Biological Optimisation

Question 1

Indirect action of cell damage?

Answer 1

Primary photon interaction producing high energy electrons

2. High energy electrons in moving through the tissue produce free radicals in water [H+ and OH-]
3. Free radicals may produce changes in DNA from breakage of chemical bonds
4. Changes in chemical bonds result in biological effects

Question 2

Shape of LET curves

Answer 2

High LET exponential function of dose

Low LET has a shoulder region for exponential

Question 3

Factors that make cells less radiosensitive?

Answer 3

Low oxygen or hypoxic state
– Low dose rates
– Fractionation
– Cells synchronised in the late S phase of the cell cycle

Question 4

What is physical and biological planning?

Answer 4

Dose and dose volume parameters

Use biological indices to reflect clinical goals

Question 5

What is biological guided radiotherapy?

Answer 5

Use of relevant information of individual patient biological response of the tumour and normal tissues to design dose distributions

– Tumour and normal tissue radio-sensitivity, oxygenation status, proliferation rate

Question 6

What is biological based treatment planning?

Answer 6

Use of feedback from biological response models in the treatment planning process

Feedback could be automated (inverse planning) or manual (forward planning)

Question 7

What is generalised EUD

Answer 7

The uniform dose that would yield the same radiobiological effect as the nonuniform dose (delivered with the same number of fractions)

Question 8

Early responding tissues?

Answer 8

Skin, mucosal layer of stomach
occurs immediately or during RT
Symptoms-pain, discomfort
Tumours

Large alpha beta (tumours) alpha dominates at low doses

Question 9

Late responding tissues?

Answer 9

Starts 6-12 months following RT
Cell depletion slowly with dividing cells (spinal cord, kidney)
Progressive and Irreversible
Late effects (small alpha beta ratio beta more influence at low doses)

Question 10

5Rs of Radiobiology

Answer 10

Radiosensitivity
Repair
Repopulation
Redistribution
Reoxygenation

Question 11

Sensitivity of high proliferating cells (tumours)

Answer 11

Not sensitive to changes in fraction size

Question 12

Sensitivity of slow proliferating cells with low a/b

Answer 12

Plenty of repair capability
Very sensitive to dose/ fraction
Late responding more sensitive to large doses/ per fraction

Question 13

Alpha/ beta ratios of early reactions

Answer 13

skin 9-12
Colon 9-11
Testis 12-13
Mucosa 9-10

Question 14

Alpha/ beta ratios of late reactions

Answer 14

Kidney 2-2.4
Rectum 2.5-5 
Lung 2.7-4
Bladder 3-7
CNS (Brain and spinal cord) 1.8- 2.2

Question 15

Fractionation schedules

Answer 15

Most rapidly proliferating
Therefore high alpha/beta (10 Gy and above)
Use large number of small dose/ fraction
Limit damage to late reacting normal tissue ( lower alpha/beta)

Question 16

Alpha beta ratios of prostate carcinoma?

Answer 16

low alpha/ beta (1.5 Gy)
Lower than rectum
likely to benefit from fewer and larger fractions

Question 17

For high alpha beta ratios?

Answer 17

Hyper fractionation is recommended
ses more than one fraction per day with a smaller dose per fraction (<1.8 Gy) to reduce long-term complications and to allow delivery of higher total tumour dose

Question 18

Low alpha beta ratios?

Answer 18

Hypofractionation increases dose per fraction minimizing tumour cell proliferation during the course of treatment