MEDICAL PHYSICS

1. OER , RBE & LET
DR. AMRITA RAKESH
DEPT. OF RADIATION ONCOLOGY

2. Law of Bergonie & Tribondeau
• Radiosensitivty of living tissues varies with maturation & metabolism;
1. Stem cells are radiosensitive. More mature cells are more resistant
2. Younger tissues are more radiosensitive
3. Tissues with high metabolic activity are highly radiosensitive
4. High proliferation and growth rate, high radiosensitivty

3. Response to RADIATION..
Two identical doses may not produce identical responses due to other modifying
factors
• Factors Affecting Radiation Response are,
Physical Factors
• Linear energy transfer
• Relative biological
effectiveness
• Fractionation &
protraction
Biological Factors
• Oxygen Effect
• Phase of cell cycle
• Ability to Repair
• Chemical Agents
• Hormesis

4. OXYGEN ENHACEMENT RATIO

5. Oxygen Enhancement Ratio
• Definition – “The ratio of doses administered under hypoxic to aerated
conditions needed to achieve the same biologic effect is called the Oxygen
Enhancement Ratio .”
• Large and important in the case of sparsely ionizing radiations , such as X-rays.
• Absent for densely ionizing radiation ; α – particles
• Intermediate value for fast neutrons.

8. • Typically ranges from 2.5-3.0 for X rays.
• For a synchronous cell population:
OER varies from according to cell cycle phase
• S phase: 2.9-3.5
• G2 and M phases: 2.3
• G1 phase: 2.5-2.6
• OER for sparsely ionizing radiation is >1 as the effects are O2 dependent ,
• However OER becomes unity for Densely ionizing particulate radiation as they are
directly ionizing (ά particles )
• Neutrons OER = 1.6
• ά Particles OER = 1

9. The time at which oxygen acts & the
mechanism of the oxygen effect:
• During or within microseconds after the radiation exposure.
• The oxygen acts at the level of free radicles.
• Absorption of radiation  fast charged particles  ion pairs  free radicals 
break chemical bonds  biologic damage.

10. Oxygen Fixation Hypothesis – oxygen
may be said to “fix” or make
permanent the radiation lesion.
• If molecular oxygen is present, DNA reacts with
the free radicals (R·).
• The DNA radical can be chemically restored to its
reduced form through reaction with a sulfhydryl
(SH) group.
• However, the formation of RO2·, an organic
peroxide, represents a nonrestorable form of the
target material; that is, the reaction results in a
change in the chemical composition of the
material exposed to the radiation.

11. Survival curves for Chinese hamster cells exposed to x-
rays in the presence of various oxygen concentrations.
Curve A is characteristic of the
response under conditions of
equilibration with air.
Curve B is a survival curve for
irradiation in as low a level of
hypoxia

12. The concentration of oxygen required :
As the oxygen concentration increases,
the biologic material becomes
progressively more sensitive to radiation,
until, in the presence of 100% oxygen,
it is about three times as sensitive as
under complete anoxia.
The rapid change of radiosensitivity occurs as the partial
pressure of oxygen is increased from 0 to about 30 mm
Hg (5% oxygen).
A relative radiosensitivity halfway
between anoxia and full oxygenation
occurs for a pO2 of about 3 mm Hg,
which corresponds to a concentration
of about 0.5% oxygen.

13. Chronic Hypoxia
• a paper by Thomlinson and Gray in 1955 ; described the phenomenon of chronic
hypoxia - study of fresh specimens of bronchial carcinoma.
• Chronic hypoxia results from the limited diffusion distance of oxygen through
tissue that is respiring.
• As the tumor cord enlarges , necrotic centre also grows ; the thickness of sheath
of viable tumor cells remain essentially constant.

15. The distance to which oxygen can diffuse is limited
largely by the rapid rate at which it is metabolized
by respiring tumor cells.
For some distance from a capillary, tumor cells are
well oxygenated (white).
At greater distances, oxygen is depleted, and tumor
cells become necrotic (black).
Hypoxic tumor cells form a layer, perhaps one or
two cells thick, in between (gray).
The distance to which oxygen can diffuse is
about 70 ᴜm at the arterial end of a capillary and
less at the venous end.

16. Acute Hypoxia
• Acute hypoxia is the result of the temporary closing of a tumor blood vessel
owing to the malformed vasculature of the tumor, which lacks smooth muscle
and often has an incomplete endothelial lining and basement membrane.
• Tumor blood vessels open and close in a random fashion, so that different regions
of the tumor become hypoxic intermittently.

