radiobiology

2.  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.
 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.  Linear energy transfer (LET) is the energy
transferred per unit length of the track.
 Unit : kiloelectron volt per micrometer
(keV/μm)of unit density material.

5.  The linear energy transfer(L) of the
charged particles in the medium is the
quotient of the 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

6.  LET can be only an average quantity
because at the microscopic level, the
energy per unit length of track varies
over such a wide range
 Track Average: calculated by dividing the
track into equal lengths and averaging
the energy deposited in each length.
 Energy Average: calculated by dividing
the track into equal energy intervals and
averaging the lengths of the track that
contain this amount of energy.

7.  Useful as a simple and naïve way to
indicate the quality of different types of
radiation
 Higher the energy, lower the LET and
lower its biological effectiveness.

9.  The National Bureau of Standards in 1954
defined RBE as:
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 the equal
biologic effects.

10.  Radiation quality
 Radiation dose
 Number of dose fractions
 Dose rate
 Biologic system or end point

11.  The survival curve for
x-rays has a large
initial shoulder
 For fast neutrons, the
initial shoulder is
smaller and the final
slope is steeper.
 Because survival
curves have different
shapes, RBE does not
have a unique value
but varies with dose,
getting larger as the
size of the dose is
reduced

12.  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
curves 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

13.  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

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

15.  Oxygen is a powerful oxidizing
agent and therefore acts as a
radiosensitizer
 Its effects are measured as the
oxygen enhancement ratio
 O.E.R. = the ratio of doses
needed to obtain a given level
of biological effect under
anoxic and oxic conditions.
 For low LET radiation the
O.E.R. is 2.5- 3.0
 For neutrons, O.E.R is about
1.6

16.  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

17.  The rapid increase in RBE and the rapid fall of OER
occur at about the same LET 100keV/μm