radiation oncology

1. THE 5 R’s OF
RADIOBIOLOGY
Dr SUVETHA CHAVARKKAD
25/8/22

2. RADIOBIOLOGY
BRANCH OF SCIENCE WHICH
COMBINES BASIC PRINCIPLES
OF PHYSICS AND BIOLOGY
AND IS CONCERNED WITH THE
ACTION OF IONISING
RADIATION ON BIOLOGICAL
TISSUES AND LIVING
ORGANISMS.
2

3. “▪ It studies the action of ionizing
radiation on healthy and diseased
tissue
▪ Radiation biologists seek to
understand the nature and sequence
of events that occur following the
absorption of energy from ionizing
radiation, the biological
consequences of any damage that
results, and the mechanisms that
enhance, compensate for, or repair
the damage.
3

4. “
▪ The ultimate goal of radiobiologic
research is to accurately describe the
effects of radiation on humans so
that radiation can be used more
safely in diagnosis and more
effectively in therapy.
▪ Most radiobiologic research seeks to
develop radiation dose-response
relationships so the effects of
planned doses can be predicted and
the response to accidental exposure
managed.

5. Law of Bergonic and Tribondeau:
• In 1906, radiologist Jean Bergonic and histologist Louis
Tribondeau observed the effects of radiation by exposing rodent
testicles to X-rays.
• The testes were selected because they contain both mature
(spermatozoa) and immature (spermatogonia and
spermatocytes) cells.
• These cells have different cellular functions and their rate of
mitosis also differs. The spermatogonia cells divide frequently,
whereas the spermatozoa cells do not divide.

6. • After irradiating the testes,Bergonie and
Tribondeau noticed the immature cells were
injured at lower doses than the mature cells.
• Supported by their observations, they proposed
a law describing radiation sensitivity for all body
cells.
• Their law maintains that actively mitotic and
undifferentiated cells are most susceptible to
damage from ionizing radiation.

7. The law of Bergonie and Tribondeau states that:
1. Stem or immature cells are more radiosensitive than
mature cells.
2. Younger tissues and organs are more radiosensitive
than older tissues and organs.
3. The higher the metabolic cell activity, the more
radiosensitive it is
4. The greater the proliferation and growth rate for
tissues, the greater the radiosensitivity.
In diagnostic imaging, the law reminds us that fetuses
are considerably more sensitive to radiation
exposure as are children compared with the mature
adults

8. ANCEL AND VINTENBERGER:
• In 1925, embryologists Paul Ancel and P. Vitemberger
modified the law of Bergone and Tribondeau.
• They suggested that the intrinsic susceptibility of
damage to any cell by ionizing radiation is identical, but
that the timing of manifestation of radiation-produced
damage varies according to the cell type

9. Their experiments on mammals demonstrated that
there are two factors that affect the manifestation
of radiation damage to the cell:
• The amount of biologic stress the cell receives.
• Pre- and post-irradiation conditions to which the
cell is exposed.
They theorized that the most significant biologic
stress on the cell is the need for cell division
They determined that a given dose of radiation will
cause the same degree of damage to all cells, but
only if and when the cell divides will damage be
demonstrated.

10. HISTORY OF FRACTIONATION:
Regaud’s Experiment:
• Tried to sterilize sheep by irradiation of their
testis.
• Testis were regarded as model of a growing
tumor & skin as dose limiting normal tissue. .
• He found that
Single dose - sterilization possible only with
unacceptable skin damage
Fractionated dose - sterilization achieved without
excessive damage to skin of scrotum. .

11. The 5 R’s of Radiobiology are the concepts to explain
the rationale behind FRACTIONATION IN
RADIOTHERAPY.

12. REPAIR:

13. DIRECT AND INDIRECT EFFECT OF
RADIATION:

14. TYPES OF RADIATION-INDUCED
DAMAGE
• Lethal Damage
• Potentially lethal Damage
• Sub-lethal Damage

15. LETHAL DAMAGE:
• Irreversible and irreparable damage that leads
to cell death.
• Eg
• Dicentric chromosome
• Ring chromosome
• Anaphase bridge

