L 5: Fractionated radiation and dose rate effect

Question 1

Lethal Damage
LD

Answer 1

• irreversable
• Irrepairable
• Leads to cell death

Question 2

Potentially lethal damage
PLD

Answer 2

• Radiation damage can be modified by post irradiation environmental conditions
• Dose would usually be lethal but modulation of post radiation environment increases the survival.
• NO PLD following high LET radiation

Question 3

Sublethal Damage
SLD

Answer 3

• Can be repaired in few hours
• Increase in survival if the radiation dose is separated by time interval.
• Can lead to lethal damage if another SLD is added to the existing one.
• Occurs in the first 2 hours following radiation exposure

Question 4

4 “r”s

Answer 4

1. Repair
2. Re-assortment
3. Re-population
4. Re-Oxygenation

Question 5

Dose effect rate

Answer 5

• Dose rate effect is most evident at 0.1-1Gy/min
• Above and below this value there is only little effect.
• Dose effect rate varies from one cell type to another.
• Decrease dose rate and exposure time extended then biological effect is reduced.
• Inverse dose rate effect: some type of HeLa cells have more killing at low dose than high dose.

Question 6

Very low dose rate

Board question

Answer 6

1. Increased survival
2. Decreased mutations
3. Decreased chromosomal aberrations
4. Increasing life-span

• Testis is the MOST sensitive organ in the body to low dose rate
• Exposure of the fetus in utero where increasing the LENGTH of exposure (at low dose-rates) increases the pathology

Question 7

Brachytherapy

Answer 7

1. Intracavity
2. Interstitial

Question 8

Intra-Cavity

Answer 8

• Temporary, takes 1-4days, dose rate is 50cGy/h, low dose rate
• Most common application: uterine cervix
• Radionuclides used in the past: Ra, Cs
• Radionuclides used now: Ir192, shorter half-life, decreased energy
• High dose-rate complications: 3-12 dose fractions, important in cervix where dose limiting problem is normal tissue (bladder, rectum), they receive lower dose
• Low dose rate: protecting surrounding organs by limiting toxicity to bladder, rectum.

Question 9

Interstitial Brachytherapy

Answer 9

• Temporary
• Usually uses Ir192, low dose-rate
• Use in tissues that have a dose-rate effect
• Most commonly used in early stage prostate cancer (80-130 radioactive seeds implanted into the prostate under transrectal ultrasound guidance)
• Minimizes effects to other tissues
• 60Gy x 7days is the standard (35.7cGy/h)