4 r’s of radiobiology

4 R’S OF RADIOBIOLOGY
Dr Nanditha Kishore

Biological basis
Cellular targets for radiation
Biological factors influencing radio sensitivity
Chemical modifiers of radiation response

DNA damage by ionizing
radiation
 Biological effects of ionizing radiation are

largely the result of DNA damage.
 A.DIRECT DAMAGE :in case of high LET

radiations such as ALPHA particles,nuetrons
 B.INDIRECT DAMAGE:IN case of low LET

radiations such as x-rays.

BIOLOGICAL BASIS
 A consideration of biological effects of

radiation should begin with three aspects
 1.Types of DNA breaks
 2.Radiosensitivity in cell cycle
 3.Advantages of dose fractionation

TYPES OF DNA DAMAGE

 DNA is a large molecule with a well known

double helical structure.
 The “backbone” of each strand consists of

alternating sugar and phosphate groups.
 Attached to this backbone are four bases, the

sequence of which specifies the genetic code.
 Pyrimidines= thymine and cytosine.
Purines= adenine and guanine

 Do DOSE=A dose of radiation that induces an






average of one lethal event per cell leaves
37% still viable .
For mammalian cells X-RAY Do dose lies
between 1-2 Gy.
Number of DNA lesions per cell
Base damage=>1000
SSB=1000
DSB=40

2.Radiosensitivity in cell
cycle

Biological basis of survival
curve

The Advantages Of Dose Fractionation Include
 Reduction in the number of hypoxic cells
through cell killing and reoxygenation.
 Reduction in the absolute number of

clonogenic tumor cells by the preceding
fractions with the killing of the better
oxygenated cells.
 Blood vessels compressed by a growing

cancer are decompressed secondary to tumor
regression.

 Fractionation exploits the difference in recovery
rate between normal, acute, and late-reacting
tissues and tumors.
 Radiation-induced redistribution of cells within
the cell cycle tends to sensitize rapidly
proliferating cells as they move into the more
sensitive phases of the cell cycle.
 The acute normal tissue toxicity of single
radiation doses can be decreased with
fractionation.
 Thus, patients' tolerance of radiotherapy will
improve with fractionated irradiation.

Biological factors influencing
radio sensitivity
 Efficacy of fractionation can be related to the “Four

Rs” of Radiobiology:

 REPAIR OF SUBLETHAL DAMAGE

 REASSORTMENT OF CELLS WITHIN THE CELL

CYCLE

 REPOPULATION
 REOXYGENATION

OPERATIONAL CLASSIFICATIONS
OF RADIATION DAMAGE

 (1) LETHAL DAMAGE= which is irreversible and

irreparable and leads irrevocably to cell death;
 (2) POTENTIALLY LETHAL DAMAGE (PLD)= The

component of radiation damage that can be
modified by postirradiation environmental
conditions.
 (3) SUBLETHAL DAMAGE (SLD)=which under
normal circumstances can be repaired in hours
unless additional sublethal damage is added

TYPES OF DNA REPAIR
 Mammalian cells have developed specialized

pathways to sense, respond to, and repair
these different types of damage.
 Different repair pathways are used to repair

DNA damage, depending on the stage of the
cell cycle.
 The stability of repair pathway determine the
radiosensitivity of cell cycle phases.

PATHWAYS OF DNA REPAIR
 Base Excision Repair (BER)

 Nucleotide Excision Repair (NER)
 DNA Double-Strand Break Repair

1. Nonhomologous End Joining (NHEJ)
2. Homologous Recombination Repair
(HRR)

 OTHERS
 Single-Strand Annealing (SSA)
 Cross-Link Repair
 Mismatch Repair

Base Excision Repair (BER)
Singlebase mutation that is first removed by a
glycosylase/DNA lyase .
Removal of the sugar residue by an AP
endonuclease
Replacement with the correct nucleotide by DNA
polymerase

completed by DNA ligase III-XRCC1-mediated
ligation

NUCLEOTIDE EXCISION REPAIR
 Nucleotide excision repair removes bulky

adducts in the DNA such as pyrimidine
dimers.
 The process can be subdivided into pathways

1.Globel genome repair(GER)
2.Transcription coupled repair(TER)
The mechanism differs only in the detection of
lesion

 STEPS

(1) damage recognition,
(2) DNA incisions that bracket the lesion,
usually between 24 and 32 nucleotides in
length
(3) removal of the region containing the
adducts,
(4) repair synthesis to fill in the gap region
(5) DNA ligation.