16. POTENTIALLY LETHAL DAMAGE:
• Causes cell death under ordinary circumstances
but can be modified by postirradiation
environmental conditions.
• If cells are prevented from dividing by creating
suboptimalVgrowth conditions for 6 hrs after
irradiation, the damage can repair.
• Invitro: by keeping cells in saline or plateau
phase

17. SUB-LETHAL DAMAGE
• Repairable in hours under ordinary circumstances
unless additional sublethal damage is added.
• Repair of sublethal damage reflects the repair of DNA
breaks before they can interact to form lethal
chromosomal abberations

18. • In a split dose experiment with cultured Chinese
hamster cells:
• A single dose of 15.58 Gy leads to a SF of 0.005.
• If the dose is divided into two approximately equal
fractions separated by 30 min SF is already
appreciably higher than for a single dose.
• As the time interval is extended, SF increases until
a plateau is reached at about 2 hours,
corresponding to a surviving fraction of 0.02.
• The increase in survival in a split-dose experiment
results from the repair of sublethal radiation
damage

20. • By splitting radiation into small parts, cells are
allowed to repair the sublethal damage
• Damage repair depends upon the ability of cells
to recognise the damage and activate the repair
pathways and cell cycle arrest
• Malignant cells often have suppressed these
pathways
• Normal tissues are able to repair by the time
next fraction is given

22. REASSORTMENT/
REDISTRIBUTION:

23. Cells may be in different phases of cell cycle during
irradiation(S phase being radioresistant and M-phase
being most radiosensitive).
Resistance and sensitivity depends upon the level of
sulfhydryl compounds(radioprotector) in the cell.
A small dose of radiation given over a short period
will kill a lot of sensitive cells and less of resistant
cells, in the dividing phase of cell cycle.
Surviving cells continue the cycle and may reach
sensitive phase when second dose of radiation is
given
23

24. ▪ The main mode of injury is mitotic cell death
▪ Cells are most sensitive in mitotic phase’
▪ Cells are resistant in the late S phase
▪ If the cell cycle is considerably long then, another phase of
resistance is obtained in early G1 phase followed by a sensitive
G2 phase.
24
BMC Proc. 2013; 7(Suppl 6): P25.
Published online 2013 Dec 4. doi: 10.1186/1753-6561-7-S6-P25
PMCID: PMC3980911
Powerful expression in Chinese Hamster Ovary cells using bacterial artificial chromosomes: parameters influencing
productivity

25. 25

26. REPOPULATION:

27. ▪ Repopulation is the process of increase in cell division seen in
normal and malignant cells after irradiation.
▪ Repopulation may help to spare normal tissue damage but may
also reduce tumour control probability.
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28. • In normal tissues, homeostatic response following radiation
injury, involve:
⚫ reduction of cell cycle time: rapid doubling
⚫ increase in growth fraction(e.g recruitment of resting cells)
⚫ decrease in cell loss factor
In tumors, rate of cell production exceeds rate of cell loss.
Repopulation may involve any of above three mechanisms.
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29. ▪ The time to onset of repopulation after irradiation and the rate
at which it proceeds vary with the tissue.
▪ • Acute-responding tissues (stem cells, progenitor cells, GI
epithelium, oropharyngeal mucosa,skin) begin repopulation
early.
▪ • Late-responding tissues (Renal tubular epithelium,
oligodendrocytes, schwann cells, endothelium, fibroblasts)
begin repopulation after completion of conventional course of
radiation.
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30. Accelerated Repopulation:
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▪ Treatment with any cytotoxic agent, including RT, triggers
surviving cells (clonogens) in a tumor to divide faster than
before
▪ Dose escalation is needed to overcome this proliferation.
▪ e.g. it starts in head & neck cancer 4wks after initiation of
fractionated RT
Implication:
▪ Treatment should be completed as soon after it is started -It is
better to delay a treatment than to introduce delay during
treatment.
▪ About 0.6Gy is needed to compensate for this repopulation.

