radiation biology

Question 1

describe the effects of radiation in DNA

Answer 1

event sequence of radiation injury:
1. absorption of radiation
- ways of interaction of photons w matter:
- no interaction 9%
- coherent scatter 7%
- comptom scatter 57%
- photoelectric absorption 27%
2. ionisation or excitation of electrons
- ionisation: neutral atom loses an electron –> forms ion pair between positive ion and electron –> causes downstream effects to body
- excitation: electron raises to higher energy level without ejecting
3. chemical alteration by direct or indirect action –> cause damage to cell DNA (mutation)
- direct action: high LET (linear energy transfer), by particulate radiation, displaced electron cause breaks in DNA strand
- indirect action: low LET, by gamma or x ray, electron reacts w water –> forms hydroxyl radical
- damage to DNA: base damage/single strand or double strand breaks/cross links to DNA or protein
4. enzymatic repair or radiation induced effects –> low dose effects (pri stoichastic) or high dose effects (pri deterministic)

Question 2

properties of ionising radiation as mutagen (4)

Answer 2

1. radiation increase incidence of mutation
2. no threshold above which mutation known to increase (LNT model)
3. spontaneous mutation background level for all organisms
4. mutation is not radiation specific

Question 3

mechanism of cell killing in high dose effect

Answer 3

1. DNA as target: chromosomal abberations –> cell death during mitosis
2. bystander effect: irradiated cells secrete molecule –> cause death of surrounding non-irradiated cells
3. apoptosis: programmed cell death = cell shrinkage and nuclear chromatin condensation
4. autophagic cell death: fuse w lysosomes –> self digestion
5. senescence: cell cycle arrest

Question 4

deterministic vs stochastic risk

Answer 4

stochastic
- no threshold dose
- severity independent to dose
- risk proportional to dose
- eg. cancer, heritable effects, in-utero effects

deterministic
- threshold dose
- severity independent to dose
- risk proportional to dose
- eg. sunburn, radiation cataracts, radiation mucositis, late tissue fibrosis, skin erythema, radiation ulcers
- eg. in uterine effects –> growth retardation, development abnormalities, intra-uterine death

Question 5

law of bergonie and tribondeau

Answer 5

most sensitive cells are those
- highly differentiated
- high metabolic rate
-high mitotic activity
- long dividing future

Question 6

describe Linear Non Threshold Model

Answer 6

• x axis = dose above background radiation
• y axis = risk of cancer
• black dots above 100msv are based on human experiences
• extrapolation of line through black dots to origin
• linear means risk of cancer is linear relationship to dose above background radiation
• no threshold means there is only no risk of cancer when there is no radiation exposed –> any dose of radiation no matter how low, will have risk of cancer
• cancer risk increase as dose increase