Lec 4 RadB 2 Flashcards
What is BED and what is it used for?
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
What is EQD2?
equivalent total dose in 2Gy fractions
What is the Equation for EQD2?
(BED)/ 1+ (2/[∂/ß])
When comparing BED3 and BED10 what are you comparing?
comparing the biological effective dose for a tissue with an ∂/ß ratio of 3 vs 10
What is the equation for BED?
BED = nd [1+ d/(∂/ß)]
where n= number of fractions
d= dose per fraction
What is the typical ∂/ß ratio for tumours?
10
What is the typical ∂/ß ratio for normal tissue?
3
What is the complex BED calculation used for?
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
What are the considerations for unscheduled interruptions?
BED pre-gap and BED post-gap
Allowable BED left to give without increasing tolerance
Head and Neck - repopulation (ie Kick in time)
What is the dominant factor in determining late effects?
fraction size is the dominant factor in determining late effects; overall treatment time has little influence
What determines the response of acutely responding tissues?
fraction size and overall treatment time both determine the response of acutely responding tissues
What is the inter-fraction interval time?
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)
What is kick time (k)?
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)
What is accelerated repopulation?
treatment with any cytotoxic agent, including radiation, can trigger surviving cells (clonogens) in a tumour to divide faster than before.
What happens if the overall treatment time is too long?
the effectiveness of later dose fractions is compromised because the surviving clonogens in the tumour have been triggered into rapid repopulation