRP 60.

Table 3

Tissue Weighting Factors

|Tissue |Tissue Weighting Factor (wT) |

|Gonads |0.20 |

|Bone marrow (red) |0.12 |

|Colon |0.12 |

|Lung |0.12 |

|Stomach |0.12 |

|Bladder |0.05 |

|Breast |0.05 |

|Liver |0.05 |

|Oesophagus |0.05 |

|Thyroid |0.05 |

|Skin |0.01 |

|Bone Surface |0.01 |

|Remainder |0.05 |

The effective dose (symbol E) is the total effective dose for all exposed organs or tissues is the sum of the tissue equivalent doses multiplied by the appropriate tissue weighting factor for those organs and tissues (see Equation 3):

[pic] [3]

where E is the total effective dose to all exposed organs and tissues

[pic]indicates the sum for each organ or tissue type T

[pic] is the equivalent dose to an organ or tissue T

[pic] is the tissue weighing factor for an organ or tissue type T

Note that the effective dose to the whole body is generally the most useful quantity in radiation protection. This is because, in most work environments, the whole body effective dose is more likely to approach dose limits than individual organ or tissue doses are. Assuming uniform irradiation to the whole body, the total effective dose can be calculated either by summing the effective doses to each individual organ or tissue, or more simply we can just assume a tissue weighting factor of 1 for the whole body.

4.2.4 Committed Dose

The quantities discussed so far have come from radiation sources outside the human body. However, as you have learnt in Module 2.3 Protection from Internal Radiation Hazards, if an intake of radioactive material occurs, it will continue to irradiate the body until either the radioactivity has decayed away or the body has excreted the substance. You will also remember that once the radioactive material is inside the body, it may be distributed throughout the body or it may concentrate in particular organs. We therefore also need to be able to indicate doses which arise from intakes of radioactive materials and these doses are known as committed doses.

Committed dose is defined as the dose accumulated by the body over 50 years following the intake (except in the case of intakes by children where it is defined as the dose accumulated until the age of 70). Committed dose can be committed absorbed dose, committed equivalent dose or committed effective dose. It is given the symbols D(50), H(50), or E(50) where 50 represents the number of years over which the dose is being calculated.

4.2.5 Summary of dosimetric quantities

Figure 1 and Table 4 summarise the dosimetric quantities and their units.

Figure 1

A Summary of Dosimetric Quantities

Table 4

Summary of Dosimetric Quantities and Their Units

|Quantity |Symbol |SI Unit |SI Unit Name |Conversion |

|Absorbed Dose |D |Joule per kilogram |gray |1 Gy = 100 rad |

| | |J kg-1 |(Gy) | |

|Equivalent Dose |H |Joule per kilogram |sievert |1 Sv = 100 rem |

| | |J kg-1 |(Sv) | |

|Effective Dose |E |Joule per kilogram |sievert |1 Sv = 100 rem |

| | |J kg-1 |(Sv) | |

Figure 2 compares the average levels of occupational and medical exposures with the average radiation levels from natural sources (see section 4.2 for the definitions of dose).

* From UNSCEAR Publication 2000

Figure 2

Comparison of Levels of Exposure

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Average Annual Occupational Exposure

(Nuclear Industry)

2.4 mSv*

Average Annual Exposure from Natural Background Radiation

Source Activity

(Bq)

Average Annual

Medical Exposure

(Diagnostic Procedures)

0.4 mSv*

Effective dose

to the whole body

[pic]

(Sv)

Absorbed dose (D)

to any medium

(Gy)

Equivalent dose

to a particular organ

[pic]

(Sv)

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