LET & RBE along with Radioprotectors

1. Linear Energy Transfer and
Relative Biological Effectiveness
By
Dr.Ayush Garg

2. Deposition Of Radiant Energy
• If radiation is absorbed in biologic material,
the events(ionization) tend to localize along
the tracks of individual particles in a
pattern that depends upon the type of
radiation involved.

3. • X-ray photons give rise to fast electrons carrying unit
electrical charge and have very less mass. The primary
events of x-rays are well separated in space and hence said
to be sparsely ionizing.
• Cobalt 60-γ-rays are even more sparsely ionizing than x-
rays
• Neutrons give rise to recoil protons carrying unit electrical
charge but mass 2000 times greater than that of electrons.
Neutrons are intermediately ionizing.
• α-particles carry 2 electrical charges and 4 times heavier
than a proton. They are densely ionizing.

4. Densely vs. Sparsely ionizing
Sparsely ionizing: ionizing events are well separately in the
space, like: X-ray

5. Time
Sparsely ionizing radiation

6. High dose sparsely ionizing radiation
Time

7. Linear Energy Transfer
• LET is the energy transferred per unit length of the track.
• Unit -kiloelectron volt per micrometer (keV/um) of unit
density material.
• In 1962, the International Commission on Radiological
Units defined this quantity as follows:
– The LET (L) of charged particles in medium is the quotient of
dE/dl, where dE is the average energy locally imparted to the
medium by a charged particle of specified energy in traversing
a distance of dl.
• That is, L= dE/dl
7

8. Low and High LET Radiations
• 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
ionizing events.
– Eg. x-rays, gamma rays
• 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

9. gamma rays
deep therapy
X-rays
soft X-rays
alpha-particle
HIGH LET
Radiation
LOW LET
Radiation
Separation of ion clusters in relation to
size of biological target
4 nm
The Spatial Distribution of Ionizing Events Varies with
the Type of Radiation and can be defined by LET

10. • High LET radiation ionizes water into H and OH radicals over a
very short track. In fig 1, two events occur in a single cell so as to form
a pair of adjacent OH radicals that recombine to form peroxide, H2O2,
which can produce oxidative damage in the cell.
• Low LET radiation also ionizes water molecules, but over a much
longer track. In fig 2, two events occur in separate cells, such that
adjacent radicals are of the opposite type: the H and OH radicals
reunite and
reform H2O.
High vs Low LET Radiations

11. • High-LET radiations are more destructive to biological material than low-LET
radiations.
• The localized DNA damage caused by dense ionizations from high-LET
radiations is more difficult to repair than the diffuse DNA damage caused by
the sparse ionizations from low-LET radiations.
• High LET radiation results in lower cell survival per absorbed dose than low
LET radiation.
• High LET radiation is aimed at efficiently killing tumor cells while
minimizing dose to normal tissues to prevent toxicity.
• Biological effectiveness of high LET radiation is not affected by the time or
stage in the life cycle of cancer cells, as it is with low LET radiation.

12. • Track Average: calculated by dividing the track into equal
lengths, calculating the energy deposited in each length, and
finding the mean.
• 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.

13. Typical LET Values
Higher the
energy
Lower is
LET
Therefore
lower its
biologic
effectiveness

14. Relative Biologic Effectiveness(RBE)
• The National Bureau of Standards in 1954 defined RBE
as:
• The RBE of some test radiation (r) compared with x-rays is defi ned 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.
• Eg. A comparison of neutrons with 250kV x-rays in lethality
of plant seedlings. The end point of observation being death
of half of plants(LD50). Suppose if LD50 for x-rays is 6Gy
and for neutrons is 4Gy then RBE of neutrons compared
with x-rays is 6:4 or 1.5

15. Factors Determining RBE
• Radiation quality
• Radiation dose
• Number of dose fractions
• Dose rate
• Biologic system or end point

16. Survival curves for mammalian cells exposed to x-rays
and fast neutrons
•X-ray survival curve has large initial
shoulder and neutron curve has smaller
shoulder and steeper final slope
•RBE increases with decrease in dose
•RBE for fractionated regimen with
neutrons is greater than for single
exposure.
•The little or no shoulder of neutron
curve indicates less wastage of dose
whereas wide shoulder of x-ray curve
indicates wastage of a part of dose
each time in fractionated regime

17. RBE for different cells and tissues
17
• The intrinsic radiosensitivity
among the various types of cells
differ from each other.
• The curves demonstrate the
variation of radiosensitivites 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.

