A major theme for the radiobiology section is the use of radiation as a model agent to study cellular responses including genomic instability, cell cycle controls, DNA damage processing, oxidative stress, senescence, and apoptosis, as well as the signaling mechanisms mediating these and other stress responses.

1. RADIATION BIOLOGY

2. DEFINITION
 Radiobiology is the study of the effects of ionozing
radiation on living systems

3. RADIATION CHEMISTRY
 Direct effect – when energy of a photon or
secondary electron ionizes biologic
macromolecules
 Indirect effect – a photon may be absorbed by
water in an organism, ionizing some of its water
molecules. The resulting ions form free radicals that
in turn interact with and produce changes in
biologic molecules

4. DIRECT EFFECT
 Biologic molecules absorb energy from ionizing
radiation and form unstable free radicals
 Generation of free radicals occurs in less than 10⁻ⁱ⁰
sec after interaction with a photon
 Free radicals are extremely active and have very
short lives, quickly reforming into stable
configurations by dissociation or cross-linking

5. DIRECT EFFECT
 RH + x-radiation R˚ + H† + e⁻
Free radical fate
 Dissociation: R˚ X + Y˚
 Cross – linking: R˚ + S˚ RS
 Approx one third of biologic effects of x ray
exposure results from direct effects

6. RADIOLYSIS OF WATER
 photon + H₂O H˚ + OH˚
 This is a complex procedure, on balance water is
largely converted to hydrogen and hydroxyl free
radicals
 When dissolved oxygen is present in irradiated
water, hydroperoxyl free radicals may also be
formed
H˚ + O₂ HO₂ ˚

7.  Hydroperoxyl free radicals contribute to formation of
hydrogen peroxide in tissues
HO₂˚ + H˚ H₂O₂
HO₂˚ + HO₂˚ O₂ + H₂O₂
Both peroxyl radicals and hydrogen peroxide
are oxidising agents and are the primary toxins
produced in tissues by ionizing radiation

8. INDIRECT EFFECT
 In this both hydrogen and hydroxyl free radicals
interact with organic molecules resulting in
formation of organic free radicals
 Two thirds of radiation induced biologic damage
results from indirect effects
 RH + OH˚ R˚ +H₂O
 RH + H˚ R + H₂
 Such reactions may involve the removal of
hydrogen

9.  OH˚ free radical is more important in causing such
damage
 Organic free radicals are unstable and transform to
form into stable, altered molecules
 These altered molecules have different chemical
and biologic properties from the original molecules

10. CHANGES IN DNA
 This is the primary cause of radiation induced cell
death, heritable mutations and cancer formation
 Radiation produces different types of alterations,
1. Breakage of one or both DNA strands
2. Cross-linking of DNA strands within the helix to
other DNA strands or to proteins
3. Change or loss of a base
4. Disruption of hydrogen bonds b/w DNA strands

11. DETERMINISTIC AND STOCHASTIC EFFECTS
 Deterministic effects – radiation injury to organisms
results from killing of no of cells
 Severity of clinical effects is proportional to dose
 The greater the dose, the greater the effect
 Probability of effect independent of dose
 Stochastic effects – sublethal damage to individual cells
that results in cancer formation or heritable mutation
 Severity of clinical effects is independent of dose
 Shows all or none response, an individual either has
effect or does not
 Frequency of effect proportional to dose
 Greater the dose, greater the chance of effect

12. DETERMINISTIC EFFECTS ON CELLS
 Effects on intracellular structures
 On nucleus
 Chromosomes aberrations
 Effect on cell replication

13. EFFECTS ON INTRACELLULAR STRUCTURES
 Results from radiation induced changes in their
macromolecules
 These changes are manifest initially as structural
and functional changes in cellular organelles
 The changes may cause cell death

14. ON NUCLEUS
 Nucleus is more radiosensitive than cytoplasm,
specially in dividing cells
 Sensitive site in nucleus is DNA within
chromosomes

15. CHROMOSOMES ABERRATIONS
 Chromosomes serve as useful markers for radiation
injury, and extent of their damage is related to cell
survival
 It is seen in irradiated cells at the time of mitosis
when the DNA condenses to form chromosomes
 It is dependent on stage of cell in cell cycle at the
time of irradiation

