Radiobiology Flashcards
What is the criteria for a radiation induced secondary tumour
Second tumour occurs in location irradiated by primary or secondary therapeutic beam
Histology of second tumour is different to first so cannot be a met
Latency period, typically of several years
Second tumour not present at time of radiation treatment
Patient does not have a cancer prone syndrome
Where is evidence for cancer induction from?
A Bomb survivors
Biological studies
Patients exposed to radiation
Occupationally exposed workers
What models should you assume for cancer induction risk for low and high doses
Low: linear no threshold model
High: non-linear model with parameters fitted from epidemiological evidence
What is ICRP 103 conclusion on cancer induction
Combined detriment to excess cancer and heritable effects is 5% per Sv
What is EAR?
Excess absolute risk
Fraction RT patients contracting Ca - Fraction non RT patients contracting cancer
What is RR?
Relative risk
Fraction RT patients contracting Ca/Fraction non-RT patients contracting Ca
Breast cancer RT patients have a RR of about 1.3 for secondary breast cancer
What is ERR?
Excess relative risk
ERR = RR - 1
Fraction or % per Gy
What has evidence from atomic bomb survivors shown?
ERR of secondary cancer induction depends on the age at exposure
Decreases with age up to 20
Increases slightly up to older age (60)
Decreases after 60
Due to initiation and promotion processes
How would you calculate risk with linear no threshold hypothesis?
Sum dose distributions from different sources of radiation dose
Apply organ specific coefficients
Age and sex dependent
Apply generalised risk coefficient (5% per Sv)
Does not account for individual factors such as smoking or diet
How does higher doses affect cancer induction
Cell sterilisation is thought to reduce response gradient and reduce the chance of cancer induction, as cell sterilisation rates overtake cell cancer induction
Linear exponential dose response curve
Where are we mostly likely to see secondary cancers according to the shape of linear exponential curves?
Near margins of RT TVs or in low dose bath volumes (not necessarily the case, it is likely that there is a plateau or slower fall off with large dose according to dose date from RT patients, may be caused by repopulation)
How does cancer induction in atomic bomb survivors differ to RT patients?
A bomb survivors suffer carcinoma ((in rapidly dividing epithelial cells) and little sarcoma
RT patients have significant induced sarcomas (MSK tissue)
Implies different response from fractionated treatment
What is OED and how is it calculated
Organ equivalent dose
For an inhomogeneous dose distribution, OED is the uniform dose with the same risk of radiation induced cancer as DVH
Uses linear exponential dose risk model
Break DVH into equal volume dose bins, lookup alpha beta values and sum over all dose bins to calculate
What is downside of IMRT in terms of radiobiology and why?
Risk of second malignant neoplasms can increase by almost twice as much, caused by:
IMRT requires more fields so larger volume is exposed to radiation
Exposure of out of field tissues form leakage x rays ig reater because IMRT requires more MUs
What is integral dose
Total energy deposited in total irradiated volume of patient in kg x Gy
ID = m.D
For inhomogeneous dose distribution, summed across dose bins
What are concomitant exposures
Exposures other than treatment exposures: imaging exposures alongside RT
What are the 5 Rs of radiotherapy
Repair (of sublethal damage)
Repopulation (following irradiation)
Redistribution (of cells within cell cycle)
Reoxygenation (of surviving cells)
Radiosensitivity (intrinsic)
What are the issues to consider with fraction timing?
Incomplete repair between fractions (repairable DNA strand breaks are not repairs, problem for normal tissue tolerance)
Treatment gaps (repopulation issue with unintentional prolongation of treatment)
When might incomplete repair occur
Accelerated radiotherapy (intervals < 24 hours, most repairable damage is repaired after 6 hours, debated evidence shows slow repair components in which case may be some unrepaired damage)
Pulsed brachytherapy (time between fractions 1 hour)
Continuous LDR BT (ongoing repair during irradiation - different BED formula)
What is an example of accelerated RT and was it effective?
CHART Trial
1.5Gy/fr 3fr/day every day instead of conventional 5fr/week
Local control/OS not improved in H&N
Significant benefit to lung patients (although more difficult for hospital to deliver)
What is expected impact of accelerated RT?
Late normal tissue reaction expected to decrease, but this happened less than expected
Acute normal tissue reaction increases with accelreated RT, more severe, peaks earlier
How is incomplete repair accounted for in equations?
Adjustment is derived from the fact that the beta d^2 term is modified such that the relative effectiveness per unit dose is greater due to incomplete repair
Fractionated radiotherapy, h term which is described by a number of functions depending on how repair is modelled
LDR, R and mu