Topic 17: radiotherapy

Question 1

Define radiation therapy

Answer 1

• Ionizing radiation = kill tumors
• Lethal dose rapidly delivered to malignant tumor cell

Question 2

Define deterministic regime

Answer 2

• Therapy doses are deterministic
• Predict fraction of tumor cells dying

Question 3

Define stochastic dose

Answer 3

• Healthy tissues exposed to high doses
• Dose stochastic to rest of body
• Random probability of effect on other cells
• Therapy small future risk vs existing life-threatening disease

Question 4

Describe how tumor regrowth is prevented

Answer 4

• Destroying enough tumor = prevent regrowth
• Radiation goal = achieve total extinction = if 1 cell left = tumor reseed
• Treatment = treating whole region surrounding cancer = healthy tissue
• Compromise between effectiveness in killing + sparing nearby tissue

Question 5

Describe the features of the dose/effect curve

Answer 5

• Threshold dose for cell killing
• Fraction of cells killed = increases with with = S-shaped curve
• There is dose which = 100% cells killed = no effect of further increasing dose

Question 6

What to do if threshold for health tissue exceeds tumor?

Answer 6

1) Treatment uses doses that kill most of tumor cells
2) Affects few health cells

Question 7

What are the undesirable consequences of variation in dose?

Answer 7

• Too high = kills many healthy cells
• Too low = spares tumor cells

Question 8

What happens whe there is an overlap between 2 curves?

Answer 8

• Choice of dose size is hard
• Killing enough tumor cells = great damage to healthy cells

Question 9

Give factors that dose-response curve depends on

Answer 9

1) Tissue where cells originate
2) Type of ionizing radiation used
3) Energy of radiation
4) Rate at which dose delivered
- Rapid = more damage + lower threshold
- Healthy cells = better DNA repair mechanisms = can use rapid to advantage

Question 10

Explain DNA repair advantage in dosing treatment

Answer 10

• High dose required = divided into many fractions
• Small doses = delivered with pause of day between fractions = time for healthy cell to recover/repopulate
• Fractions = damages tumor cell = changes point in cell cycle where cell is irradiated

Question 11

Give factors cure rate + effectiveness of radiation depends on

Answer 11

1) Cancer type
2) Specific individual case

Question 12

What is the use of radiation therapy if cure is unlikely?

Answer 12

• Palliative measure

Question 13

Give the 4 R’s of fractionation

Answer 13

1) Repair: few hrs = sub-lethal damage
2) Redistribution: few hrs-days = cell move to radiation sensitive phase of cell cycle
3) Re-oxygenation: few hrs-days = tumor hypoxia + greater radiation resistance seen
4) Repopulation: few weeks = healthy + malignant cell repopulate after irradiation

Question 14

Give the 4 fractionation schemes

Answer 14

1) Conventional
2) Hyper-fractionation
3) Accelerated fractionation
4) Hypo-fractionation

Question 15

Describe conventional fractionation

Answer 15

• Dose/fraction = 1.8 -2.2 Gy
• Fractions/week = 5
• Total dose = 45.0 – 50.4 Gy
• Issues = can be too slow for fast-growing tumors + dose too low for resistant cancers