16.  Chromatid abberation - If radiation exposure occurs
after DNA synthesis, in G2 or mid or late S, only
one arm of affected chromosome is broken
 Chromosome abberation - If radiation induced
break occurs before DNA has replicted, in G1 or
early S phase, the damage manifests as a break in
both arms

17. EFFECT ON CELL REPLICATION
 Radiation to rapidly dividing cell systems will cause
a reduction in size of irradiated tissue as a result of
mitotic delay and cell death
 Reproductive death – loss of capacity for mitotic
division in a cell population
 The three mechanisms of reproductive death are
DNA damage, bystander effect, and apoptosis

18. DNA DAMAGE
 Cell death is caused by damage to DNA, which
inturn causes chromosome abberations, which
cause the cell to die during the first few mitosis after
irradiation
 It is the rate of cell replication in various tissues and
thus the rate of reproductive death
 In a sample of slowly dividing cells is irradiated,
larger doses and longer time intervals are required
for induction of deterministic effects

19. BYSTANDER EFFECT
 Cells that are damaged by radiation and release
molecules into their immediate environment that kill
nearby cells
 This effect causes chromosome abberations, cell
killing, gene mutations and carcinogenesis

20. APOPTOSIS
 Also known as programmed cell death, occurs
during normal embryogenesis
 Cells round up, draw awayfrom their neighbors and
condense nuclear chromatin
 This can be induced in both normal tissue and in
some tumors
 It is most common in hemopoietic and lymphoid
tissues

21. RECOVERY
 Cell recovery from DNA damage and bystander
effect involves enzymatic repair of single strand
breaks of DNA
 So, a higher total dose is required to achieve a
given degree of cell killing when multiple fractions
are used

22. RADIOSENSITIVITY AND CELL TYPE
Most radiosensitivity cells have foll characteristics
 A high mitotic rate
 Undergo many future mitoses
 Are most primitive in differentiation

23. RELATIVE RADIOSENSITIVITY
Characteristic
s
HIGH: Divide regularly,
Long mitotic figures,
Undergo no or little
differentiation b/w
mitosis
INTERMEDIATE:
Divide
occasionally in
response to
demand for more
cells
LOW: Highly
differentiated,
when mature are
incapable of
division
Examples Spermatogenic and
erytroblastic stem cells,
Basal cells of OMM
Vascular
endothelial cells,
fibroblasts, acinar
and ductal
salivary gland
cells,
parenchymal cells
of liver, kidney
and thyroid
Neurons, striated
mm cells,
squamous ept
cells, erythrocytes

24. DETERMINISTIC EFFECTS ON TISSUES AND
ORGANS
 Short term effects on tissue is determined primarily
by sensitivity of its parenchymal cells
 When continuously proliferating tissues are
irradiated with moderate dose, cells are lost
primarily by reproductive death, bystander effect
and apoptosis
 Extent of cell loss depends on damage to stem cell
pools and proliferative rate of cell population
 Effect becomes apparent as a reduction in no of
mature cells in a series

25. DETERMINISTIC EFFECTS ON TISSUES AND
ORGANS
 Long term effects: results in loss of parenchymal cells
and replacement of fibrous connective tissue
 This is caused by reproductive death of replicating cells
and by damage to fine vasculature
 Damage to capillaries leads to narrowing and eventually
obliteration of vascular lumens
 This impairs the transport of oxygen, nutrients and
waste products and results in death of all cell types
 Thus both dividing and non dividing parenchymal cells
are replaced by fibrous connective tissue, a progressive
fibroatrophy of the irradiated tissue

26. MODIFYING FACTORS
 The response of cells, tissues and organs depends
on exposure conditions and cell environment
 Dose
 Dose rate
 Oxygen
 Linear energy transfer

27. DOSE
 Severity of deterministic damage is dependent on
amount of radiation received
 In all individuals, receiving doses above threshold
level, the amount of damage is proportional to dose

28. DOSE RATE
 Dose rate indicates the rate of exposure
 Exposure to a dose at a high dose rate causes
more damage than exposure to the same total dose
given at a lower dose rate
 Low dose rate allows for opportunity to repair the
damage

