Radiobiology Lecture 1

Question 1

Differences between malignant and normal cells?

Answer 1

larger nucleus
varies in shape
loss of specialised cell features/function
poorly differentiated (don't look like normal cells)
poorly defined tumour boundary

Question 2

What is Direct Action radiation? (direct hit)

Answer 2

Radiation hits the DNA strand causing a direct break

Question 3

What is Indirect Action radiation?

Answer 3

Radiation hits a water molecule producing a free radicle that then hits the DNA causing a break

Question 4

What are the 4 stages of the Cell Cycle?

Answer 4

G1- Growth
S- DNA synthesis
G2- Growth and preparation for mitosis
M- Mitosis

Question 5

Which stage of the cell cycle is radioresistant and which are radiosensitive?

Answer 5

S-phase (DNA synthesis) is radioresistant

G2/M (Growth and preparation for mitosis and Mitosis) are radiosensitive

Question 6

What dose plating efficiency describe?

Answer 6

estimates the number of cell colonies, from single cells, that have originated (if they had not been irradiated)

PE= (Number of colonies counted/ number of cells seeded) x 100

Question 7

What is Cell Survival?

Answer 7

the number of cells that survive after exposure to different Gy’s of radiation

Question 8

What is a cell survival curve?

Answer 8

Graph of number of surviving cells vs dose

Question 9

What factors affect the shape of the cell survival curve?

Answer 9

Shape of graph affected by:
Low LET (e.g. x-rays)
High LET (alpha, low-energy neutrons)
Radiation variables: (particle size, particle charge, dose rate)
Tissue Variables: (mitotic rate, cell cycle phase, oxygenation)

Question 10

What does a higher a/B ratio mean?

Answer 10

higher a/B therefore in the linear part of the LQ model, therefore, single electron for 2 breaks therefore requires lower dose to kill the cell.
(early responding tissue)

Question 11

What dose a lower a/B ratio mean

Answer 11

lower a/B therefore in the quadratic part of LQ model, therefore 2 electrons required for 2 breaks, therefore the cells are more radioresistance therefore require a higher dose to kill cells
(late responding tissue)

Question 12

What are late and early responding tissues and what are their a/B ratios?

Answer 12

Late: a/B = 3 e.g. prostate therefore require higher dose
Early: a/B = 10 e.g. most tumours therefore require less dose

Question 13

What is Biological Effective Dose?

Answer 13

compares the effectiveness of different treatment regimes

Question 14

How does BED relate to a/B ratios and LQ model?

Answer 14

depending on the a/B ratio e.g. low a/B ratio more dose is required therefore cells are more radioresistant

Question 15

What is repair of sublethal damage?

Answer 15

repairing damaged cells so they don’t die

Question 16

How does fractionation affect repair of sublethal damage?

Answer 16

a higher fractionation means giving the normal cells time to repair their damage between fractions

Question 17

How does fractionation affect tumour cell damage?

Answer 17

higher fractionation means that the cell cycle can change between fractions and allow the cycle to move from radioresistant phase into the radiosensitive phase. Also affects reoxygenation

Question 18

How does fractionation affect toxicities?

Answer 18

higher fractionation spares early reactions

Question 19

What is the negative of increased fractionation?

Answer 19

excessive prolongation allows the surviving tumour cells to proliferate

Question 20

What are the 5R’s of Radiobiology?

Answer 20

1. Repair of sublethal damage
2. Reoxygenation of hypoxic tissue
3. Redistribution (or Reassortment) of cells within the cell cycle
4. Repopulation (or Regeneration)
5. Radiosensitivity

Question 21

What is repair of sublethal damage?

Answer 21

cells can repair through P53 gene or cell cycle arrest

Question 22

How do malignant cells affect P53 gene?

Answer 22

malignant cells can suppress the repair pathway by inhibiting the TP53 gene thereby preventing repair and tumour suppression

Question 23

What is Reassortment of cells within the cell cycle?

Answer 23

S-phase more radioresistant
G2 and M phase more radiosensitive
Over time the surviving cells will continue to cycle

Question 24

What is repopulation/ regeneration of cells within the cell cycle?

Answer 24

Following irradiation some cell population will exhibit increased cell division
(repopulation tends to begin more quickly in normal early responding tissues than in tumours (therrfore repopulation favours survival of normal early responding tissues over tumours)

Question 25

What is the reoxygenation of cells within the cell cycle?

Answer 25

Oxygen makes tumour more radiosensitive

tumours in hypoxic regions may not be supplied well with blood therefore are irradiation the tumour will reoxygenate

Question 26

Radiosensitivity

Answer 26

there is an intrinsic radiosensitivity or radioresistance in different cell types

Question 27

What are examples of radiosensitive cells?

Answer 27

haemotological cells
epithelia stem cells
gametes and tumour cells from haemotological or sex organ origin

Question 28

What are examples of radioresistant cells?

Answer 28

neurons and tumour cells such as melanoma or sarcoma

Question 29

What is the Oxygen Effect?

Answer 29

cells are much more sensitive to x-rays in the presence of molecular oxygen
(DNA reacts with the free radicals)

Question 30

What is the Oxygen Enhancement Ratio?

Answer 30

the ratio of doses under hypoxic to aerated conditions necessary to produce the same level of cell killing is called OER

Question 31

What is the oxygen fixation hypothesis?

Answer 31

Radiation-induced damage can be permanently ‘fixed’ by molecular oxygen (oxygen fixes into the break thereby preventing the damage from being repaired, therefore the cell dies/eaten)