18. RBE as a function of LET
•As the LET increases from about
2keV/µm for x-rays upto 150 keV/µm
for α-particles, the survival curve
becomes steeper and the shoulder of the
curve becomes progressively smaller.
•Larger shoulder indicates the
accumulation and repair of the large
amount of sub-lethal radiation
damage

19. 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/µm. Between 10. and 100
keV/µm, the RBE increases rapidly
with increasing LET and in fact
reaches a maximum at about 100
keV/µm. Beyond this value for the
LET, the RBE again falls to lower
values.

20. The Optimal LET
• LET of about 100keV/µm is optimal in terms of producing
biologic effect
• At this density of ionization the average separation between
the ionizing events just about coincides with the diameter of
DNA double helix(2nm) and has highest probability of
causing DSBs by passage of a single charged particle.

21. In x-rays, probability of a single track causing a DSB is low and
requires more than one track.
Much more densely ionizing radiations (eg. LET of 200keV)
readily produce DBSs but energy is wasted as events coincide with
each other

22. The Oxygen Effect and LET
• Oxygen enhanced ratio(OER) is the ratio of doses of
radiation administered under hypoxic to aerated conditions
needed to achieve the same biologic effect.
• OER for different types of radiations are as follows:
– X-rays: 2.5
– Neutrons: 1.6
– 2.5-MeV particles:1
– 4-MeV particles: 1.3

23. Survival curves for cultured cells of human origin in hypoxic
and aerated conditions determined for four different types of
radiation.
23

24. OER AS A FUNCTION OF LET
At low LET (x- or y-rays)
with OER between 2.5 and
3, as the LET increases, the
OER falls slowly until the
LET exceeds about 60
keV/µm, after which the
OER falls rapidly and
reaches unity by the time
the LET has reached about
200keV/µm.

25. OER AND RBE AS A FUNCTION OF LET
•The rapid increase in RBE
and the rapid fall of OER
occur at about the same
LET 100keV/µm .
•Two curves are virtually
mirror images of each
other.

26. Example
• To achieve 50% survival fraction, 250 kV
x-ray needs 2 Gy, but the tested particle
needs 0.66 Gy only
RBE = D250/Dt 2 = 2 / 0.66 = 3
RBE at survival fraction of 0.5 for the
tested particle is 3.

27. Radioprotectors

28. WAYS TO IMPROVE THE PROTECTION OF
NORMAL TISSUES

29. IDEAL RADIOPROTECTOR
• Preservation of the anti-tumor efficacy of radiation
• Wide window of protection against all types of toxicity
• High theraputic ratio
• High efficacy/toxicity profile(Low intrinsic toxicity profile)
• Easy and comfortable administration
• Reasonable cost-effectiveness

30. Chemical Radioprotectors
Radioprotector: A chemical compound that reduces
the biologic consequences of radiation
Some may protect whole animals as they cause
vasoconstrictions/upset metabolism to <O2
concentration. e.g. Sodium Cyanide, CO,
epinephrine, serotonin, histamine

31. • The most remarkable group of true
radioprotectors is the sulfhydryl (SH)
compounds.
• The simplest is cysteine, an SH compound
• containing a natural amino acid, the
structure of which is

32. • Bacq and his colleagues
• in Europe independently discovered that
cysteamine
• could also protect animals from total
• body irradiation. This compound has a
structure represented by

33. • Animals injected with cysteamine to
concentrations of about 150 mg/kg require doses
of x-rays 1.8 times larger than control animals to
produce the same mortality rate. This factor of 1.8
is called the dose reduction factor (DRF), defined
as

34. MECHANISM OF ACTION
• The mechanisms most implicated in SH mediated
cytoprotection include:
• 1. Free-radical scavenging that protects against
oxygen-based free radical generation by ionizing
radiations or chemotherapy agents such as
alkylating agents
• 2. Hydrogen atom donation to facilitate direct
chemical repair at sites of DNA damage

35. • Radioprotectors containing
a sulfhydryl group exert
their effect by scavenging
free radicals and by
reducing free-radical
damage to DNA. They are
most effective for
radiations characterized by
low linear energy transfer
(LET), becoming
progressively less effective
with increasing LET
because the amount of local
damage is so great.