17. There is clinical evidence
that in addition to causing
radioresistance, hypoxia may
play an important role in
malignant progression and
in metastasis

18. Reoxygenation
• phenomenon, by which hypoxic cells become oxygenated after a dose of
radiation, is termed reoxygenation.
• Putten and Kallman - When groups of tumors were exposed to five daily doses of
1.9 Gy delivered Monday through Friday, the proportion of hypoxic cells was
determined on the following Monday to be 18%.
• In another experiment, four daily fractions were given Monday through Thursday,
and the proportion of hypoxic cells measured the following day, Friday, was found
to be 14%.

19. • 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,
most cells in the tumor are hypoxic.
• the preirradiation pattern tends to return because of
reoxygenation.
• If the radiation is given in a series of fractions
separated in time sufficient for reoxygenation to occur,
the presence of hypoxic cells does not greatly influence
the response of the tumor.

20. • The “slow” component is caused by the reoxygenation of chronically hypoxic cells
as the tumor shrinks.
• The “fast” component of reoxygenation is caused by the reoxygenation of acutely
hypoxic cells as tumor blood vessels open and close.

21. Percentage of hypoxic cells in a transplantable mouse
sarcoma as a function of time after a dose of 10 Gy of x-rays
Immediately after irradiation, essentially 100% of the
viable cells are hypoxic because such a dose kills a large
proportion of the aerated cells.
By 6 hours after irradiation, the percentage of hypoxic
cells has fallen to a value close to the preirradiation
level.

22. HYPOXIC COMPONENT OF TUMOUR
• BIPHASIC SURVIVAL CURVE
• Powers & Tolmach investigated the
response to radiation of solid tumours
• The early steep part of curve was due
to killing of oxygenated cells
• Remaining anoxic cells were relatively
resistant with a D0 of 2.6 Gy as
compared to initial slope with a D0 of
1.1 Gy

23. • When this shallow portion of the curve is extrapolated back to the ordinate,
a value of ~ 0.01 or 1% is noted – this roughly represents the proportion of
hypoxic cells in the tumor
• Evidence that a solid tumor could contain cells that were sufficiently hypoxic to
be protected from cell killing by x-rays but still clonogenic and capable of
providing a focus for tumor regrowth

24. LINEAR ENERGY TRANSFER

25. Linear Energy Transfer
• Linear Energy Transfer (LET) is the rate at which energy is deposited as a
charged particle travels through matter by a particular type of radiation.
• Linear Energy Transfer (LET):the energy deposited per unit track.
• Unit is keV/m.

26. • LET is a function of:
1. mass (m)
2. charge (Q)
• LET is inversely proportional to the square of the velocity
• LET is directly proportional to the square of the charge
• LET ∝ Q2/V2

27. 1. LOW LET RADIATION-
EM radiation ( x and gamma rays) has no mass or charge, the Interacts with
matter by producing fast electrons which have small mass and -1 charge.
Because of the electron's fast speed and low mass the interactions that are
produced are far apart from each other,
Hence EM radiation is called Low LET radiation.

28. 2. MEDIUM LET RADIATION-
Neutrons even though they have no charge are highly ionizing particles because
of their mass.
3. HIGH LET RADIATION-
Alpha particles because of their mass and charge are even more highly ionizing
than neutrons .
These types of particles produce many ionizations in a short distance and hence
called as High LET radiation.

29. • The most commonly used method is to calculate the track average, which is
obtained by dividing the track into equal lengths, calculating the energy
deposited in each length, and finding the mean.
• The energy average is obtained by dividing the track into equal energy
increments and averaging the lengths of track over which these energy
increments are deposited.

31. LET < 10 keV / mm low LET
LET > 10 keV / mm high LET
250 kVp X rays 0.25 keV/μm.
Cobalt-60 g rays 0.3 keV/μm.
3 MeV X rays: 0.3 keV/μm
1 MeV electrons 0.25 keV/μm.
14 MeV neutrons 12 keV/μm.
Heavy charged
particles
100–200 keV/μm
1 keV electrons 12.3 keV/μm
10 keV electrons 2.3 keV/μm.

32. • With low LET radiation the
interactions that are produced
are relatively far apart from each
other, therefore, they will be
spread throughout the cell,
making for a more uniform dose
distribution throughout the cell.
With high LET radiation the particles give rise to well defined tracks
of ionization which cause extensive damage along the path.