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32. 32
CARCINOMA TREATMENT TIME
HEAD AND NECK 99 DAYS
BREAST 212 DAYS
CERVIX 56 DAYS

33. REOXYGENATION :

34. ▪ Tumours under I mm size are fully oxic but beyond this size
they develop the region of hypoxia.
▪ Hypoxia in tumours can result from two different mechanisms.
1. Acute Hypoxia
2. Chronic Hypoxia
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35. ACUTE HYPOXIA:
▪ Develop in tumour as a result of the temporary closing or
blockage of a particular blood vessel owing to the malformed
structure which lacks smooth muscle and often has incomplete
endothelial lining and basement membrane.
▪ At the moment when a dose of radiation is delivered, a
proportion of the tumor cells may be hypoxic, but if the
radiation is delayed until a later time, a different group of cells
may be hypoxic.
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36. CHRONIC HYPOXIA:
⚫ It results from the limited diffusion distance of oxygen in
respiring tissue that is actively metabolizing oxygen.
⚫ The distance oxygen can diffuse in respiring tissue is about
70µm.
⚫ Cells that are hypoxic for long periods become necrotic and die.
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37. 37

38. REOXYGENATION:
▪ The phenomenon by which hypoxic cells
become oxygenated after a dose of radiation.
▪ This is important because it helps to fix the free
radical injury caused by radiation.
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39. PROCESS OF REOXYGENATION:
⚫Tumors contain a mixture of aerated and
hypoxic cells.
A dose of x-rays kills a greater proportion of
aerated cells than hypoxic cells because
aerated cells are more radiosensitive.
• Therefore, immediately after irradiation,
most15% cells in the tumor are hypoxic.
• Hypoxic cells get reoxygenated during a
fractionated course of treatment
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40. MECHANISM:
Reoxygenation in tumours have:
1.Fast component:
-seen in acute hypoxia
-occurs within hours
-reoxygenation occurs when temporarily closed vessels reopen
2.Slow component:
-seen in chronic hypoxia
-occurs within days
-reoxygenation occurs when the tumor shrinks in size and the
surviving cells that were previously beyond the range of oxygen
diffusion, come closer to a blood supply
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41. RADIOSENSITIVITY :

42. ▪ •Apart from previous 4 R's, there is an intrinsic radiosensitivity
or radioresistance in different cell types.
▪ •The radiosensitivity of the tumor cells is now thought to be
the primary determinant of tumor response to radiation.
42

43. CELL TYPES & RADIOSENSITIVITY:
▪ The cells of a tissue system are identified by their rate of proliferation
and their stage of development
▪ Immature cells are called undifferentiated cells,precursor cells, or stem
cells.
▪ As a cell matures through growth and proliferation, it can pass through
various stages of differentiation into a fully functional and mature cell.
▪ The sensitivity of the cell to radiation is determined somewhat by its
state of maturity and its functional role.
▪ Stem cells are more sensitive to radiation than mature cells.
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45. 45