 Defective NER increases sensitivity to UV-

induced DNA damage and anticancer agents
such as alkylating agents that induce bulky
adducts.
 Germline mutations in NER genes lead to

human DNA repair deficiency disorders such
as xeroderma pigmentosum

Nonhomologous End Joining
(NHEJ)
Steps
(1) end recognition(Ku hetero dimer and DNA
PKcs)
(2) end processing(Artemis protein)
(3) fill-in synthesis or end bridging(DNA
polymerase µ)
(4) ligation (XRCC4/DNA ligase IV complex )

Homologous Recombination
Repair (HRR)
 Homologous recombination repair (HRR) is a

High-fidelity mechanism of repairing DNA
DSBs.
 Its function primarly in late S/G2 is to repair
and restore the functionality of replication
forks with DNA double-strand breaks.
 HRR requires physical contact with an
undamaged chromatid or chromosome (to
serve as a template) for repair to occur.

STEPS
1. Recognition of damage(ATM protein kinase)
2. Recruitment of proteins(H2AX, BRCA1,
SMC1, Mre11, Rad50, and Nbs1)
3.Resection of DNA(MRE11 )

4.Strand exchange(BRCA2 and RAD51)

5. DNA synthesis(Using undamaged strand as
primer)
6. Resolution of HOLIDAY junctions.(MMS4
and MUS81 by non-crossing over)
7.Gap filling
8.ligation

HEREDITARY SYNDROMES THAT
AFFECT RADIOSENSITIVITY
 Ataxia-Telangiectasia (AT)

 Ataxia-Telangiectasia-Like Disorder (ATLD)
 Nijmegen Breakage Syndrome (NBS)
 Fanconi Anemia (FA)

Split dose repair
 split dose repair (SDR) that manifests its
importance during fractionated radiotherapy.
 SDR describes the increased survival found if a
dose of radiation is split into two fractions
compared to the same dose administered in one
fraction.
 Molecular mechanisms for SDR are unknown,

experimental evidence suggests that this repair
is due to DNA double-strand break rejoining.

INCUBATED AT NORMAL GROWTH
CONDITIONS

 This simple experiment, performed in vitro,

illustrates three of the “four Rs” of
radiobiology: repair, reassortment, and
repopulation.
 Reassortment and repopulation appear to

have more protracted kinetics in normal
tissues than rapidly proliferating tumor cells.

RE-ASSORTMENT
 Cells change in their radiosensitivity as they

traverse the cell cycle.
 After exposure of asynchronous population of

cells to radiation those in the sensitive phase
are killed thus becomes partly synchronised.
 If allowed time between fractions they

become SELF SENSITISED.

 This phenomenonof SELF SENSITIZATION

due to movement through cell cycle is called
RE-DISTRIBUTION or RE-ASSORTMENT.
 This will occur only in a proliferating cell

population.
 Thus therapeutic ratio can be enhanced .
 The differential is greater the smaller the

dose per fraction and proportional to number
of fractions.

Dose rate effect and inverse
dose rate effect.

 Dose rate is one of the principal factors that






determine the biological consequences of
absorbed radiation.
As dose rate
exposure time increases
Biological effect generally
This Is due to SUB LETHAL DAMAGE
REPAIR

IDEALIZED FRACTIONATION
EXPERIMENT

SURVIVAL CURVES AT WIDE DOSE
RATES
 AS Dose rate is reduced survival curve

becomes shallower and shoulder tends to
disappear.
 This is most dramatic between o.o1 and 1Gy

/min.
 Magnitude of dose rate effect varies among

types of cells

Cell lines from human origin
tends to fan out at LDR

INVERSE DOSE RATE EFFECT
 IN converse with usual phenomenon

increased cell killing is seen with decrease in
dose rate called the INVERSE DOSE RATE
EFFECT.
 This is due to phenomenon of RE-

DISTRIBUTION.

RE-POPULATION
 It occurs as a homeostatic response to cell

depletion caused by treatment.
 It is mainly observed in
 1.Acute –Responding normal tissurs
 2.Tumors With high rate of cell production.
The cell loss after each fraction of radiation
induces compensatory cell regeneration the
extent of which determines tissue tolerance.

POTENTIAL DOUBLING TIME
 It Is The Pre Irradiation Proliferative Activity
measured by time required for the number of
clonogenic cells to double assuming cell loss
factor as zero.
 Tpot = c Ts/LI

GROWTH FRACTION=It increases following
cytoreductive therapies and leads to
ACCELERATED RE-GROWTH.

 It is assessed by increase in dose required for

tumor control as duration of treatment
increases.
OR
 For a constant dose decrease in tumor control
rate as treatment time is extended

Supporting observations
1.Time to recurrence
 One tumor cell must undergo 30 doublings to
become detectable as recurrence.
 Most recurrences in HEAD & NECK cancer
occur within 12 months after radiation
therapy.
 Median doubling time at presentation is
usually 2 months.

2.Split course treatment
These Schedules resulted in lower local control rates .

3.Protracted treatment
It resulted in decreased rate of locoregional control and
led to worse outcomes in several analyses.

