OER LET RBE

1. OER, LET & RBE
Presenter : Dr. KIRAN DOBRIYAL
Moderator: Dr. SWARUPA MITRA

2. Incident photon
Fast electron(e-)
Ion radical
Free Radical
Chemical changes from breakage of bonds
Biologic effects
10-15 second
10-10 second
10-9 second
10-5 second
Hours, days, months, years
Time Line(life span)
Mechanism of action of X-rays or Gamma rays

3. OER
• The oxygen enhancement ratio (OER) is the ratio of
doses under hypoxic to aerated conditions that
produce the same biologic effect.
• The presence or absence of molecular oxygen
dramatically influences the biologic effect of x-rays.
• Oxygen presence (aerated cells) increases radiation
effectiveness for cell killing.
• Lack of oxygen (hypoxic cells) results in more radio
resistant cells.

4. Oxygen Effect ❑ To produce its effect,
molecular oxygen must
be present during the
radiation exposure or
5
during the
of the freeat least
lifetime
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.

5. 7
Radio-sensitivity and O2 Concentration
❑ Most of the change of
sensitivity occurs as the
oxygen tension increases
from 0 to 30 mm Hg.
❑ A further increase of
oxygen content has little
further effect.
❑ A relative radio-sensitivity
halfway between anoxia and
full oxygenation occurs for a
pO2 of about 3
mm Hg, which
to a
of about
corresponds
concentration
0.5% oxygen.
7
Fig: The dependence of radio-sensitivity on oxygen
concentration

6. OER Effect
❑ OER varies from 2-3,
increasing with dose
❑ Low-LET radiations
❑ oxygen effect is more
pronounced
❑ High-LET radiations
❑ oxygen effect is non-
existent (OER = 1)
Low-LET radiation
8

7. OER
For sparsely ionising radiation(X-rays or Gamma Rays) :
OER (2.5-3.5)
At Low doses ~ 2.5
At high doses ~3.5

8. OER
For Densely Ionising radiation
Alpha particle: 1
Neutron : 1.6

9. Hypoxia in tumor tissue
Acute hypoxia:
Temporary closing of a tumor blood vessel due to
• malformed vasculature of the tumor, which lacks smooth muscle
• Incomplete endothelial lining and basement membrane.
Chronic hypoxia:
limited diffusion distance of oxygen through tissue that is respiring

10. Chronic Hypoxia
Chronic hypoxia results from the limited diffusion distance of oxygen through tissue that is respiring
Thomlinson and Gray

11. Reoxygenation
Hypoxic cells become
oxygenated after a dose of
radiation, is termed
reoxygenation.
if reoxygenation is
efficient between dose
fractions, the presence of
hypoxic cells does not have a
significant effect on the
outcome of a multi fraction
regimen.

12. Time to Reoxygenation

13. Linear Energy Transfer (LET)
Radiation is absorbed in biologic material, ionizations and excitations occur that are not
distributed at random but tend to be localized along the tracks of individual charged particles
in a pattern that depends on the type of radiation involved
The average energy deposited per unit length of track is called linear energy transfer
(LET).
The linear energy transfer (L) of charged particles in medium is the average energy locally
imparted to the medium(dE) by a charged particle of specified energy in traversing a
distance of dl.
L=dE/dL

14. Calculation of LET
LET is an average quantity , at microscopic level ,
the energy per unit track varies over a wide range.
➢ Track average is obtained 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.

15. The deposition of radiation energy
The spatial distribution of the ionizing events produced by different particles varies
enormously, as the charge-to-mass ratio is different for every particle.
If the primary events are well separated in space, it is called as sparsely ionizing.
On the other hand, if dense column of ionization is produced, then it is called as densely
ionizing.

16. LET
GAMMA RAYS
X-RAYS
ALPHA PARTICLES
IONS OF HEAVY NUCLEI
CHARGED PARTICLES
LOW ENERGY NEUTRONS
LOW LET HIGH LET

17. Low LET radiations
Are produced from X-rays or gamma rays
Short wavelength, high energy waves
Sparsely ionizing
Damage is usually caused indirectly
▪ Free radicals are formed
Damage is usually sublethal
▪ Repair enzymes reverse the damage

18. High LET Radiation
High-LET radiation includes particles that possess substantial mass
and charge.
These radiations cause dense ionization along their length of track.
Ex: Alpha article, Ions of heavy nuclei,
Because high-LET radiations deposit more energy per unit of biologic
tissue traversed, they are more destructive to biologic matter than are
low-LET radiations.

19. Typical Linear Energy Transfer Values

20. RELATIVE BIOLOGIC EFFECTIVENESS(RBE)
Biologic damage produced by radiation increases as the LET of radiation
increases.
Identical doses of radiations of various LETs do not produce the same biologic
effect.
Relative biologic effectiveness (RBE) : the relative capabilities of radiation with
differing LETs to produce a particular biologic reaction.

21. RBE(Relative Biological Effectiveness)
RBE of a test radiation (r)
=
D 250: doses of 250 kv x rays
D r : doses of test radiation.
to produce same biological effect
D250
Dr

22. RBE and Fractionated Doses
In fractionated dose, the
shoulder of the survival curve is
re-expressed after each dose
fraction; since the shoulder is
larger for x-ray than for neutron,
this result in an enlarged RBE for
fractionated treatment

23. ✓ As the LET increases, the RBE increases
slowly at first, and then more rapidly as
the LET increases beyond 10keV/µm.
✓ Between 10 &100 keV/µm, the RBE
increases rapidly with increasing LET
✓ RBE reaches a maximum at about 100
keV /µm.
✓ Beyond this value for the LET, the RBE
again falls to lower values
Relationship between LET & RBE

24. The Optimal LET & RBE
LET at ~100 keV/mm has the greatest RBE
producing most biological effect per unit dose.
At this density of ionization, the average
separation between ionizing events just about
coincides with the diameter of the DNA double
helix (20 Å or 2nm).
Radiation with this density of ionization has the
highest probability of causing a double-strand
break by the passage of a single charged
particle

25. RBE
RBE depends on :
Radiation quality i.e. LET
Radiation dose and dose rate
Number of dose fractions
Biological or system endpoint

26. Factors that determine RBE
Radiation quality (LET) : the type of radiation and its energy, whether
electromagnetic or particulate and whether charged or uncharged.
Radiation dose :RBE depends on the dose level and the number of dose fractions (
the dose per fraction) because the shape of the dose-response relationship varies for
radiations that differ substantially in their LET.
Number of dose fractions & Dose rate :RBE can vary with the dose rate because
the slope of the dose-response curve for sparsely ionizing radiations varies with the
dose rate.
Biological system or end point:RBE values are high for tissues that accumulate and
repair a great deal of sub-lethal damage and low for those that do not.

27. LET, RBE and OER
• In this graph OER & RBE are plotted
as a function of LET.
• These two curves are virtually mirror
image of each other.
• The rapid increase in RBE & the rapid
fall of the OER occur at about the
same LET, 100keV/mm.

28. Thank YouTHANK YOU