46. ▪ Cell radiosensitivity depends upon what part of the cell cycle
the cell is in.
▪ Mitosis, and the passage from late G1 into early S-phase, are
judged the most radiosensitive phases of the cell cycle
whereas, Mid- to late S-phase is considered to be the most
radioresistant cell cycle phase.
▪ Numerous experiments have determined that the nucleus of a
cell is considerably more radiosensitive than is the cytoplasm of
the cell.
▪ DNA is the most radiosensitive part of the cell whereas, RNA
radiosensitivity is intermediate between that of DNA and
protein.
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47. Physical Factors That Affect Radiosensitivity:
▪ The response of the tissue is determined principally by the
amount of energy deposited per unit mass the radiation dose in
Gy (rad).
▪ However, even under controlled experimental conditions, when
equal doses are delivered to equal specimens, the response
may not be the same because of other modifying factors
▪ A number of physical factors affect the degree of radiation
response, some of them are:
I. Linear Energy Transfer (LET)
II. Relutive Biologic Effectiveness (RBE)
III. Protraction and fractionation
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48. LET:
▪ LET is the product of the average energy transferred per ion
pair and the specific ionization (number of ion pairs per unit
length)
▪ It is the energy transferred by radiation per unit path length in
soft tissue
▪ It is a physical quantity that is useful for defining the quality of
an ionizing radiation beam
▪ The LET of a charged particle is proportional to the square of
the charge and inversely proportional to the particle's kinetic
energy
▪ The unit used for the LET is keV/um.
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49. High and Low LET Radiations:
▪ High LET Radiation:
▪ This is a type of ionizing radiation that deposit a large amount
of energy in a small distance.
▪ Eg. Neutrons, alpha particles
▪ Low LET Radiation:
▪ This is a type of ionizing radiation that deposit less amount of
energy along the track or have infrequent or widely spaced
lonizing events.
▪ Eg. x-rays, gamma rays
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50. ▪ High LET radiation lonizes water into H and OH radicals over a
very short track. In fg. two events occur in a single cell so as to
form a pair of adjacent OH radicals that recombine to form
peroxide, H₂O, which can produce oxidative damage in the cell.
▪ Low LET radiation also ionizes water molecules, but over a
much longer track. In fig, two events occur in separate cells,
such that adjacent radicals are of the opposite type: the H and
OH radicals reunite and reform H,O.
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51. RBE:
▪ Although all ionizing radiations are capable of producing the
same types of biologic effects, the magnitude of the effect per
unit dose differs
▪ To evaluate the effectiveness of different types and energies of
radiation and their associated LETS, experiments are performed
that compare the dose required for the test radiation to
produce the same specific biologic response produced by a
particular dose of a reference radiation typically, x-rays
produced by a potential of 250 kV)
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52. ▪ The term relating the effectiveness of the test radiation to the
reference radiation is called the relative biological effectiveness
(RBE).
▪ The RBE is defined, for identical exposure conditions, as follows:
▪ RBE=
𝐷𝑜𝑠𝑒 𝑜𝑓 𝑠𝑡𝑎𝑛𝑑𝑎𝑟𝑑 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑛𝑒𝑐𝑒𝑠𝑠𝑎𝑟𝑦 𝑡𝑜 𝑝𝑟𝑜𝑑𝑢𝑐𝑒 𝑎 𝑔𝑖𝑣𝑒𝑛 effect
𝑑𝑜𝑠𝑒 𝑜𝑓 𝑡𝑒𝑠𝑡 𝑟𝑎𝑑𝑖𝑎𝑡𝑖𝑜𝑛 𝑛𝑒𝑐𝑒𝑠𝑠𝑎𝑟𝑦 𝑡𝑜 𝑝𝑟𝑜𝑑𝑢𝑐𝑒 𝑡ℎ𝑒 𝑠𝑎𝑚𝑒 𝑒𝑓𝑓𝑒𝑐𝑡
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53. PROTRACTION & FRACTIONATION:
▪ If a dose of radiation is delivered over a long period of time
rather than quickly, the effect of that dose is less,
▪ If the dose is delivered continuously but at a lower dose rate, it
is said to be protracted
▪ If the dose is delivered at the same dose rate, in equal fraction,
all separated by similar time interval, the dose is said to be
fractionated.
▪ Dose protraction and fractionation cause less effect because
time is allowed for intracellular repair and tissue recovery.
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54. BIOLOGICAL FACTORS THAT AFFECT RADIOSENSITIVITY:
▪ A number of biologic conditions alter the radiation response of
tissue.
▪ Oxygen Effect
▪ Age
▪ Recovery
▪ Chemical Agents
▪ Hormesis
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55. OXYGEN EFFECT:
▪ The 1940s brought experimentation with oxygen.
▪ Geneticist Charles Rick determined oxygen to be a
radiosensitizer because it increases the cell-killing effect of a
given dose of radiation.
▪ This occurs as a result of the increased production of free
radicals when ionizing radiation is delivered in the presence of
oxygen
▪ This was named the oxygen effect
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57. CHEMICAL AGENTS:
▪ Some chemicals can modify the radiation response. To be
effective, they must be present at the time of irradiation.
Radiosensitizers:
▪ Agents that enhance the effect of radiation are called
sensitizing agents.
▪ Examples include:
▫ Oxygen,
▫ halogenated pyrimidines,
▫ methotrexate,
▫ actinomycin D,
▫ hydroxyurea, and
▫ vitamin K.
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58. Radioprotectors:
▪ Radioprotective compounds include molecules that contain a
sulfhydryl group, such as cysteine and cysteamine
▪ The most successful synthetic radioprotector is amifostine,
which is believed to have been carried by the Apollo astronauts
to protect them in case of a large radiation dose due to a solar
event."
▪ Amifostine is Food and Drug Administration (FDA) approved for
clinical use, where it has been successful in reducing side
effects from radiation therapy in head and neck cancer patients.
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59. AGE:
▪ Humans are most sensitive before birth.
▪ After birth, sensitivity decreases until
maturity, at which time humans are most
resistant to radiation effects.
▪ In old age, humans again become
somewhat more radiosensitive.
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60. RECOVERY:
▪ In vitro experiments show that human cells
can recover from radiation damage
▪ If the radiation dose is not sufficient to kill
the cell before its next division (interphase
death), then given sufficient time, the cell will
recover from the sublethal radiation damage
it has sustained
▪ This intracellular recovery is attributable to a
repair mechanism inherent in the
biochemistry of the cell
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61. ▪ If a tissue or organ receives a sufficient radiation dose, it
responds by shrinking. This is called atrophy
▪ If a sufficient number of cells sustain only sub-lethal damage
and survive, they may proliferate and repopulate the irradiated
tissue or organ
▪ The combined processes of intracellular repair and repopulation
contribute to recovery from radiation damage.
▪ Recovery = Intracellular repair + Repopulation
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62. HORMESIS:
▪ A separate and small body of radiobiologic evidence suggests
that a little bit of radiation is good.
▪ Some studies have shown that animals given low radiation
doses live longer than controls because a little radiation
stimulates hormonal and immune responses to other toxic
environmental agents.
▪ Regardless of radiation hormesis, we continue to practice
ALARA vigorously as a safe approach to radiation management.
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63. THE 6TH “R” :
▪ Reactivation of Anti-Tumor
Immune Response
▪ RT is able to modify the tumor
micro environment (TME) and to
induce a local and systemic
(abscopal effect) immune
response
▪ The abscopal effect was first
described in the early 1950s by
RH Mole when he coined the
term after he observed a clinical
response to irradiation at distant
sites that were not exposed to
RT
63
Mole, R. H. "Whole body irradiation—radiobiology or
medicine? The British Journal of Radiology 26.305
(1953): 234-241.

