Quantities Of Dosimetry

USE OF RADIATION EFFECT IN BODY TISSUES SKILLS IN DETERMINING EXPOSURES DOSE TO PATIENT (QUANTITIES RELATED TO DOSIMETRY)

RADIATION SCIENCES

Quantities Related to Dosimetry

• Radiation Exposure and Dose

Roentgen (R)

Absorbed Dose

Equivalent Dose

Effective Dose

• Flux or Fluence Rate
• Integrated Flux or Fluence

Kerma, Cema, and Terma

ABSORBED DOSE

E- Mean energy imparted to matter of mass m

• Unit: SI unit= Gray 100 rads=1 Gy

Old unit=Rads 1 Gy= 1J/kg

Apart from Gy, there is another unit of absorbed dose called rad.

Even though rad has mostly been replaced by Gy it is still found in some modern literature.

Rad was introduced in 1953 to replace Roentgen, which was the unit of exposure due to x-rays or γ-rays only.

It is defined as the dose equivalent to the absorption of 0.01 joule of energy per kilogram of tissue

Imparted energy

• It is the total amount of energy deposited in matter
• It is the product of dose & mass over which the energy is imparted

Unit: J

Equivalent Dose

• Absorbed dose is not capable of characterizing the biological effect of radiation

Absorbed dose is not capable of characterizing damage to any medium All it tells us is how much energy has been absorbed by the medium and not what this deposited energy has done to the medium For this case of absorbed dose, there is no difference between a photon and an α-particle if they deposit the same amount of energy.

When it comes to the effects of radiation, one can not use absorbed dose as the relevant quantity.

Since dosimetry is primarily concerned with the safety of personnel, therefore a quantity called equivalent dose has been defined that characterizes the damaging effect of radiation on tissue

Equivalent dose is the product of absorbed dose received by tissue (T) from radiation (R) and radiation weighting factor

• It is denoted by HT.R

HT.R= DTR X WR

WR= Radiation weighting factor

Formely called quality factor basically defines the quality of the radiation that is interacting with the matter Quality of a radiation depends upon two parameters LET & RBE

HT.R s the equivalent dose due to radiation type R,

• DTR is the mean absorbed dose delivered by radiation R, and
• WR is the radiation weighting factor.
• The radiation weighting factor is given by

WR = QR · NR

where QR and NR are the quality and modified factors for the radiation type R respectively

Quantities Of Dosimetry

In case of mixed field, the total equivalent dose can be obtained by simply summing the contribution due to individual types of radiation, that is HT = R wR · DTR.

Quantities Of Dosimetry

Example: In a mixed radiation environment, a person receives 20 mGy of γ-ray dose and 2 mGy of slow neutron dose. Calculate the total equivalent dose received by the person.

Solution

As the source is external, we can take NR = 1 and the weighting factors for the two radiation types as given as wγ = 1 and wn = 5.

The equivalent doses due to γ-rays and neutrons are given by

• HT,γ = wγ · DT,γ = (1)(20) = 20 mSv

HT,n = wn · DT,n = (5)(2) = 10 mSv

The total dose received by the person is then sum of these individual doses, that is HT = HT,γ + HT,n = 20 + 10 = 30 mSv

EFFECTIVE DOSE

The equivalent dose as described above can be used for one tissue type only as it does not address the sensitiveness of tissue types to the same type of radiation.

The question is, how we can determine the whole body equivalent dose to estimate the risk associated with a certain type of radiation environment?.

Or how one can estimate the whole body dose if the dose received by a particular organ is known.

This is done by using the quantity effective dose

Effective dose

Can be defined as the product of equivalent dose and tissue weighting factor Since all tissues doesn’t have the same sensitivity towards radiation, so each tissue is given a factor based on the response to radiation This factor is called tissue weighting factor It is denoted by E

• E=Sum(HTR X WT)

WT= Tissue weighting factor

Unit= SI unit= Sievert (Sv)

Quantities Of Dosimetry

Example: During a CT scan of the stomach, that had to be repeated several times, a patient receives a total absorbed dose of 0.3 Gy. 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

The tissue weighting factor for stomach is WT = 0.12 as given in Table

The effective dose is then give by E = WT · HT,R = (0.12)(0.3) = 0.036 Sv = 36 mSv.

The usual effective dose received during a typical CT scan of abdomen is around 10 mSv, which means that this patient received more than three times the usual dose.

KERMA

Acronym for kinetic energy released in matter K= Sum of kinetic energy all charged particle liberated by ionizing radiation per unit weight of matter K= dE/dm

• Unit is J/kg

It is different from absorbed dose as some of the K.E escapes from the absorbing volume