Radiation in medicine

State the meanings of the terms exposure, absorbed dose, quality factor (relative

biological effectiveness) and dose equivalent as used in radiation dosimetry.

Students should be able to discuss the significance of these quantities in radiation dosimetry.

Exposure

Intensity of Radiation (Exposure)The roentgen (R) is a measure of radiation intensity of x-rays or gamma rays. It is formally defined as the radiation intensity required to produce and ionization charge of 0.000258 coulombs per kilogram of air. It is one of the standard units for radiation dosimetry, but is not applicable to alpha, beta, or other particle emission and does not accurately predict the tissue effects of gamma rays of extremely high energies. The roentgen has mainly been used for calibration of x-ray machines.

X = Q/mCoulombs per Kg

Absorbed dose (J/kg)The charge deposited per Kg of air does not take into account the type of tissue and therefore the energy deposition.

The absorbed dose is equal to the energy deposited per unit mass of the medium, and so has the unit J/kg, which is given the special name Gray (Gy).

Note that the absorbed dose is not a good indicator of the likely biological effect.

The dose equivalent is a measure of the radiation dose to tissue where an attempt has been made to allow for the different relative biological effects of different types of ionising radiation. Equivalent dose is therefore a less fundamental quantity than radiation absorbed dose, but is more biologically significant. Equivalent dose has units of sieverts.

    Radiation type Quality factor

       X -rays            1

      gamma            1

       beta            1

       alpha          20

    slow neutrons             5

     fast neutrons           10

        protons              5

Discuss the precautions taken in situations involving different types of radiation.

Students should consider shielding, distance and time-of-exposure factors. They should be familiar with the film badge.

TOK: They should appreciate that current practice is determined from a gradual increase in available data.

There is a light-proof packet of photographic film inside the badge. The more radiation this absorbs, the darker it becomes when it is developed. To get an accurate measure of the dose received, the badge contains different materials that the radiation must penetrate to reach the film. These may include aluminium, copper, lead-tin alloy and plastic. There is also an open area at the centre of the badge.

Alpha Particles (2n, 2p)

Beta Particles (e-or+)

Photons (hv)

(x or gamma rays)

Paper Concrete

Penetrative power

Minimizing Radiation Minimizing Radiation ExposureExposure

Basic ConceptsBasic Concepts

• TimeTime

• DistanceDistance

• ShieldingShielding

Minimizing Exposure - TimeMinimizing Exposure - Time

• Minimize the Minimize the amount of time amount of time spent near spent near sources of sources of radiation.radiation.

Minimize Exposure by Minimize Exposure by Maximizing DistanceMaximizing Distance

As the distance from a radioactive source doubles, the exposure rate

decreases by a factor of four.

Minimizing Exposure By Minimizing Exposure By Using ShieldingUsing Shielding

Lead shielding Lead shielding around radiation around radiation sourcessources

Surry Power Station

The type of shielding required depends on the type of radiation present.

Loss of Life ExpectancyLoss of Life ExpectancyActivity or Behavior LLE (DAYS)__________________

Recreational swimming 40 Being 15 percent overweight 900 Smoking 20 cigarettes per day 1,600 Using pesticides at home 12 Being exposed to radon in a home 35

Living within 10 miles of a nuclear power plant 0.4 Riding a bicycle 6 Driving a car 200

Skydiving 25 Consuming alcohol (U.S. average) 230

Discuss the concept of balanced risk.

Aim 8, Int, TOK: Students should appreciate that codes of practice have been developed for conduct involving the use of radiations.

Alara principle

Distinguish between physical half‑life, biological half-life and effective half‑life.

Students should be able to calculate the effective half-life from the physical half-life and the biological half-life.

Radiation DoseRadiation Dose Dose or radiation dose is a generic term for a measure of radiation exposure. In radiation protection, dose is expressed in millirem.

X-Ray Machine Image

(film)Subject is not radioactive but has been exposed to a radiation dose (single chest x ray = 5-10 mrem).

After

External Dose

ContaminationContamination Contamination is the presence of a radioactive Contamination is the presence of a radioactive

material in any place where it is not desired, material in any place where it is not desired, and especially in any place where and especially in any place where

its presence could be harmful.its presence could be harmful.

Yuck!

Solve problems involving radiationdosimetry.

1. This question is about radiation used in medicine.(a) Define the terms exposure and absorbed dose. (2)Exposure: .........................................................................................................

