OAR Limits, modelling retreatments and standard Radiobiological reporting

Question 1

Why are OAR dose limits important?

Answer 1

Need to know the probability of doing harm to the patients - limits the dose and technique used to treat the patient

Question 2

What four pieces of information are needed to use dose limits in radiobiology calculations?

Answer 2

Total dose to OAR
Number of fractions OAR dose is given in
Effective volume of OAR that receives the dose
alpha/beta used in analysis in the source being used

Question 3

What are the main sources of data used for OAR dose limits?

Answer 3

Emami data
Data from clinical trials
QUANTEC data
Upcoming data

Question 4

What does the Emami data include?

Answer 4

Literature review up to 1991 for 28 dose limiting complications
Tabulated TD50/5 and TD5/5 - doses that give 50% and 5% NTCP at 5 years
Tabulated volume dependencies - partial irradiation of volume fraction v of organ where v = 1, 2/3, and 1/3
Fit of a volume dependence to a power law and calculation of NTCP using Lyman model

Question 5

What does the Emami 2013 paper include?

Answer 5

Summarised updates to his previous dose limits in light of QUANTEC data and new techniques

Question 6

What are the QUANTEC papers?

Answer 6

DVH driven outcome data for 16 critical organs - gives consensus guidlines and lit review

Question 7

How are the data sets in the QUANTEC papers presented?

Answer 7

Different summary charts as well as some overview papers and summary tables

Question 8

Why should the data in the QUANTEC papers not always be used directly?

Answer 8

Some data states fraction regime which is good
Some specify ‘standard fractionation’ to the tumour- could be between 1.8-2Gy/# - there is no fixed relationship between OAR and tumour doses so need to find out fraction regime used

Question 9

What is the formula for calculating the dose limit for the OAR locally from the QUANTEC papers and clinical trials?

Answer 9

X(1+(X/(nref.(alpha/beta))) = D(1+(D/(nlocal.(alpha/beta)))
Where X = dose in paper
nref = number of fractions in paper
D = local dose
nlocal = local number of fractions

Question 10

What needs to be considered for patients with previously treated areas?

Answer 10

If they have have one course or multiple courses before
What recovery has been made by the specific organ - literature gives organ specific retreatment outcomes and recommended limits

Question 11

What is the standard method of reporting radiobiology?

Answer 11

EQD2 or EQDX

Question 12

What is the Withers formula?

Answer 12

EQD2 = (D(1+d/(alpha/beta)))/(1+(2/(alpha/beta))) = D(((alpha/beta)+d)/((alpha/beta)+2))

Question 13

What is the new EQDX(alpha/beta) used for?

Answer 13

ICRU are working on it as a comprehensive and consistent framework for radiobiological modelling with EQD2 and BED as special cases