29. OXYGEN
 The radioresistance of many biologic systems
increases by a factor of 2 or 3 when exposure is
made with reduced oxygen
 The cell damage in the presence of oxygen is
related to formation of hygrogen peroxide and
hydroperoxyl free radicals
 It is important coz hyperbaric oxygen therapy may
be used during radiation therapy of tumors having
hypoxic cells

30. LINEAR ENERGY TRANSFER
 The dose required to produce a certain biologic
effect is reduced as the LET of the radiation is
increased
 Thus, higher LET radiations are more efficient
indamaging biologic systems coz their high
ionization density is more likely than x rays to
induce double strand breakage in DNA
 Low LET radiations such as x rays deposit their
energy in the absorber and thus are more likely to
cause single strand breakage and less biologic
damage

31. RADIOTHERAPY IN THE ORAL CAVITY
 Effect on oral tissues
1. Oral mucous membrane
2. Taste buds
3. Salivary glands
4. Teeth
5. Radiation caries
6. Bone
7. Musculature

32. RATIONALE
 Fractionation of the total x ray dose into multiple small
doses provides greater tumor destruction than is
possible with a large single dose
 Fractionation allows for increased cellular repair of
normal tissues, also allows for increasing the mean
oxygen tension in irradiated tumor, rendering the tumor
cells radiosensitive
 Results in killing rapidly dividing tumor cells and
shrinking the tumor mass after first few fractions,
reducing the distance that oxygen must diffuse from fine
vasculature through tumor to reach remaining viable
tumor cells

33. EFFECT ON ORAL MUCOUS MEMBRANE
 It contains a basal layer of rapidly dividing,
radiosensitive stem cells
 Near the end of 2nd week of therapy, as some cells
die, mm begins to show areas of redness and
inflammation
 As therapy continues, mm begins to separate from
underlying CT, with formation of white to yellow
pseudomembrane
 At end of therapy, mucositis is most severe,
discomfort is at maximum, and food intake is
difficult
 Topical anesthetics may be required at meal time
 Complication: secondary yeast infection by C .
Albicans

34.  After irradaition is complete, mucosa begins to heal
rapidly
 Healing is usually complete by about 2 months
 Later, mm tends to become atrophic, thin, &
relatively avascular
 This long term atrophy results from progressive
obliteration of the fine vasculature and fibrosis of
the underlying CT
 These changes complicate denture wearing coz
they cause oral ulcerations of compromised tissue

35. TASTE BUDS
 Are sensitive to radiation
 Therapeutic dose cause extensive degeneration of the
histologic architecture of taste buds
 Pts often notice a loss of taste acuity during 2nd or 3rd
week of radiotherapy
 Bitter and acid flavors are more severly affected when
posterior 2/3rd of tongue is irradiated
 Salt and sweet is lost when anterior 1/3rd is irradiated
 Taste acuity decreases by a factor of 1000 to 10,000
during radiotherapy course
 Taste loss is reversible and recovery takes 60 to 120
days

36. SALIVARY GLANDS
 Parenchymal component of salivary gland is
radiosensitive
 A marked and progressive loss of salivary
secretion is usually seen in the first few weeks
after initiation of radiotherapy
 Extent of reduced flow is dose dependent and
reaches zero at 60 Gy
 Mouth becomes dry and tender and swallowing
is difficult and painful
 Pts with irradiation of both glands are more
likely to c/o dry mouth and difficulty with
chewing and swallowing

37.  Serous cells are more r’sensitive than mucous cells
and residual saliva is more viscous
 The saliva has a reduced pH, 1 unit less
 This pH is sufficient to cause decalcification
 Buffering capacity of saliva falls as much as 44%
 If some portions of salivary gland are spared,
dryness usually subsides in 6 to 12 months coz of
compensatory hypertrophy
 In months later, inflammatory response become s
chronic and glands demonstrate progressive
fibrosis, adiposis, loss of fine vasculature and
concommitant parenchymal degeneration

38. TEETH
 Childrens subjected to radiation therapy may
show defects in permanent dentition such as
retarded root development, dwarfed teeth or
failure to form one or more teeth
 If exposure precedes calcification, irradiation
may destroy the tooth bud
 Irradiation after calcification has begun may
inhibit cellular differentiation, causing
malformations and arresting general growth
 Eruptive mechanism is totally r’resistant

