Radiation & Clinical applications

Contents SUBJECT

CHAPTER

1

Introduction

2

Atomic Structure

3

Radioactive Transformation

4

General characteristics of Alpha (α) Beta (β) and Gamma (γ) radiation

5

Decay Kinetics

6

Half-Life time

7

Interaction of ionizing radiation with matter

8

Radiation Dose Units

9

Clinical application

10

Radiation protection

Introduction Man-Made Radiation Sources: Medical sources Industrial sources Nuclear Explosions Nuclear Power Nuclear and Radiation accidents Total of 18% Natural Radiation Sources: Terrestrial Radiation(sources from ground: Th, U, Ra) Radon-222 (Gas) formed by the decay of U-238 Radon-220 (Gas) , produced during the decay series of Th-232 Both may cause lung cancer since they may inhale and decay in close contact with lung tissue) Cosmic Rays (Source from outer space in the form of cosmic rays depends on altitude)

Total of 82%

U= Uranium Th= Thorium H= Hydrogen Be= Beryllium

Ra= Radium K= Potassium C= Carbon

Atomic Structure Carbon is the sixth most abundant element in the universe. . Atomic number (Z) = number of protons = 6 Neutron Number (N) = 6 The mass number (A), also called atomic mass, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus.

Mass number (A) of carbon = 12 (6 protons + 6 neutrons)

Types of Atoms Stable 4 He 2 2 Helium Protons= 2 Neutrons= 2 Mass number= 4

Metastable A=Mass number Z= Atomic number

99

43Tc56

Technetium Protons = 43 Neutrons= 56 Mass number= 99

Unstable 235 U 92 143

Uranium Protons= 92 Neutrons= 143 Mass number=235

- For nucleus to be stable a certain number of neutrons = number of protons is

required if this number is increased or decreased than required, then the nucleus will become unstable and start to disintegrate (decay) to reach a stable state. - Metastable radionuclide decay by emitting γ-rays only, and its daughter nucleus differs from its parent in having less energy. -The difference between metastable and unstable atoms is that unstable atoms will not return back to its initial state, while metastable type could return back to its initial state.

Radioactive Transformation When the number of protons (Z) is in the range of 1-20 are known as stable atoms have n = p hence n/p = 1. After Z = 20, the ratio n/p starts to increase up to 1.6 and more. A figure illustrating this ratio is called the belt of stability. Nuclear binding energy (E) The force that holds the nucleus constituents together. The nuclear binding energy is computed by calculating the difference in mass of a nuclide, and the sum of the masses of the number of free neutrons (n) and protons (p). E =Δmc² [Einstein’s equation ] Mass defect (Δm) = (sum of masses of protons and neutrons) - (actual mass of nucleus)

∆m = [(mp+ mn) – mnuclide ] The actual mass of the nucleus (mnuclide) is always less than the sum of the individual masses of protons and neutrons (mp+ mn). c = speed of light in vacuum = 3 x108 (m/sec).

Isotopes

What’s an isotope?

Two or more varieties of an element having the same number of protons but different number of neutrons. Certain isotopes are “unstable” and decay to lighter isotopes or elements. Isotopes of a given element are chemically identical [they participate in the same chemical reactions but could be distinguished from each other because their mass number (A = Z+N) are different].

Nuclei with equal number of neutrons (N) but different A & Z are called isotones Examples:

17

7N10,

18

8O10,

and 199F10

Nuclei with equal mass number (A) but different Z & N are called isobars

Examples

18 N , 18 O 7 11 8 10

and 189F9

Protium (hydrogen-1), Deuterium and Tritium are isotopes of hydrogen. In addition to the 1 proton, they have 1 and 2 additional neutrons in the nucleus respectively. Protium is the most common

hydrogen isotope with an abundance of more than 99.98%. Protium Deuterium Tritium

12C 6

6

13C 7

6

14C 8

6

(for big atoms)

Left of the belt

of the belt If the mass Right difference between parent and • daughter is less than (<)1.022 MeV (the mass of 2 electrons) a proton-rich nucleus may still convert protons to neutrons by the process of electron capture, Positron is positively charged electron

If the mass difference between parent and daughter is less than 1.022 MeV a proton-rich nucleus may still convert protons to neutrons by the process of electron capture, in which a proton simply captures one of the atom's K orbital electrons, emits an electron-neutrino (ve), and becomes a neutron (i.e., number of protons decrease while, number of neutrons increase)

Proton-rich nucleus 7

4Be3

7

3Li4

Nuclides Chart

Unstable nuclei undergo transformation by the emission of energetic radiation such as: (1) alpha (α) particles, which are high-speed helium nuclei consisting of two protons and two neutrons; (2) beta (β) particles, which are very high-speed electrons; and (3) gamma (γ) rays, which are highly energetic photons.

