Cellular targets for radiation
Biological factors influencing radio sensitivity
Chemical modifiers of radiation response
DNA damage by ionizing
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.
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
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
Blood vessels compressed by a growing
cancer are decompressed secondary to tumor
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
Thus, patients' tolerance of radiotherapy will
improve with fractionated irradiation.
Biological factors influencing
Efficacy of fractionation can be related to the “Four
Rs” of Radiobiology:
REPAIR OF SUBLETHAL DAMAGE
REASSORTMENT OF CELLS WITHIN THE CELL
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
(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
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
Base Excision Repair (BER)
Singlebase mutation that is first removed by a
glycosylase/DNA lyase .
Removal of the sugar residue by an AP
Replacement with the correct nucleotide by DNA
completed by DNA ligase III-XRCC1-mediated
NUCLEOTIDE EXCISION REPAIR
Nucleotide excision repair removes bulky
adducts in the DNA such as pyrimidine
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
(1) damage recognition,
(2) DNA incisions that bracket the lesion,
usually between 24 and 32 nucleotides in
(3) removal of the region containing the
(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
Germline mutations in NER genes lead to
human DNA repair deficiency disorders such
as xeroderma pigmentosum
Nonhomologous End Joining
(1) end recognition(Ku hetero dimer and DNA
(2) end processing(Artemis protein)
(3) fill-in synthesis or end bridging(DNA
(4) ligation (XRCC4/DNA ligase IV complex )
Homologous recombination repair (HRR) is a
High-fidelity mechanism of repairing DNA
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.
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
6. Resolution of HOLIDAY junctions.(MMS4
and MUS81 by non-crossing over)
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
Molecular mechanisms for SDR are unknown,
experimental evidence suggests that this repair
is due to DNA double-strand break rejoining.
This simple experiment, performed in vitro,
illustrates three of the “four Rs” of
radiobiology: repair, reassortment, and
Reassortment and repopulation appear to
have more protracted kinetics in normal
tissues than rapidly proliferating tumor cells.
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
Thus therapeutic ratio can be enhanced .
The differential is greater the smaller the
dose per fraction and proportional to number
Dose rate effect and inverse
dose rate effect.
Dose rate is one of the principal factors that
determine the biological consequences of
As dose rate
exposure time increases
Biological effect generally
This Is due to SUB LETHAL DAMAGE
SURVIVAL CURVES AT WIDE DOSE
AS Dose rate is reduced survival curve
becomes shallower and shoulder tends to
This is most dramatic between o.o1 and 1Gy
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
This is due to phenomenon of RE-
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
It is assessed by increase in dose required for
tumor control as duration of treatment
For a constant dose decrease in tumor control
rate as treatment time is extended
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
Median doubling time at presentation is
usually 2 months.
2.Split course treatment
These Schedules resulted in lower local control rates .
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
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
These values are independent of treatment
duration up to about 28 days, after which
they increase rapidly (consistent with 0.6
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.
Influence of regeneration
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
High initial doses shorten the LAG PERIOD.
It confers benefit by reducing toxicity in acute
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
Like acute responding normal tissues tumors
accelerate their growth in response to injury.
It is division of surviving clonagens of cells in
a tumor at a faster rate than before being
triggered by any cytotoxic agent including
PRACTICALS IMPLICATIONS OF
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
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
“ Rapidly growing tumors must be treated
Treatment should never be unnecessarily
protracted because it is difficult to predict
the accelerated repopulation response of
Effect Of Oxygen On
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
MECHANISM OF THE OXYGEN EFFECT
The Oxygen Fixation Hypothesis
Absorption of radiation
fast charged particles
number of ion pairs
R+DNA=R.+ O2 =R02(FIXATION OF
break chemical bonds
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
This phenomenon, by which hypoxic cells become
oxygenated after a dose of radiation, is termed
Time sequence of REOXYGENATION(mouse sarcoma
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