33. THE OPTIMAL LET
• LET of about 100 keV/μm is optimal in terms of producing a biologic effect.
• At this density of ionization, the average separation in ionizing events is equal to
the diameter of DNA double helix which causes significant DSBs.
• DSBs are the basis of most biologic effects.
• The probability of causing DSBs is low in sparsely ionizing radiation such as x-rays
that has a low RBE.

35. Effect of LET on cell survival
• Survival curves for cultured cells of human
origin exposed to 250-kV X-rays, 15-MeV
neutrons, and 4-MeV alpha-particles.
• As the LET of the radiation increases, the
survival curve changes: the slope of the
survival curves gets steeper and the size of the
initial shoulder gets smaller

39. RELATIVE BIOLOGIC EFFECTIVENESS

40. • ABSORBED DOSE – measure of the enrgy absorbed per unit mass of the tissue.
• Equal doses of different types of radiation do not, however, produce equal
biologic effects.
• For example, 1 Gy of neutrons produces a greater biologic effect than 1 Gy of X-
Rays.
• The key to the difference lies in the pattern of energy deposition.
• In comparing different radiations, it is customary to use x-rays as the standard.
• RBE = dose in Gy from 250 keV X-rays / dose in Gy from another
radiation source to produce the same biologic response

41. • The National Bureau of Standards in 1954 defined relative biologic
effectiveness (RBE) as follows:
The RBE of some test radiation (r) compared with x-rays is
defined by the ratio D250/Dr, where D250 and Dr are, respectively,
the doses of x-rays and the test radiation required for equal biologic
effect.
• Reference : 250 KV X-Ray.

42. RBE is End-Point Depndent
• The survival curves for x-rays has a large
initial shoulder;
• For fast neutrons, the initial shoulder is
smaller and the final slope is steeper.
• RBE does not have a unique value but varies
with dose,getting larger as the size of the
dose is reduced.

43. RBE & Fractionated Doses
• RBE for a fractionated regimen with neutrons is greater than for a single exposure.
• Because a fractionated schedule consists of several small doses and the RBE is large
for small doses.
• Neutrons become progressively more efficient than X-rays as the dose per fraction
is reduced and the number of fractions is increased.
• The shoulder of the survival curve is re-expressed after each dose fraction; the fact
that the shoulder is larger for x-rays than for neutrons, results in an enlarged RBE
for fractionated treatments.

45. RBE for different cells and tissues
• The intrinsic radiosensitivity among the various types
of cells differ from each other.
• The curves demonstrate the variation of
radiosensitivities for x-rays and markedly less variation
for neutrons.
• X-ray survival curves have large and variable initial
shoulder whereas for neutrons ; it is small and less
variable.
• Hence,RBE is also different for different cell lines.

46. RBE as a function of LET
• As the LET increases, the RBE increases slowly at first and then more rapidly as the
LET increases beyond 10 keV/um.
• Between 10 and 100 keV/ μm, the RBE increases rapidly with increasing LET and
reaches the maximum at about 100 keV μm.
• Beyond this value for the LET, the RBE again falls to lower values.

47. In the case of x-rays, which
are more sparsely ionizing,
the probability of a single
track causing a DSB is low,
and in general, more than
one track is required.
much more
densely ionizing
radiations (with
an LET of 200
keV/um, for
example) readily
produce DSBs,
but energy is
“wasted” because
the ionizing
events are too
close together.
Because RBE is
the ratio of doses
producing equal
biologic effect,
this more densely
ionizing radiation
has a lower RBE
than the optimal
LET radiation.

48. Curves 1, 2, and 3
refer to cell-survival levels of 0.8, 0.1, and 0.01, respectively, illustrating that the
absolute value of the RBE is not unique but depends on the level of biological damage
and therefore, on the dose level.