Alpha Particles (α) Radioactivity is associated with the transmutation of the nucleus from one element to another. For example, when radium (Ra) emits an alpha (α) particle, the nucleus is transformed into radon (Rn).

Radium

Radon

226 88 Ra138

222Rn

88 protons 138 neutrons

86

136

86 protons 136 neutrons

+

n p p n

α (42He2) 2 protons 2 neutrons

The alpha-particle (α) is a Helium nucleus(42He2). It’s the same as the element Helium, with the electrons stripped off !

Beta Particles (β-) Carbon A 14 C8 Z 6

Nitrogen 14 7 N7

6 protons 8 neutrons

7 protons 7 neutrons

+

e-

+ νe

electron (β- particleelectron Negatron)antineutrino

The neutrons from the 14C nucleus is “converted” into a proton and electron, then election leaves the nucleus as beta minus (-β) rays. The remaining nucleus 7p and 7n, which is a nitrogen nucleus [714N7], new element is formed. n  p + e- + n e In beta minus decay, the mass number (A =14) has not changed, since the neutron and proton have nearly the same mass. -Converts one neutron into a proton and emit electron [negative beta particle (e- = β-= negatron)]. This is because neutrons are more massive than protons. mp = 938.2723 (MeV) and mn = 939.5656 (MeV).

-Release of electron antineutrino (νe).

Particles (β+) positron (β+ particle)

Sodium 22 11 Na11 11 protons 11 neutrons

Mass number 22

10Ne12

+

e+

+

νe Electron neutrino

10 protons 12 neutrons

A proton from the 22Na nucleus is “converted” into a neutron and as a result a positron (β+) was ejected. The remaining nucleus contains 10p and 12n, which is a “neon” nucleus. In symbolic notation, the following process occurred: p  n + e+ + neutrino Note that in beta decay, the atomic mass not change, since the neutron and proton have nearly the same mass. - Converts one proton into a neutron and positron, + + - positive beta particle (e = β ). -The conversion of protons to neutrons is the result of a weak (nuclear) force. - No change of A = 22 and a new element is formed. - Release of electron-neutrino (ve).

me = 0.5110 (MeV)

Gamma emission (γ) Neon 20 Ne 10 10

Neon 20 Ne 10 10

+ Gamma rays

10 protons 10 neutrons (in excited state)

10 protons 10 neutrons (lowest energy state)

A gamma (γ) is a high energy electromagnetic radiation of every high energy. Example for γ emission, when cobalt decay into nickel. (Nickel in the ground state)

Penetration power of radiation

Alpha particles (γ) may be completely stopped by a sheet of paper, beta particles (β) by aluminum shielding. Gamma rays can only be reduced by much more substantial mass, such as a very thick layer of lead. Note: Neutrons (n0) is the most penetrating because neutrons are electrically neutral

Ray Gamma (γ) Beta (β) Alpha (α)

Mass = energy/c2 (MeV/c2) 0 ~0.5 ~3752

Charge

Shield

0 -1 or +1 +2

Lead Aluminum Paper

Interaction of ionizing radiation with matter What is Ionizing Radiation? Ionizing radiation have received their name owing to their ability of ionizing atoms and molecules in an irradiated material. When radiation interact with substance, which gives rise to loss of electrons. In this case the remaining part of the atom or molecule acquires a positive charge and becomes a positive ion. This process is called ionization.

Excitation occurs when the radiation excites an electron from an occupied orbital into an empty, higher-energy orbital. Excitation transfers enough energy to an orbital electron to displace it further away from the nucleus. It results in the alteration of the atom from the condition of lowest energy (ground state) to one of higher energy (excited state).