For treatment durations of 30-55 days
EACH 1 DAY EXTENSION=O.6 Gy INCREASE IN TOTAL
DOSE to achieve constant rate of tumor control

4.Accelerated treatment


If accelerated tumor growth contributes to
treatment failure, acceleration of standard
treatment may benefit some tumors.

 In nonrandomized studies this improved the

local control in inflammatory breast cancer ,
melanoma metastases to brain.
 Randomized studies of accelerated

treatment of head and neck cancer validated
the benefit.

TCD50 analysis.
 These values are independent of treatment

duration up to about 28 days, after which
they increase rapidly (consistent with 0.6
Gy/day).
 Head and neck SCCs exhibit a lag period of 3

to 4 weeks before beginning to repopulate
with an average doubling time of 3 to 4 days.

Re population in oro
pharyngeal cancers

Influence of regeneration
Normal tissues
 Time to onset of repopulation after
irradiation and rate at which it proceeds vary
with the tissue.
 In humans tissue turn over kinetics are slower
than mice.
 High initial doses shorten the LAG PERIOD.
 It confers benefit by reducing toxicity in acute
responding tissues.

Growth factors may be useful in protecting
normal tissues from irradiation by shortening
the apparent lag phase and accelerating
recovery in irradiated tissues.
 Hematopoietic growth factors
 Keratinocytic growth factors

 Tumor tissues
 Like acute responding normal tissues tumors

accelerate their growth in response to injury.
ACCELERATED REPOPULATION
 It is division of surviving clonagens of cells in
a tumor at a faster rate than before being
triggered by any cytotoxic agent including
radiation.

PRACTICALS IMPLICATIONS OF
RE-POPULATION
1.Protracting treatment longer than necessary
will likely be a disadvantage.
 using 1.8 Gy rather than 2 Gy fractions given
five times per week extends overall treatment
time by about 10% .
 RESERVED FOR SITUATIONS IN WHICH
1.Acute responses limiting dose accumilation.
2.Ih homogenicities in dose distribution

2. If a break in treatment is necessary because
of acute toxicity, it should be kept as short as
is tolerable.
3.Planned split-course therapy is inadvisable
unless it is part of an accelerated treatment
protocol that ultimately shortens the overall
treatment duration .
4.Breaks in therapy for nonmedical reasons
(machine breakdown, holidays) may merit
catch up treatments in patients being treated
for cure

“ Rapidly growing tumors must be treated
rapidly”
Treatment should never be unnecessarily
protracted because it is difficult to predict
the accelerated repopulation response of
individual tumors.

Effect Of Oxygen On
Radiosensitivity
 The oxygen effect was observed as early as 1912 in

Germany by Swartz. In England in the 1930s, Mottram
explored the question of oxygen in detail.
 oxygen enhancement ratio (OER).

doses administered under hypoxic condition
doses administered under aerated conditions
to achieve the same biological effect.
 It varies with 1.Type of radiation

2.Dose assay

MECHANISM OF THE OXYGEN EFFECT
 The Oxygen Fixation Hypothesis
Absorption of radiation

 fast charged particles





number of ion pairs
free radicals

R+DNA=R.+ O2 =R02(FIXATION OF
DAMAGE)

break chemical bonds
chemical changes



Final expression of biologic damage

FRACTIONATION TO TACKLE HYPOXIA
 The oxygen status of cells in a tumor is not static; it is

dynamic and constantly changes.
 Proportion of hypoxic cells in the tumor is about the

same at the end of a fractionated radiotherapy regimen
as in the untreated tumor.
 During the course of the treatment hypoxic cells become

oxygenated.
 This phenomenon, by which hypoxic cells become

oxygenated after a dose of radiation, is termed
reoxygenation..

Time sequence of REOXYGENATION(mouse sarcoma
cells)

 RATE and EXTENT of Re-oxygenation varies
with type of tumors.
 If this is appropriately studied it can be used

in designing fractionatation shedules.
 Since pattern is not known for many human
tumors it can considered that doses on the
order of 60 Gy (6,000 rad) given in 30
treatments argues strongly in favor of
reoxygenation.

Re –oxygenation in various
tumors

Mechanism of Re-Oxygenation
 Opening up of blood vessels
 Decreased diffusion distance(70um-150um)

 Revascularization of tumor

Chemotherapy Induced
Radiosensitization

4 r’s of radiobiology

Empfohlen

Empfohlen

Weitere ähnliche Inhalte

Was ist angesagt?

Was ist angesagt? (20)

Andere mochten auch

Andere mochten auch (19)

Ähnlich wie 4 r’s of radiobiology

Ähnlich wie 4 r’s of radiobiology (20)

Kürzlich hochgeladen

Kürzlich hochgeladen (20)

4 r’s of radiobiology