39. RADIATION CARIES
 It is a rampant form of dental decaypts
receiving r’therapy show acidogenic saliva
and plaque and show an increase in S
mutans, Lactobacillus and Candida
 Caries is due to reduced salivary flow,
decreased pH, reduced buffering capacity,
increased viscosity and altered flora
 Reduced saliva has a low concentration of
Ca†², resulting in greater solubility of tooth
structure and reduced remineralization

40. 3 TYPES OF RADIATION CARIES
 Widespread superficial lesions attacking buccal,
occlusal, incisal and palatal surfaces
 Another type involves primarily the cementum and
dentin in cervical region. These lesions may
progress circumferentially and result in loss of
crown
 Last type, appears as a dark pigmentation of the
entire crown, incisal edges may be markedly worn

41. TREATMENT
 Daily application for 5 minutes of viscous topical 1%
neutral NaF gel in custom made applicator trays
 Use of topical fluoride causes a 6 month delay in
irradiation induced elevation of S. mutans
 Restorations, excellent oral hygiene, cariogenic
food restriction and NaF application
 Grossly carious or periodontally involved teeth are
extracted before irradiation

42. BONE
 Primary damage to mature bone results from
radiation induced damage to vasculature of
periosteum and cortical bone
 Radiation also acts by destroying osteoblasts and
to a lesser extent osteoclasts
 Normal marrow may be replaced with fatty marrow
and fibrous CT
 The marrow becomes hypovascular, hypoxic and
hypocellular
 The endosteum becomes atrophic, showing a lack
of osteoblastic and osteoclastic activity

43.  Reduced degree of mineralization leading to
brittleness
 When these changes are so severe that bone death
results and bone is exposed – osteoradionecrosis
 Osteoradionecrosis is the most serious clinical
complication
 The decreased vascularity of mandible renders it
easily infected by microorganisms from the oral
cavity

44.  This bone infection results from radiation induced
breakdown of oral mucous membrane, by
mechanical damage to the weakened mm such as
from a denture sore or tooth extraction, through a
periodontal lesion or radiation caries
 More common in mandible than maxilla

45. MUSCULATURE
 Radiation may cause inflammation and
fibrosis resulting in contracture and trismus
in muscles of mastication
 Usually masseter or pterygoid mm are
involved
 Restriction in mouth opening usually starts
about 2 months after radiotherapy is
completed and progresses thereafter
 An exercise program may be helpful in
increasing opening distance

46. DETERMINISTIC EFFECTS OF WHOLE BODY
IRRADIATION
Acute Radiation Syndrome:
 It is a collection of signs and symptoms
experienced by persons after acute whole body
irradiation
DOSE (Gy) MANIFESTATION
1 to 2 Prodormal symptoms
2 to 4 Mild hematopoietic
symptoms
4 to 7 Severe hematopoietic
symptoms
7 to 15 Gastrointestinal symptoms
50 CVS & CNS symptoms

47. PRODORMAL PERIOD
 Within the first few minutes to hours after exposure to whole
body irradiation of about 1 .5 Gy an individual experiences
anorexia, nausea, vomitting, diarrhea, weakness and fatigue
 The higher the dose, the more rapid the onset and the
greater the severity of symptoms

48. LATENT PERIOD
 A period of well being during which no signs and
symptoms of radiation sickness occurs
 This period extends from hours or days after
supralethal exposures to a few weeks after
exposure

49. HEMATOPOIETIC SYNDROME
 Whole body exposures of 2 to 7 Gy cause injury to
the hematopoietic stem cells of bone marrow and
spleen
 The high mitotic activity of these cells makes bone
marrow a highly radiosensitive tissue
 Doses in this range cause a rapid fall in the nos of
circulating granulocytes, platelets and finally
erythrocytes

50.  Clinical signs includes infection, hemorrhage and
anemia
 Death from hematopoietic syndrome occurs in 10-
30 days after irradiation

