RBE

1. RELATIVE BIOLOGICAL
EFFECTIVENESS
DR.ASMITA RAYAMAJHI
1ST YR RESIDENT

2.  Ratio of biological effectiveness of one
type of radiation relative to
another,given the same amount of
absorbed energy.
 Eg,.1 Gy of neutrons produces a greater
biologic effect than 1 Gy of Xrays.
 Empirical values that varies depending on
the particles,energies involved.

3.  Comparison of different radiation it is
customary to use x-rays as standard.
 The National Bureau of Standards
in 1954 defined RBE as
Ratio of D250 and Dr where D250 :dose
of x rays
Dr :dose of test radiation required for
equal biologic effect.

4.  Example:
 Comparison:RBE of fast neutron vs
250kV x-rays
 Test system:plant seedlings
 Observation:dose of x rays that result in
death of half of the plants and then of
neutrons that result in death of half of the
plants.
 If LD50 for xray:6Gy and LD50 for
neutrons:4Gy
 RBE:6:4 or 1.5

5. RBE DEPENDS ON
 Radiation Quality
 Radiation Dose
 Number of dose fraction
 Dose rate
 Biologic system or end point

6.  LET:Energy transferred per unit length
of tract.
 LE:dE/dL, where dE is average energy
locally imparted to the medium by the
charge particle of specified energy in
traversing distance of dL.

7.  As LET increases
from about
2keV/μm for xrays
upto 150keV/μm
for alpha
particles,the
survival curve
becomes steeper
and the shoulder
of curve becomes
progressively
smaller.

8. RBE AS A FUNCTION OF
LET
 For cell to be killed enough energy
must be deposited to produce
sufficient double strand breaks
 Low LET radiation is inefficient coz
more than one particle may have to
pass through the cell to produce
enough DNA breaks

9.  Densely ionizing radiation also is
inefficient coz more densely ionizing
radiation readily produces DNA break but
energy is wasted as are too close
together.
 Radiation of optimal LET deposits right
amount of energy per cell
 Optimal LET is around 100keV/μm.
 Radiation having optimal LET are few
hundred kilo-electron volts,low energy
protons and α particle.

10.  As LET increases,the
RBE increases slowly
at first,and then
more rapidly as LET
increases beyond
10keV/μm.
 RBE reaches
maximum at about
100keV/μm
 Beyond this value
RBE falls.

11. THE OPTIMAL LET
 At 100 keV/μm LET
is optimal?
At this density of
ionization average
separation
between the
ionizing events
coincides with
diameter of DNA
helix(2nm) causing
DNA break.

12. RBE DEPENDS UPON
DOSE
 Xrays and neutron
survival curve have
different shapes,the
xray survival curve
having an initial
shoulder and
neutron curve being
an exponential
function of
dose,resultant RBE
depends on the dose
level chosen

13. RBE AND FRACTIONATED
DOSES
 X ray curve has
larger initial
shoulder and
neutron curve has
smaller shoulder
and steeper slope.
 RBE increases
with decrease in
dose.
 RBE is larger for
small dose.

14. RBE FOR DIFFERENT CELLS
AND TISSUES
 RBE varies greatly
according to tissues
 Depends on
intrinsic
radiosensitivity that
differs from each
other.
 X ray survival curve
have large and
variable initial
shoulder whereas
for neutrons it is
small and less
variable.

15.  Cells characterized by x ray survival
curve with large shoulder,indicates
they can accumulate and repair,shows
larger RBE
 For neutrons it is small and less
variable
 RBE is different for different cells.

16. OXYGEN ENHANCEMENT
RATIO
 Ratio of the doses under hypoxic
condition to those under oxygenated
conditions to produce same biologic
effect.
 OER is measure of radiosensitizing
effect of oxygen

17.  As LET increases
OER falls slowly at
first.
 LET exceeds
60keV/μm,OER falls
rapidly
 Rapid increase in RBE
and rapid fall of OER
occurs at same LET
100keV/μm
 Two curves are
virtually mirror image
of each other.

18. RADIATION WEIGHTING
FACTOR
 Term radiation weighting factor has
been introduced by International
Commision of Radiological protection.
 Independent of tissue type
 Equivalent dose is the product of
absorbed dose and the radiation
weighting factor
 If absorbed dose is expressed in Gy,
 Unit of equivalent dose sievert.

19.  Radiation weighting factor is set at unity
for low LET radiations(x rays,γ rays and
electrons)with a value of 20 for
maximally effective neutrons and α
particles.
 Equivalent dose is designated by ICRP as
the limiting quantity to specify exposure
limits
 Used for risk and exposure assessment in
radiology and nuclear assessment.

20. Quality factor for different type of
radiation
TYPE OF RADIATION Q FACTOR
X RAY 1
BETA PARTICLES 1
GAMMA RAY 1
THERMAL NEUTRONS 5
FAST NEUTRONS 10
ALPHA PARTICLES 20

21. THANK YOU