Natural Decay Law • Variation of decay rate with time • The number of α, β or  rays emitted per unit time is called the decay rate or radioactivity. • Activity of sample has an exponential relation as: A = λN= Aoe-λt N = No e-λt λ = ln2/T1/2 = 0.693/T1/2

Activity of sample (Bq) versus time

-A is the activity (disintegration/second), Ao is the initial activity at t = 0, λ (hour-1) is decay constant = ln2/T1/2. -t is the time in hour. Unit of activity is Becquerel (Bq = decay/s) or Curie (Ci= 3.7x1010 Bq). - No is number of nuclei at t = 0 - N is number of nuclei at time t

Half life (t1/2) of a radionuclide: Every radionuclide is characterized by its halflife, time, which is defined as “the time required to reduce the initial radioactivity into one half”. It is usually denoted by (t1/2) and it is unique for a given radionuclide. T1/2 = ln2/λ = 0.693/λ Example Calculate the decay constant of iodine-131 if the half life time is T1/2 = 8.02 days. Solution λ = ln2/T1/2 λ = 0.693/8.02 = 0.086 day-1 Example Calculate the half life time of iodine-131 if the decay constant is λ = 0.086 day-1.

T1/2 = ln2/λ = 0.693/λ

How to calculate effective half life time (Teff)? Physical half-life time(Tp): Called the radioactive half-life. It is defined as the time required for one half of the original number of atoms in a given radioactive sample to disintegrate. Biological half-life time (Tb): Biologic half-life (Tb) is the time required for the body to eliminate one half of the dose of any radioactive substance by regular processes of elimination. Effective half-life time (Teff): Because both the physical and biologic half-lives must be taken into consideration when predicting the amount of radiation that is absorbed per unit mass of tissue.

Teff

TP x Tb = -------TP + Tb

Example: Determine the effective half-life of iodine-131 (Tp = 8.02 days) in the human thyroid if it is removed with a biological half-life (Tb) of 120 days. Note: Iodine-131 is administered orally as a liquid or capsules in the treatment of thyroid cancer.

Solution: Teff = ? TP = 8.02 d Tb =120 d Teff

TP x Tb = -------TP + Tb

Teff = (8.02x120) ÷ (8.02+120) = 962.4/128.02 = 7.5 (days)

Clinical application Nuclear medicine is a medical specialty involving the application of radioactive substances in the diagnosis and treatment of disease. Nuclear medicine scans are usually conducted by Radiographers. Nuclear medicine, in a sense, is "radiology done inside out" or "endoradiology" because it records radiation emitting from within the body rather than radiation that is generated by external sources like X-rays

Diagnostic techniques in nuclear medicine Gamma camera

•Radioactive tracers are those which emit gamma rays from the body after injection. These tracers are generally short-lived isotopes linked to chemical compounds which permit specific physiological processes to be scanned. They can be given either by injection, inhalation or orally. Tracer is normally a gamma emitter which can be detected and monitored. •Gamma rays are chosen since alpha and beta particles would be absorbed by

tissues and cannot be detected outside the body. •Technitium-99 is most widely used because it has a half-life (t1/2) of only 6 hours.

Radiotherapy Treatment Irradiation Using High Energy Gamma Rays

•Gamma rays are emitted from a cobalt-60 source - a radioactive form of cobalt. •The cobalt source is kept within a thick, heavy metal container. •This container has a slit to allow a narrow beam of gamma rays to emerge.

Positron Emission Tomography (PET) • PET is a more recent development • A positron-emitting radionuclide is introduced, usually by injection, and accumulates in the target tissue. • As it decays it emits a positron, which promptly combines with a nearby electron resulting in the simultaneous emission of two identifiable gamma rays in opposite directions.

Radionuclide therapy (RNT) •Rapidly dividing cells are particularly sensitive to damage by radiation. For this reason, some cancerous growths can be controlled or eliminated by irradiating the area containing the growth. Radiation therapy is commonly applied to the cancerous tumor because of its ability to control cell growth. Ionizing radiation works by damaging the DNA of exposed tissue leading to cellular death. To spare the normal tissues, shaped radiation beams are aimed from

several angles of exposure to intersect at the tumor, providing a much larger absorbed dose there than in the surrounding, healthy tissue.

• External irradiation (teletherapy) can be carried out using a gamma beam from a radioactive cobalt-60 source. • Internal radionuclide therapy is by administering or planting a small radiation source, usually a alpha or beta emitter • Example: -Iodine-131 is commonly used to treat thyroid cancer. -Iridium-192 implants are used especially in the head and breast.

Radiation protection Radiation protection is based on: Justification: The use of radiation should produce a benefit to the exposed individual to offset the harmful effect it causes. Optimization: Exposures to ionizing radiation should be kept As Low As Reasonably Achievable (ALARA) Dose limits and constraints: Exposures of individuals to radiation should be subjected to dose limits and dose constraints.