49. FACTORS THAT DETERMINE RELATIVE BIOLOGIC EFFECTIVENESS
• Radiation quality (LET)
• Radiation dose
• Number of dose fractions
• Dose rate
• Biologic system or end point

50. RADIATION WEIGHTING FACTOR (Wr)
• To have a simpler way to consider differences in biologic effectiveness of different radiation.
• The term radiation weighting factor(Wr) has been introduced for this purpose by International
Commission on Radiological Protection (ICRP).
• The quantity produced by multiplying the absorbed dose by the weighting factor is called the
equivalent dose.
• The unit of absorbed dose is the gray, and the unit of equivalent dose is the sievert (Sv).
• The radiation weighting factor is set at unity for all low-LET radiations (x-rays, y-rays, and
electrons).
• value of 20 for maximally effective neutrons & a-particles.
• Representative RBE at low dose and low dose rate for biologic effects relevant to radiation
protection, such as cancer induction and heritable effects. It is used in radiologic protection to
reduce radiations of different biologic effectiveness to a common scale.

51. THE OXYGEN EFFECT AND LINEAR ENERGY TRANSFER
• At low LET, corresponding to x- or y-
rays, the OER is between 2.5 and 3.
• as the LET increases, the OER falls
slowly at first, until the LET exceeds
about 60 keV/um, after which the
OER falls rapidly and reaches unity
by the time the LET has reached
about 200 keV/um.

53. Points to summarise
• The presence or absence of molecular oxygen dramatically influences the biologic effect of x-rays.
• The OER is the ratio of doses under hypoxic to aerated conditions that produce the same biologic
effect.
• The OER for x-rays is about 3 at high doses and is possibly lower (about 2) at doses less than
about 2 Gy.
• The OER decreases as linear energy transfer increases. The OER approaches unity (i.e., no oxygen
effect) for -particles. For neutrons, the OER has an intermediate value of about 1.6.
• To produce its effect, molecular oxygen must be present during the radiation exposure or at least
during the lifetime of the free radicals generated by the radiation.
• Oxygen “fixes” (i.e., makes permanent) the damage produced by free radicals. In the absence of
oxygen, damage produced by the indirect action may be repaired.
• Only a small quantity of oxygen is required for radiosensitization; 0.5% oxygen (pO2 of about 3
mm Hg) results in a radiosensitivity halfway between hypoxia and full oxygenation.

54. • There are two forms of hypoxia that are the consequence of different mechanisms: chronic
hypoxia and acute hypoxia.
• Chronic hypoxia results from the limited diffusion range of oxygen through respiring tissue.
• Acute hypoxia is a result of the temporary closing of tumor blood vessels and is therefore
transient.
• In either case, there may be cells present during irradiation that are at a sufficiently low oxygen
tension to be intransigent to killing by x-rays but high enough to be viable.
• Most transplantable tumors in animals have been shown to contain hypoxic cells that limit
curability by single doses of x-rays. Hypoxic fractions vary from 0% to 50%, with a tendency to
average about 15%.
• Hypoxia in tumors can be visualized by the use of hypoxia markers such as pimonidazole or
hypoxia-inducible factors.
• Reoxygenation is the process by which cells that are hypoxic at the time of irradiation become
oxygenated afterward.
• The extent of reoxygenation and the rapidity with which it occurs vary widely for different
experimental animal tumors.
• If reoxygenation is rapid and complete, hypoxic cells have little influence on the outcome of a
fractionated radiation schedule.

55. • The “slow” component is caused by the reoxygenation of chronically hypoxic cells as the tumor
shrinks. The “fast” component of reoxygenation is caused by the reoxygenation of acutely hypoxic
cells as tumor blood vessels open and close.
• X- and y-rays are said to be sparsely ionizing because along the tracks of the electrons set in
motion, primary ionizing events are well separated in space.
• a-particles and neutrons are densely ionizing because the tracks consist of dense columns of
ionization.
• RBE increases with LET to a maximum at about 100 keV/um, thereafter decreasing with higher
LET.
• For radiation with the optimal LET of 100 keV/um, the average separation between ionizing
events is similar to the diameter of the DNA double helix (2 nm), so that DSBs can be most
efficiently produced by a single track.
• The RBE of high-LET radiations compared with that of low-LET radiations increases as the dose per
fraction decreases. This is a direct consequence of the fact that the dose-response curve for low-
LET radiations has a broader shoulder than for high-LET radiations.
• RBE values are high for cells or tissues that accumulate and repair a great deal of sublethal
damage, so that their dose-response curves for x-rays have a broad initial shoulder.

56. • RBE depends on the following:
Radiation quality (LET)
Radiation dose
Number of dose fractions
Dose rate
Biologic system or end point
• The OER has a value of about 3 for low-LET radiations, falls when the LET rises more than about
30 keV/um and reaches unity by an LET of about 200 keV/um.