51. GASTROINTESTINAL SYNDROME
 In the range of 7 to 15 Gy, causing extensive damage to
GI system in addition to hematopoietic syndrome
 It causes extensive injury to the rapidly proliferating
basal ept cells of intestinal villi and leads to rapid loss of
ept layer of intestinal mucosa
 Coz of denuded mucosal surface, there is loss of
plasma and electrolytes, loss of efficient intestinal
absorption and ulceration of mucosal lining with
hemorrhage into the intestines
 These changes are responsible for diarrhea,
dehydration and loss of weight
 Endogenous intestinal bacteria rapidly invade the
denuded surface causing septicemia

52.  When damage to GI system reaches a
maximum, effect of bone marrow
depression is beginning to be manifested
 Result is marked lowering of bodys defense
against bacterial infection and a decrease in
effectiveness of clotting mechanism
 Combined effects of both symptoms causes
death within 2 weeks from fluid and
electrolyte loss, infection and possibly
nutritional impairment

53. CVS & CNS SYNDROME
 Exposures in excess of 50 Gy usually cause
death in 1 or 2 days
 At this level collapse of circulatory system
with a precipitous fall in blood pressure in
the hours preceding death
 Victims also may show intermittent stupor,
incordination, disorientation and convulsions
suggestive of extensive damage to the
nervous system

54. MANAGEMENT
 Antibiotics are indicated when granulocyte count
falls
 Fluid and electrolyte replacement as necessary
 Whole body transfusions to treat anemia and
platelets to arrest thrombocytopenia

55. RADIATION EFFECTS ON EMBYROS AND
FETUSES
 They are more radiosensitive than adults because most
embryonic cells are relatively undifferentiated and
rapidly mitotic
 Exposures in the range of 2 to 3 Gy during the first few
days after conception cause undetectable death of
embryo
 The first 15 weeks includes the period of organogenesis
when major organ systems form
 Symptoms in early gestation include, reduced growth,
microcephaly, mental retardation, small birth size,
cataracts, genital and skeletal malformations and
micropthalmia
 The period of maximal sensitivity of brain is 8 to 15
weeks after conception

56. LATE EFFECTS
 Growth & development – children showed reduced height,
weight and skeletal development.
 The younger the individual at the time of exposure, the more
pronounced effects are seen
 Cataracts – threshold ranges from 0.6 Gy when a single
dose is given and ˃5 Gy when received in multiple doses
over a period of weeks
 Life span shortening – reduction ranges from 2 months upto
2.6 yrs with overall mean of 4 months

57. STOCHASTIC EFFECTS
 Results from sublethal changes in DNA of individual
cells
 Most important consequence is carcinogenesis and
less likely heritable effects are seen

58. CARCINOGEESIS
 Causes cancer by modifying DNA
 Mechanism is radiation induced gene mutation
 Radiation acts as a initiator or promoter, stimulating
cells to multiply
 Finally, converts premalignant cells to malignant
ones

59. LEUKEMIA
 Incidence of leukemia rises after exposure of bone
marrow to ir coz of radiation
 Leukaemias appear sooner than solid tumors coz of
higher rate of cell division and differentiation of
hematopoietic stem cells
 Persons younger than 20 yrs are more at risk

60. THYROID CANCER
 Incidence increases after exposure
 Susceptibility is increased in childhood
 Females are 2 to 3 times more susceptible
 Esophageal cancer

61. BRAIN AND NERVOUS SYSTEM CANCERS
 Pts exposed to diagnostic x ray examinations in
utero and to therapeutic doses in childhood or as
adults show excess nos of malignant and benign
brain tumors

62. SALIVARY GLAND CANCER
 It is increased in pts treated with irradiation for
diseases of head and neck
 Risk is highest in persons receiving full mouth
examinations before 20 yrs of age
 Individuals receiving a cumulative parotid dose of
0.5 Gy or more showed a significant correlation b/w
dental radiography and salivary gland tumors

63. CANCER OF OTHER ORGANS
 Other organs sucha as skin, paranasal sinuses and
bone marrow show excess exposure after exposure

64. HERITABLE EFFECTS
 Are changes seen in the offspring of irradiated
individuals
 It is a consequence of damage to the genetic
material of reproductive cells
 Doubling dose: a way to measure the risk from
genetic exposure is by determining this dose
 It is the amount of radiation a population requires
to produce in the next generation as many
additional mutations as arise spontaneously
 In humans, the genetic doubling dose is estimated
to be 1 sievert