64. ABSCOPAL EFFECT:
▪ Ab- Away from, Scopal- target
▪ Defined as a reaction within an organism that had not been
directly exposed to irradiation, but cause tumour regression of
the non-irradiated tumours
▪ It is the ability of localized radiation to induce an antitumor
response throughout the body at sites that were not subjected
to targeted RT.
▪ It is thought to be mediated by activation of the immune
system via cytokines.
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65. ▪ Local irradiation of a tumor nodule may lead to immunogenic
forms of tumor cell death and liberation of tumor cell-derived
antigens.
▪ These antigens can be recognized and processed by antigen-
presenting cells within the tumor (dendritic cells and
macrophages).
▪ Cytotoxic T cells which recognize these tumor antigens may in
turn be primed by the tumor antigen-presenting cells.
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66. ▪ In contrast to the local effect of irradiation on
the tumor cells, these cytotoxic T cells
circulate through the blood stream and are
thus able to destroy remaining tumor cells in
distant parts of the body which were not
irradiated.
▪ Accordingly, increases in tumor-specific
cytotoxic T cells were shown to correlate with
abscopal anti-tumor responses in patients.
66
Vanpouille-Box C, Diamond JM, Pilones KA, et al. TGFbeta is
a master regulator of radiation therapy-induced antitumor
immunity. Cancer Res. 2015;75:2232-2242.

67. ▪ Abscopal effects of ionizing radiation are often blocked by the
immunosuppressive microenvironment inside the irradiated
tumor which prevents effective T cell priming.
▪ This is why the effect is so rarely seen in patients receiving
radiotherapy alone.
▪ In contrast, the combination of immunomodulatory drugs such
as ipilimumab and pembrolizumab can partially reconstitute
systemic anti-tumor immune reactions induced after local
tumor radiotherapy.
67
Hiniker SM, Reddy SA, Maecker HT, et al. A prospective clinical trial combining radiation therapy with systemic
immunotherapy in metastatic melanoma. Int J Radiat Oncol Biol Phys. 2016;96:578-588.

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69. THANK YOU!!!
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