36. HISTORY OF DEVELOPMENT OF
RADIOPROTECTERS

37. • After World War II, a development
programme was initiated in 1959 by the U.S.
Army at the Walter Reed Institute of
Research to identify and synthesize drugs
capable of conferring protection to
individuals in a radiation environment, but
without the debilitating toxicity of cysteine or
cysteamine.
• Over 4,000 compounds were synthesized and
tested.

38. Historically known fact
NH2
HS-CH2-CH
COOH
Problem was their toxicity
• Nausea and Vomiting
General structure:
i. A free SH group at one end
ii. Strong basic function, i.e. an amine or
guanidine at other

39. Toxicity of the compound decreased because
the phosphate group is stripped inside the
cell, and the SH group begins scavenging for
free radicals.
• The important discovery was made that the
toxicity of the compound could be greatly
reduced if the SH group was covered by a
phosphate group.

41. AMIFOSTINE (WR-2721) AS A
RADIOPROTECTOR IN
RADIOTHERAPY
• The only radioprotective drug approved by the U.S. Food and
Drug Administration (FDA) for use in radiation therapy is
amifostine (WR-2721), sold under the trade name Ethyol for use
in the prevention of xerostomia in patients treated for head and
neck cancer.
• The RTOG conducted a phase III randomized clinical trial,
which demonstrated the efficacy of amifostine in reducing
xerostomia in patients with head and neck cancer receiving
radiotherapy without prejudice to early tumor control.
• The drug was administered daily, 30 minutes before each dose
fraction in a multifraction regimen. Three months post treatment,
the incidence of xerostomia was significantly reduced

42. >30 min---NO
difference

43. <30 min--- Difference
present

44. Head & Neck Cancers
• SCC of H&N
• 75% parotid gland was present in the fields
• Dose was 200 mg/m2 daily,15–30 minutes
before each fraction of radiation therapy
(1.8 –2.0 Gy/day, 5 days per week for 5–7
weeks, to a total dose of 50–70 Gy).

46. • Amifostine is a phosphorothioate that is nonreactive and
does not readily permeate cells, primarily because of its
terminal phosphorothioic acid group.
• It is therefore a “prodrug.” When dephosphorylated by the
enzyme alkaline phosphatase, which is present in high
concentrations in normal tissues and capillaries, it is
converted to the active metabolite designated WR-1065.
• This metabolite readily enters normal cells by facilitated
diffusion and scavenges free radicals generated by ionizing
radiations or by drugs used in chemotherapy such as
alkylating agents.

47. Amifostin (WR-2721)
Phosphorothioate prodrug-inactive, does not readily
permeate cells.
Dephosphorylation by ALP(expressed on
endothelial cell lining & proximal renal tubular
cells)
Active thyol (WR 1065)
OxidationEnter in cell by facillited
diffusion
WR – 33278(polyamine like disulphide
metabolite)
Radioprotection

48. RADIOPROTECTORS AND
CHEMOTHERAPY
The experimental clinical use of amifostine
has shown that the compound offers
significant protection against nephrotoxicity,
ototoxicity, and neuropathy from cisplatin and
hematologic toxicity from cyclophosphamide.

49. DIETARY SUPPLEMENTS AS
COUNTERMEASURES TO
RADIATION
• Example is the soybean-derived serine protease
inhibitor known as the Bowman-Birk inhibitor
(BBI), which has long been proposed as a cancer
chemopreventive agent.
• Another possibility is a cocktail of common
antioxidants, including
• L-selenomethionine
• Ascorbic acid
• Nacetyl cysteine
• Alpha-lipoic acid
• Vitamin E succinate
• Coenzyme Q10.

50. THANK YOU