Radiation Measurements Units

Ionizing radiations are used to treat cancer. Receiving radiations in not without risk. This why the radiation delivered to a patient must be appropriate, mainly in quantity. There are many different physical quantities that can be used to express the amount of radiation delivered to a human body. One distiguishes two quantity: the concentration of radiation at certain point and the total radiation delivered to the entire body. We distinguishes also two units: the conventional Units that are the roentgen, rad and rem, and the SI: Système International d'unités. The conventional unit roentgen continue to be used.

1. Exposure

It expresses the concentration of radiation of x-rays or gamma-rays at some specified point in air. The concentration of photons over a surface is called fluence. The exposure is easily detected and measured in ionization chamber.

For this purpose, we use two units: the conventional unit is the roentgen (R) and the SI unit is the coulomb/kg of air (C/kg of air)

The cgs (centimeter-gram-second) unit roentgen is defined as the amount of ionization produced in 1 cm³ of air = 0.001293 g of air.

1 dyne = the force that gives to a mass of 1 gram an acceleration of 1 centimetre per second squared = = cm·g·s^-2 cgs = 1 x 10-5 N SI

1 e.s.u. = the charge that exerts a force of 1 dyne on an identical charge 1 cm distant in a vacuum. ε₀ ≈ 8.85 x 10^-12 C²/N.m² (1/4πε₀ x q²/1 cm² = 1 dyne Hence: q ≈ 3.336 x 10^-10C 1 e.s.u. = 3.336 x 10^-10C

Then:
1 roentgen = 1 esu/1 cm3 of air = 3.3356×10-10 C/0.001293 g = 2.58 x 10^-4 C/kg of air.

1 roentgen (R) = 2.58 x 10^-4 C/kg of air

If the exposure is uniform, the total exposure over the entire exposed area is called Surface Integral Exposure. It is the product of the exposure (at a certain point) and the surface of the body. It is expressed in R.cm². In the case that the exposure is not uniform, we sum or integrate the individual exposure concentration over the total exposed area.

2. Air Kerma

Kerma stands for Kinetic Energy Released per unit of MAss (of air). As the exposure, it expresses the radiation concentration delivered to a point. It fits into the SI scheme, because It is the energy (in Joules) deposited in 1 kg of air. It is, like the exposure, easily detected and measured in ionization chamber. An ionization chamber can be calibrated to read air kerma and exposure values.

3. Absorbed dose

If the medium is other than the air, we talk about absorbed dose (as in tissues or organs). The Kerma, could be then understood, as the absorbed dose in air. It is directly related to biological effects.T he absorbed dose is the radiation energy absorbed by a medium. It is expressed in the conventional units in radand in the SI units in gray (G) = 1 J/kg. The rad is equivalent to 100 ergs of energy absorbed in a gram of target material(tissue) and the gray is one joule of energy absorbed per kilogram of the material (tissue). The conversion is :
1 gray (Gy) = 100 rads
The total energy absorbed by the entire body is called the Integral dose. It is expressed in gram-rad (traditional units), or Joules (SI units).

4. Measurements in Medical Physics

If the exposure, air kerma, or SIE, in air, are relatively easy to measure, mainly by the mean of an ionization chamber device which we can palce in any location of our interest; It is not the case for medical treatements (human body). It is not practical to measure an absorbed dose in a tissue or an organ by inserting a device. We use instead indirect external means. These means include dosimeter that we can place on a surface of interest to treat, Mammography Mean Glandular Dose, and Computed Tomography (CT) procedures. These two special techniques use the established measurement to determined the final estimation of the absorbed dose.

4.1. Mammography Mean Glandular Dose

In mammography, the breast considered as containing 50% of glandular tissue and 50% of fat, the tretment focuses mainly in the glandular tissue because it is the tissue at risk from radiation. The special dose quantity used in then the Mean Glandular Dose (MGD). This method to determine this average dose takes into account the size and other features of the glandular tissue. Hence published factors are set. We determine first the entrance surface exposure or air kerma over the breast (tissue or organ of interest), using a dosimeter or other equipment device, and then use those published dose factors to calculate the related dose.

4.2. Computed Tomography (CT)

The CT uses the method of phantom. The phantom is a block of water or plasic that has the same radiation absorption properties as tissue, the same shape and size as the section of the body to be determined. A dosimeter is inserted in the middle of a phantom and gives the value of the absorbed dose. We correct this measured dose using related factors of a known radiation exposure.

In the CT treatment, the x-ray beam is rotated around the patient and passes through all sides. The distribution of absorbed dose within each slice is relatively uniform. The measured dose in the centre of a slice gives a good estimation of the absorbed dose, taking also into account the exposure from other adjacent slices. This procedure is not completely precise, this why we use the term CT dose index (CTDI). The total radiation to the whole patient body is the dose length product (DLP). The DLP is the product of the CTDI value and the length of the body scanned area.

5. Dose equivalent and effective dose

5.1. Dose equivalent

Even though the the physical quantities, such as absorbed doses, are the same, they might not give the same biological effect within aa irradiated body. It depends on the type of the used radiation. To have the same biological effect after irradiation, the unique dose to deliver is the dose equivalent. It is the product of the absorbed dose and the weighting factor W_R specific for each type of radiation (equal to 1 for radiations used in medical imaging as x-ray, gamma, beta, or positron). It is expressed in Sievert (Sv).

Dose Equivalent (Sv) = Dose (Gy) x W_R

5.2. Effective dose

The main idea here is that the organs and tissues of the human body have not the same effect when irradiated by a same radiation. Each member of the body has its own risk. Gonads are more sensitive. We define the Tissue weighting factor (W_T) for each specific area (tisue) of the body. The effective dose is the product of the absorbed dose and the weighting factor W_T.

Effective Dose (Gy) = Absorbed Dose (Gy) x w_T
The total effective dose is the sum of the effective doses for each exposed area, and the body total weighting factor is equal to 1.