Cancer Treatment: Dose Fractionation & Palliative Care

Question 1

linear quadratic model

Answer 1

• alpha is SSB
• beta is DSB
• shoulder is sublethal damage
• shoulder exists because of 6 Rs
• describes relationship between total isoeffective dose and dose number from 1-6Gy

Question 2

6 Rs of radiobiology

Answer 2

• repair of sublethal damage
• reassortment to more radiosensitive phases
• reoxygenation
• repopulation
• redistribution
• radiosensitivity

Question 3

How does radiotherapy cause cell death

Answer 3

• free radicals released from water
• strand breaks
• mitosis alterations
• cell cycle arrest in interphase
• apoptosis

Question 4

why do we fractionate

Answer 4

allows us to limit normal tissue exposure

Question 5

biologically equivalent dose

Answer 5

• allows us to compare potency of various fractionation regimens
• alpha:beta ratios used to compare the damage types produced

Question 6

EQD2

Answer 6

• equivalent dose in 2-GY fractions
• using linear quadratic model, compares effectiveness of schedules with different total doses into an equivalent schedule in 2-Gy fractions which would give the same biological effect

Question 7

alpha:beta ratios

Answer 7

• low ratio= late responding normal tissues
• high ratio= early responding and rapidly proliferating carcinomas

Question 7

conventional fractionation RT

Answer 7

• 1.8-2 Gy per day, once a day
• around 5 weeks (leave weekends to recover)
• macroscopic disease = 40-70Gy
• microscopic disease = 40-50 Gy, [60 Gy if hypoxic]

Question 8

hyperfractionation

Answer 8

• < 1.8-2 Gy moree than one time per day
• quicker OTT
• increased LC and survival but increases side effects

Question 9

combatting higher OTTs

Answer 9

• treatment times longer than 4 weeks, loss of radiation due to proliferation is 0.6Gy per days

Question 10

continuous hyper fractionated accelerated RT

Answer 10

• shortens OTT
• no break days (7 days a week)
• 2 weeks (12 days)
• 1.5 Gy per day three times a day

Question 11

CHART and hyper-fractionantion

Answer 11

• increased late side effects due to increased early side effects
• need gaps of 6 hrs between fractions

Question 12

split-course RT

Answer 12

• no longer done
• have gap in middle of treatment
• allows recovery of early responding healthy tissues
• LC decreased

Question 13

hypofractionation

Answer 13

• used most in palliative treatment
• high doses per fraction but lower number of fractions
• low alpha:beta ratios respond best
• mild hypo-fractionation used after breast cancer surgery

Question 14

stereotactic RT (sbrt/sabr)

Answer 14

• extreme ablative doses
• > 6 Gy per fraction
• very few fractions <8
• affects endothelial cells of vascular supply

Question 15

bone mets

Answer 15

• pain palliation in 80% patients with bone mets
• equivalent being single fraction or multi-fraction

Question 16

stereotaxic radiosurgery (srs)

Answer 16

• BRAIN
• 1 fraction
• very high dose (18-22)
• ablative
• aims to hit tumour while avoiding normal tissue
• no radio-resistant phenotypes