Lec 4 RadB 2

Question 1

What is BED and what is it used for?

Answer 1

the ratio of the parameters ∂ and ß in the linear quadratic model
used to quantify the fractionation sensitivity of tissues

(BED is a measure of the true biological dose delivered by a particular combination of dose per fraction and total dose to a particular tissue characterized by a specific ∂/ß ratio

Question 2

What is EQD2?

Answer 2

equivalent total dose in 2Gy fractions

Question 3

What is the Equation for EQD2?

Answer 3

(BED)/ 1+ (2/[∂/ß])

Question 4

When comparing BED3 and BED10 what are you comparing?

Answer 4

comparing the biological effective dose for a tissue with an ∂/ß ratio of 3 vs 10

Question 5

What is the equation for BED?

Answer 5

BED = nd [1+ d/(∂/ß)]
where n= number of fractions
d= dose per fraction

Question 6

What is the typical ∂/ß ratio for tumours?

Answer 6

10

Question 7

What is the typical ∂/ß ratio for normal tissue?

Answer 7

3

Question 8

What is the complex BED calculation used for?

Answer 8

calculating the BED following a treatment interruptions
BED = Nd x [1+ d/(∂/ß)] - K x (T - -Tdelay)
K = repopulation constant
T = Overall treatment time
T delay = time lag from the beginning of treatment before rapid repopulation occur

Question 9

What are the considerations for unscheduled interruptions?

Answer 9

BED pre-gap and BED post-gap
Allowable BED left to give without increasing tolerance
Head and Neck - repopulation (ie Kick in time)

Question 10

What is the dominant factor in determining late effects?

Answer 10

fraction size is the dominant factor in determining late effects; overall treatment time has little influence

Question 11

What determines the response of acutely responding tissues?

Answer 11

fraction size and overall treatment time both determine the response of acutely responding tissues

Question 12

What is the inter-fraction interval time?

Answer 12

if multiple fractions per day are used, the inter-fraction interval should be at least 6 hours (the characteristic halftime of recovery is 4-5 hrs)

Question 13

What is kick time (k)?

Answer 13

some cancers e.g. head and neck have rapid repopulation of 28 days after the initiation of radiotherapy in a fractionated regimen
(therefore it may be better to delay initiation of treatment than to introduce delays during treatment)

Question 14

What is accelerated repopulation?

Answer 14

treatment with any cytotoxic agent, including radiation, can trigger surviving cells (clonogens) in a tumour to divide faster than before.

Question 15

What happens if the overall treatment time is too long?

Answer 15

the effectiveness of later dose fractions is compromised because the surviving clonogens in the tumour have been triggered into rapid repopulation

Question 16

What is hyperfractionation?

Answer 16

Radiation treatment in which the total dose of radiation is divided into small doses and treatments are given more than once a day

Question 17

What is hypofractionation?

Answer 17

A treatment schedule in which the total dose of radiation is divided into large doses and treatments are given once a day or less

Question 18

What is accelerated fractionation?

Answer 18

a conventional number of dose fractions is delivered in a significantly shortened overall treatment time

Question 19

What is the advantage of a prolonged treatment?

Answer 19

spare early reactions

to allow for adequate reoxygenation in tumours

Question 20

What is a disadvantage of excessive prolongation?

Answer 20

allows the surviving tumour cells to proliferate during treatment

Question 21

What are the advantages of hyperfractionation?

Answer 21

1. a large number of significantly reduced dose fractions is used to give a greater total dose in a conventional overall treatment time
2. increased opportunity for tumour cell redistribution and reoxygenation between dose fractions
3. possibly lower oxygen enhancement ratio with small incremental doses
4. different sparing of late reacting normal tissue with small dose fractions

Question 22

What is the method of accelerated treatment?

Answer 22

interpose a rest period in the middle of the treatment (e.g. 2 weeks) or to reduce the dose slightly with acute effects as the limiting factor

Question 23

Why are the advantages of accelerated treatment?

Answer 23

to reduce repopulation in rapidly proliferating tumours.
there should be little or no change in the late effects because the number of fractions and the dose per fraction are unaltered

Question 24

What are some advantages and disadvantages of CHART (Continuous Hyperfractionated Accelerated Radiation Therapy)
(36 fractions over 12 consecutive days, three fractions delivered daily with interfraction interval of 6 hours)?

Answer 24

strategy based on a low dose per fraction to minimize late effects and a very short overall time to minimize tumour proliferation

• good local control but severe acute reactions
• lower incidence of late effect
• but damage to spinal cord

Question 25

What is the dosage for CHART?

Answer 25

three fractions per day: 1.4 to 1.5Gy per fraction, 50 to 54Gt total

Question 26

What has been found with hypofractionation vs standard radiation treatment?

Answer 26

equal efficacy and toxicity as well as noninferiority, compared with standard
BUT hypofractionation might result in unacceptably higher rates of acute and late toxicities

Question 27

What is the estimated ∂/ß ratio for breast cancer?

Answer 27

4 Gy