Radiobiology 6

Question 1

How does fractionation impact the cell survival curve?

Answer 1

• treating in multiple fractions allows sub-lethal repair to take place, so the shape of the curve repeats from where the previous curve ended

Question 2

Therapeutic ratio and assessing cell kill…

Answer 2

• NTCP may look different to TCP

- They have different thresholds

Question 3

What are the 5 R’s?

Answer 3

• Radiosensitivity
• Repair
• Repopulation
• Reoxygenation
• Re-distribution

Question 4

Radiosensitivity

Answer 4

• Intrinsic radiosensitivity differs between cells and normal tissues and strongly determines final surviving fraction
• Accounts for variable tumour response, more radiosensitive, less dose needed and therefore could be better for normal tissues

Question 5

Repair

Answer 5

• Cells differ in capacity to repair, this is usually more effective in non-proliferating cells
• repair for normal tissues is important, so six hours is allowed so as not to compromise this repair

Question 6

Repopulation

Answer 6

Surviving cells in many tumours and acute-responding tissues proliferate more rapidly once treatment has begun

• More time allowed, means less time for tumour to grow/repopulate
• Acute effects will worsen

Question 7

Reoxygenation

Answer 7

• Hypoxic cells, which occur especially in tumours are quite resistant to radiation. Hypoxic surviving cells begin to reoxygenate
• allowing cells to reoxygenate is a good thing (become more radiosensitive), lack of oxygen could lead to resistance
• further from blood vessel = more oxygenated as treated, shrink and move toward oxygen area, cycle

Question 8

Redistribution

Answer 8

• Cells in certain phases are quite resistant

- Time between fractions means cells will redistribute themselves across the cell cycle

Question 9

How do we optimise fractionation?

Answer 9

• Spare normal tissues, kill tumour tissues
• alpha/beta ratio for tumour is usually higher than normal tissues - in a small dose per fraction, large impact on tumour, less on normal cells - large dose per fraction, more impact on normal tissues

Question 10

BED means?

Answer 10

• Biologically effective dose
• Unit is Gy
• When using BED, assume delivering same dose to tumour and to NT
• Also does not take into account repopulation.