L20: Radiation Biology

Question 1

What is radiation biology?

Answer 1

the study of the action of ionizing radiation on live organisms

Question 2

How does x-ray interact with matter?

Answer 2

when x-rays pass through the body, some of them are absorbed by tissues and deposit their energy locally

• amount of energy attenuated varies depending on the specific tissue
• the denser the material is, the more x-ray it attenuates

Question 3

What are the 4 ways that x-ray interacts with matter?

Answer 3

• photoelectric effect – can happen in a diagnostic energy x-ray
• compton scatter – can happen in a diagnostic energy x-ray
• rayleigh/coherent scatter – can happen in a diagnostic energy x-ray
• pair production

Question 4

What is the photoelectric effect?

Answer 4

emission of electrons when electromagnetic radiation (such as light) hits a material

• atom bombarded by x-rays loses an electron, and the newly freed photoelectron will go on to ionize more atoms (ionizing radiation)
• incoming photon hits an electron in an atom’s K-shell – ejects photoelectron
• higher energy electron ‘falls’ to fill the gap in the K-shell
• there is an attraction force between x-rays and electrons

Question 5

What is a photon?

Answer 5

particle that travels at the speed of light – energy only, no mass

Question 6

What is a photoelectron?

Answer 6

electron with high energy

Question 7

What is the compton scatter?

Answer 7

x-rays or gamma rays are scattered on a material with an increase in wavelength (and decrease in energy)

• independent of atomic number (mass element) of an atom – ie. more electrons do not influence this phenomena like photoelectric effect

Question 8

What is the compton scatter the source of?

Answer 8

source of imaging artifacts – do not know where x-ray is going

• scatter causes noise and trouble
• results in decrease in image quality

Question 9

What is the principal target of ionizing radiation?

Answer 9

DNA

Question 10

What are the two methods of DNA damage?

Answer 10

• direct action
• indirect action

Question 11

What is direct action DNA damage?

Answer 11

radiation directly hits the DNA, knocking out the electron – around 5% of damage

Question 12

What is indirect action DNA damage?

Answer 12

free radical formation – around 95% of damage

• x-ray → water molecule → free radicals

Question 13

What are free radicals?

Answer 13

molecular species with an unpaired electron in an atomic orbital

• anything with a charge is unstable

Question 14

What is linear energy transfer (LET)?

Answer 14

average (radiation) energy deposited per unit path length along the track of an ionizing particle

• LET = (change in energy) / (change in distance)
• more energy deposited locally → more damage

Question 15

What does high LET mean?

Answer 15

low penetrating power – energy deposits easily on tissue and can damage it

• protons
• alpha particles (helium nucleus)
• neutrons
• heavy charge particles

Question 16

What does low LET mean?

Answer 16

high penetrating power

• photons (x-rays and gamma rays) – gamma rays require denser materials to stop it
• electrons (beta-rays)

Question 17

What particles can penetrate through paper?

Answer 17

• beta
• neutron
• gamma

Question 18

What particles can penetrate through hands?

Answer 18

• beta
• neutron
• gamma

Question 19

What particles can penetrate through metal?

Answer 19

• neutron
• gamma

Question 20

What particles can penetrate through water?

Answer 20

• gamma

Question 21

What particles can penetrate through concrete?

Answer 21

• gamma

Question 22

What particles can penetrate through lead?

Answer 22

none

Question 23

What is absorbed dose?

Answer 23

energy absorbed in the human body from exposure to radiation

• measured in gray (Gy)
• 1 Gy = 1 joule of energy deposited in a kilogram of substance
• 1 Gy of alpha radiation is more harmful than 1 Gy of x-ray

Question 24

What is relative biological effectiveness (RBE)?

Answer 24

how different types of radiation with different LET produce the same biological effect

• usually compare to 250 keV x-rays

Question 25

What does damage caused by radiation depend on?

Answer 25

• effectiveness of the radiation
• sensitivity of the specific organ

Question 26

What are some organs that are more sensitive to radiation?

Answer 26

• bone marrow
• colon
• lung
• stomach
• breast
• etc.

Question 27

What are some organs that are less sensitive to radiation?

Answer 27

• bone surface
• brain
• skin
• etc.

Question 28

What is a sievert (Sv)?

Answer 28

international system of units (SI) to measure the relative biological damage in the human body

• determines how dose affects certain tissues
• not every tissue is the same – certain ones are more sensitive to radiation

Question 29

What are some key sievert (Sv) measurements of imaging?

Answer 29

• 10 µSv – average daily dose received from natural background
• 20 µSv – chest x-ray
• 600 µSv – mammogram
• 3650 µSv (3.65 mSv) – average yearly dose received from natural background
• 5800 µSv (5.8 mSv) – chest CT scan
• 20,000 µSv – single full-body CT scan
• 1,000,000 µSv (1000 mSv) – 5% increase in cancer risk
• 5,000,000 µSv (5000 mSv) – dose that kills a human with a 50% risk within 30 days (LD50/30) if the dose is received over a very short duration

Question 30

What are the types of ionizing radiation that are used for both imaging and therapy?

Answer 30

photons

• x-rays
• gamma rays

Question 31

What are the types of ionizing radiation that are used for therapy only?

Answer 31

• electrons (beta-rays)
• protons
• alpha particle (helium)
• neutrons
• heavy charged particles