Biological Effects Of Radiation, LET, RBE, Tissue Tolerances, DRL

Question 1

Cell structure - dna

Answer 1

Most DNA is located within the nucleus. Small amounts of DNA is found in the mitochondria. Each cell contains 23 pairs of chromosomes.

Question 2

DNA structure

Answer 2

DNA is 2 linked strands create a double helix each strand has a sugar phosphate backbone attached to each sugar is 4 bases - ATGC

Question 3

Cell cycle mitosis

Answer 3

1. Interphase- G1,S,G2: dna replicates to form 2 copies of each chromosome

2.Mitosis
-Prophase: nuclear membrane disappears and chromosomes condense
-metaphase: chromosomes line up on the equator if the cell
-Anaphase: sister chromates pulled towards opposite poles of the cell
-telophase: nuclear envelope reforms

3.cytokinesis: cytoplasm divides to make 2 identical daughter cells

Question 4

Which phase of the cell cycle is most sensitive to radiation damage?

Answer 4

MITOTIC PHASE

Question 5

2 mechanisms of radiation damage

Answer 5

1.direct damage
2.indirect damage

Question 6

What is Direct damage

Answer 6

Occurs when radiation directly damages the DNA causing either
- single or double strand breaks.
Breaks in DNA causes disruption to cell replication

Question 7

What are the 4 things direct damage to DNA can lead to?

Answer 7

1. cell may be undamaged.

2.cell may be repaired and work normally (single strand break)

3.cell may be repaired abnormally. As strands try to repair they may join via the wrong bases instead of complimentary base pairing so this leads to mutations and leads to malignancy of cancer. E.g Adenine joins with Guanine instead of thymine

4.Cells may die instead of repairing

Question 8

What is indirect damage to DNA?

Answer 8

Occurs when radiation interacts with non critical target atoms within the cell, usually water, This then produces free radicals which attacks other parts of the cell (dna).

Question 9

What are the 3 stages if indirect damage to dna?

Answer 9

1. Radiation interaction
   2.free radical production
2. Indirect dna damage

Question 10

Indirect damage 1.radiation interaction

Answer 10

• x rays interact with non critical molecules in a cell
  -x rays interact with water instead of DNA in a cell
  -70% of cells are made up of H20

Question 11

Indirect damage 2.free radical production

Answer 11

-this interaction produces free radicals by breaking water
-X-rays break water H20 into 2H+O2
-one of the OH becomes a free radical
-radicals are highly reactive atoms with an unpaired electron

Question 12

Indirect damage to dna stage 3

Answer 12

Free radicals diffuse into the cell and attack dna
Damaging dna indirectly

Question 13

What is linear energy transfer

Answer 13

• Primary mechanism for the biological effects of ionisation

-LET determines the biological consequence of x-ray production

-As radiation interacts with matter it loses energy through interactions with atoms.

-defined as the average radiation energy deposited per unit path length along the track of the ionising particle.

-measured in KeV per micrometer

-LET depends on the type of radiation and the materials travelled through.

Question 14

LET definition

Answer 14

-measure of how much energy a charged particle transfers to a material per unit distance it travels

-helps to describe the biological effects of radiation

Question 15

High LET definition

Answer 15

Deposits a higher concentration of energy in a small distance
Leads to more intense localised damage
Causes lots of biological damage in a small area
E.g. protons =, alpha particles, heavy ions.

Question 16

LOW LET defnition

Answer 16

Deposits its energy over a long distance
More disperses energy deposits
Leads to less damaging events and less intense biological damage.
E.g x-rays, gamma rays, beta particles

Question 17

Protons have high/low LET?

Answer 17

Protons are a form of cancer treatments so they have high LET.
Protons deposit all their energy at the end of their trajectory.
High Let is more effective in damaging biological systems and causing cell death or mutations.
This also makes it more effective in cancer treatment, but also more hazardous in terms of radiation protection.

Question 18

Photons have high/low LET

Answer 18

-Photons are x rays and differ to protons with a low LET
Energy deposits are spread out so less damaging events occur in only one cell from a single x-ray.

-X-rays have low let sp causes less biological damage as energy is dispersed

-effective for taking diagnostic images to identify pathology.

Question 19

What are the 3 types of Dose Quantities?

Answer 19

1. Absorbed dose
2. Equivalent dose
3. Effective dose

Question 20

1. Absorbed dose

Answer 20

• Deposited energy is absorbed energy
• Its the amount of radiation deposited into cells / tissues
• Energy deposited in KG of substance by the radiation
• Radiation exposure is equal to the energy deposited in a substance by radiation
• Measured in Gray (Gy)
• Absorbed dose is not a good indicator of the likely biological effect or damage as it would be much greater to tissues on the entrance of the surface than the dose absorbed in deeper regions

Question 21

2.equivalent dose

Answer 21

Modifying energy for type of radiation is equivalent dose
Effect of radiation on human tissue
Uses radiation weighing factors for the different types of radiation
Not all radiation has the same biological effect for the same amount of absorbed dose.
Every type of radiation has urs Owen type of weighing factor

EQD- measure of biological damage calculated for individual organs and adjusted for the type of radiation

Expressed in sieverts (SV)

Takes the absorbed dose and applies a weighing factor for the type of radiation used.

Radiation weighing factor (Wf) x absorbed dose to organ (Gy)

Question 22

3. Effective dose

Answer 22

Modifying energy for radiation and tissue its effective dose.
It’s the probability of a harmful effect from radiation exposure depending on the organs exposed by it.

Risk of da,age leading to cancer is greater in tissues where cells divide frequently

E.g bone marrow is much more radio sensitive than muscle or nerve tissue

To obtain an indication of how exposure can affect overall health the EQD is multiplied the Tissue weighting factor (Wt)

Tissue weighting factor = measure of the organ sensitivity to radiation

Effective dose= EQD x Wt every type of tissue has its own type of weighting factor

Question 23

RBE:

Answer 23

• relative biological effectiveness
• measure of damage per given unit in tissues
• ability of radiation to cause biological damage

-relative measure of damage caused by a type of radiation per unit of energy deposited in tissues

-ability of radiation to cause biological damage

Question 24

RBE is affected by

Answer 24

1.type of radiation
2.dose rate
3.tissue type

As LET goes up RBE goes up
High LET high RBE

Question 25

High LET HIgh RBE

Answer 25

Lower dose of energy deposited over a distance
More energy deposited in one area at the end of the trajectory
Higher damage

Question 26

Low LET low RBE

Answer 26

Energy dose deposits are dispersed
Higher dose
Minimal damage

Question 27

What is tissue tolerance

Answer 27

Each type of tissue has a a radiation dose which it can ‘tolerate’
Some tissues are more radio sensitive than other e.g bone marrow over muscle tissue

Question 28

Below this tolerance dose…

Answer 28

Any damage to tissues due to radiation can be repaired
Tissue isn’t affected harmfully

Question 29

Above tolerance levels….

Answer 29

Damage cannot be repaired

Question 30

What is caused by overdosing tissues based on tolerance levels

Answer 30

It can lead to erythmia:
-radiation burns
-skin reddening
-skin tears
-blisters
-puss

Question 31

Knowledge of tissue tolerances led to the creation of Dose reference levels

Answer 31

Dose reference levels were created which is dose that should not be exceeded for a specific examination

E.g chest x ray DRL should not be exceeded

List of DRL is formed by the government

Question 32

What tissues are the most sensitive to radiation

Answer 32

Tissues that are frequently reproducing
E.g Reproductive organs
-ovaries
-consider DRL for ovaries
-high radiation could cause infertility

E.g. lenses/epithelial cells