.........................................................................................................Absorbed dose: .........................................................................................................

.........................................................................................................(b) Explain, with reference to α and γ radiation, the distinction between absorbed dose and dose equivalent..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (3) (c) Explain why, when using radioactive tracer elements in the treatment of cancer, it is better to use radioactive isotopes that have a long physical half-life and a short biological half-life..................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (2)(Total 7 marks)

1. (a)exposure:total ionised charge produced in unit mass of airby a particular radiation;

absorbed dose: energy absorbed per unit mass; 2 (b) dose equivalent is the amount of energy absorbed;but a quality factor is introduced to describe the effects of differenttypes of radiation;α is absorbed more that γ radiation and so has a much higher Q factor; 3Do not look for this precise wording but look for the understanding. (c) if the biological half-life is long then the tracer can do a lot of damageto healthy cells;with a short biological half-life and long physical half-life the tracerwill have a high activity during the time it is in the body; 2Again do not look for this precise wording but look for the understanding.[7]

2. This question is about dosimetry.(a) Describe what is meant by the term relative biological effectiveness (quality factor)........................................................................................................................................................................................................................................................................... (2)

The whole body of a person of mass 70 kg is exposed to monochromatic X-rays of energy 200 keV. As a result of this exposure, the person receives a dose equivalent of 500 μSv in 2.0 minutes.(b) Deduce that the person absorbs about 1010 X-ray photons per second............................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................... (4)(Total 6 marks)

2.(a) a factor that compares (the effectiveness) of different types of radiationto that of X-rays;some detail e.g. because different radiations (of the same intensity)produce different amounts of ionization / cause different amounts of damage; 2Second marking point can be implied in first point. (b) D = 5 × 10–4 J kg–1 (Q = 1);E = m × D = 70 × 5 × 10–4 J;1 photon has energy 200 × 103 × 1.6 × 10–19 J;number of photons in 1 second = » 1010; 4[6]

Outline the basis of radiation therapy for cancer.

Radiation TherapyRadiation TherapyUsed for treating cancer. Why does it work? Used for treating cancer. Why does it work?

External Beam Brachytherapy (implants)

Image courtesy of

Photo by Karen Sheehan

X-rays: RadiotherapyThis should include the differential effects on normal and malignant cells, as well as a description of the types of sources available.

Cancer occurs when cells divide too quickly. The dividing cells become a tumour which can damage surrounding tissue or spread to the rest of the body

X-rays: Radiotherapy

• X-rays or other radiation can damage the DNA in cells and kill them

• This is why radiation can be dangerous

• But cells which are dividing rapidly are more likely to be killed

• So we use x-rays to kill the rapidly-dividing cancer cells

• We must still ensure that healthy tissue is undamaged

X-rays: Radiotherapy

A linear accelerator generates x-rays. It rotates around the body, irradiating the tumour

from all directions

X-rays: Radiotherapy

X-ray CT scan of chest shows lungs, heart and tumour (red)

X-rays: Radiotherapy

A medical physicist decides which angles to shine x-rays fromto destroy tumour and minimise damage to other tissue

X-rays: Radiotherapy

The treatment plan lists the directions the x-rays will come from and calculates the radiation dose to the tumour (in purple) and rest of body (grey)

X-rays: Radiotherapy

Solve problems involving the choice of radio‑isotope suitable for a particular diagnostic or

therapeutic application.

Students should be familiar with a variety of techniques. Where reference is made to a specific technique, sufficient description will be given forthe student to be able to answer any questions on that technique.

Things to consider;

Type of radiation. How far do you want it to travel in the body?

Half life. How long do you want the substance to be radioactive for?

Solve problems involving particulardiagnostic applications.

For example, assessment of total blood volume. Where reference is made to a specific technique, sufficient description will be given for the studentto be able to answer any questions on that technique.

Nuclear MedicineNuclear MedicineDiagnostic ProceduresDiagnostic Procedures

• Radioactive injectionRadioactive injection• Short half-life Short half-life

radionuclide radionuclide • Pictures taken with Pictures taken with

special gamma cameraspecial gamma camera• Many different studies:Many different studies:

ThyroidThyroidLungLungCardiac Cardiac White Blood CellWhite Blood Cell Photo by Karen Sheehan

Bone ScansBone Scans

Image courtesy of

Tsokos

Page 717 Q’s 1-9.

IB review pack Q’s 4,9,17,20,25,30.