Units of Measurements

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Units of Measurements

Units of measurements are different for different medium (air, water).  Till now below mentioned units are used to find the absorbed dose and also to represent the dose limits in the field of radiotherapy. They are,

1. Exposure

2. Absorbed Dose

3. Equivalent Dose

4. Effective Dose

Exposure (X):

 It is defined as the quotient of dQ by dm where dQ is the absolute value of the total charge of the ions of one sign produced in air when all the electrons (negatrons and positrons) liberated by photons in air of mass dm are completely stopped in air.

X= dQ/dm

The SI unit of exposure is C/Kg and special unit is Roentgen.

Absorbed Dose:

It is defined as the absorbed dose, or simply dose, is the quotient  where dE/dm is the mean energy imparted by ionizing radiation to material of mass dm (1). The old unit of dose is rad (an acronym for radiation absorbed dose) and represents the absorption of 100 ergs of energy per gram of absorbing material.

The SI unit of absorbed dose is Gy and it is defined as ,

1 Gy = 1J/Kg

The relationship between the Gray and Rad is

1Gy = 100rad

Equivalent Dose (H):

As we know that different types of radiation are used for the treatment of cancer, all the radiation will not produce the same biological effect.  It will be different for different radiation like alpha, beta, and gamma etc. so, that when we are encounter the absorbed dose to the person should include this phenomenon also to consider.

It is defined as the product of absorbed dose and the radiation weighting factor (wr).

                H = D x wr    

The SI unit of Equivalent Dose is Sievert (Sv). One Sievert is equal to 100 rem

The radiation weighting factors for different types of radiation are mentioned below Radiation    
                                                 weighting factors

1. X-rays, gamma rays, beta particles, and electrons–––––––––  1       
2. Protons (>2 MeV) and charged pions ––––––––––––––––––––––––– 2      
3. Alpha particles and other multiple-charged particles –––––– 20

4. Neutrons––––––––––––-- A Continuous function of Neutron Energy

Effective Dose (E):

Not only that the different types of radiation produce different biological effect , that biological effect is also vary according to the sensitive organ or tissue for the radiation. For that ICRP 60 introduced the tissue weighting factors for different organs and tissues are shown in the table below.

Effective dose is defined as the product of Equivalent Dose and Tissue weighting factors (wt)

                E = ∑T  x wT x HT    

The SI unit of effective dose is same as equivalent dose.

Recommended tissue weighting factors:

Tissue                Tissue weighting factor, wT                Sum of wT values

Bone-marrow (red),             0.12             0.72
colon, lung,stomach,
 breast, remainder tissues (a)

Gonads                              0.08             0.08

Bladder, oesophagus,                                  0.04             0.16
liver, thyroid.
Bone surface, brain,                          0.01                                               0.04
 salivary glands, skin.                                              
                                                       Total                     1.00                  

(a-    Remainder tissues: Adrenals, extrathoracic (ET) region, gall bladder, heart, kidneys,lymphatic nodes, muscle, oral mucosa, pancreas, prostate, small intestine, spleen, thymus, uterus/cervix.)


1. What is the radiation weighting factor for alpha particles

a) 20

b) 10

c) 15

d) All

2. What is the SI unit of Exposure?

a) C/Kg
b) Roentgen
c) Gray
d) Sievert


1. a) 20
2. a) C/Kg


1. The Physics of  Radiation Therapy. 4th Edition, by F.M.Khan

2. The Essential  Physics of Medical Imaging by J.T. Bushberg, J.Antony Siebert, Edwin M. Leidholdt

3. http://www.icrp.org

4. http://iopscience.iop.org

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