Radiobiology Flashcards
Influence of fraction size and overall treatment time
Fraction size mainly influences late effects, overall treatment time has little influence
But, fraction size and overall treatment time both determine the response of acutely responding tissue
What is kick in time?
Repopulation is accelerated at about 28 days (kick-in time). They need to finish treatment in prescribed time period. Better to delay the initiation of treatment than to introduce delays during treatment
Different fractionation schedules
- Standard fractionation 1.8 - 2Gy.
- Hyperfractionation-Radiation treatment in which the total dose of radiation is divided into small doses (less than 1.8Gy) and treatments are given more than once a day.
- Hypofractionation- A treatment schedule in which the total dose of radiation is divided into large doses and treatments are given once a day or less often
- Accelerated fractionation -a conventional number of dose fractions is delivered in a significantly shortened overall treatment time (by-daily
Why do multiple fractions per day?
Spare early reaction and allow adequate reoxygenation in tumours
Overall treatment time reduction: if overall treatment time is too long, it could decrease acute reactions without sparing late injury and allows the surviving tumour cells to proliferate
Why use hyperfractionation schedule?
- Increased opportunity for tumour cell redistribution and reoxygenation between dose fractions
- A possibly lower OER with small incremental doses
- Different sparing of late reacting normal tissue with small dose fractions
Why use Hypofractionation?
- A treatment schedule in which the total dose of radiation is divided into large doses and treatments are given once a day or less often
have equal efficacy and toxicity, as well as noninferiority, compared with standard radiation treatment.
Concern - higher rates of acute and late toxicities
Why are unscheduled interruptions bad?
reduction of local control of 1-1.4%. Treatment should not exceed 42 days
Major causes of unscheduled disruptions:
- Machine and staff availability -Adequate staff,
- Public holidays -Ideally all patients should be treated on public holidays, but this may not be practicable.
- Transport problems -Efficient communication with local ambulance service
- Medical problems -Management of acute side effects
- Social circumstances -that lead to a patient’s failure to attend for treatment as scheduled - Psychological and social work support
Which patients get prioritised?
Categorise patients from most at risk of loss of tumour control/cure rates from unscheduled interruptions
Category 1: priority for patients with radical intent and have evidence of prolongation effect on their cancer type, SCC of lung, and H+N
Category 2: 2 days prolongment is ok, TCC bladder, SCC Cervix
Managing interruptions that have occurred:
- Accelerated scheduling: treat twice daily on some days remaining between interruptions and end of treatment. BIDs not recommended for fraction sizes larger than 2.2Gy.6 hour difference between two fractions
- Biological allowance: increase total dose or dose per fraction. Requires radiobiological calculations
Additional considerations for managing interruptions
- Concurrent Chemotherapy
- Previous treatment
- SBRT/SRS techniques- very high doses per fraction
- Proton Treatments- Limited data • General principles are the same as for photon treatments. • In a proton dept, there should be plans in place to minimize interruptions in treatment.
- Brachytherapy- a variety of dose/fractionation schedules are used
- Combination treatments – integration of EBRT with other modalities e.g. brachy
- Radioprotectors
What are radioprotectors
chemicals that reduce the biologic effects of radiation
radioprotectors containing a sulfhydryl
Group exert their effect by scavenging free radicals and by reducing free-radical damage to DNA
Most effective for radiations characterised by LET
Retreatment after RT
- If its after 10 years, it is approved to be treating
- Use highly conformal techniques
- Reirradiation is possible in various sites with reduced doses and with a high price in terms of morbidity
Reirradiation with 50 to 60 Gy within a few years of the initial treatment improves local control and possibly survival, but with severe toxicity and functional sequelae
What is BED
Biologic effective dose: measure of true biological effect of dose
ND X (1+d/a/b)
What is EQD
Biological effective dose converted to standard dosing
EQD= BED/1+2/A/B
