X-rays and neutrons interact differently with biological material based on their ionizing ability. X-rays produce sparse ionization while neutrons produce more dense ionization. Linear energy transfer (LET) quantifies the energy deposited over track length and is used to compare radiation types. Higher LET radiation like alpha particles are more biologically effective due to producing denser ionization over shorter tracks. The relative biological effectiveness (RBE) of radiation depends on factors like dose, fractions, and biological system and is calculated as the ratio of doses needed for equal effect compared to a reference radiation like x-rays. RBE increases with increasing LET up to 100keV/μm then decreases with further increases in LET. Oxygen enhancement ratio (

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