Session II.2.9a
Part II Quantities and Measurements
Module 2 Dosimetric Calculations and
Measurements
Session 9a Microdosimetry
3/2003 Rev 1
IAEA Post Graduate Educational Course
Radiation Protection and Safe Use of Radiation Sources
II.2.9a – slide 1 of 25
Overview
 Microdosimetry theory and principles will be
discussed
 Students will learn about microdosimetry
theory and principles, track descriptive
approach, limitations on the concept of linear
energy transfer (LET), site relevant approach,
and various stochastic quantities created to
support microdosimetry
3/2003 Rev 1
II.2.9a – slide 2 of 25
Content
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

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Theory of microdosimetry
Track descriptive approach
Limitations on concept of LET
Site relevant approach
Stochastic quantities (energy deposit,
energy imparted, specific energy
imparted, and lineal energy)
3/2003 Rev 1
II.2.9a – slide 3 of 25
Theory of Microdosimetry
 Microdosimetry most generally means
determination of an absorbed dose on a
microscopic scale of spatial distribution
 More specifically, it is the science that deals
with the spatial, temporal, and energyspectral distributions of energy imparted in
cellular and subcellular biological structures
3/2003 Rev 1
II.2.9a – slide 4 of 25
Theory of Microdosimetry
 It includes the relationship of such
distributions to biological effects
 Microdosimetry seeks to express the
"quality" of radiation in terms of subtle
physical parameters sufficient to allow
quantitative prediction of biological effects
for different types of ionizing radiation
3/2003 Rev 1
II.2.9a – slide 5 of 25
Track-Descriptive Approach
 Earliest approaches to microdosimetry
focused on the rate of energy loss of the
charged particles that deliver dose
 Linear energy transfer (L) was defined and
applied to radiobiological target theories
 L is a complex descriptive parameter
3/2003 Rev 1
II.2.9a – slide 6 of 25
Limitations on L
 Range effect:
 L says nothing about the range of the
particle or whether it can traverse a given
biological target volume
 If the particle stops in the target volume,
L is obviously irrelevant
3/2003 Rev 1
II.2.9a – slide 7 of 25
Limitations on L
 Delta-ray production:
 L does not describe the diameter of a
charged-particle track, only the rate of
energy loss along the track
 For example, a 10-MeV proton and a
167-MeV alpha particle have the same L
but the velocity ( = v/c) of the alpha
particle is twice that (0.145) of the proton
3/2003 Rev 1
II.2.9a – slide 8 of 25
Limitations on L
 Thus, the maximum energy of the -rays generated
by the alpha particle is 92 keV, while that for the
proton is 21 keV
 In water these  -rays would have ranges of about
120 and 9 m, respectively
 The diameter of the alpha-track would be 13 times as
large as that of the proton track in this case, and the
average energy density per unit volume in the alpha
track would be less than 1% of that in the proton
track
3/2003 Rev 1
II.2.9a – slide 9 of 25
Limitations on L
 Random variations (energy-loss straggling):
 L describes the expectation value of the rate of
energy loss by a charged particle of a given type
and energy
 It does not address the random nature of energy
losses along a track, which may leave zero
energy in a small target volume or give it orders
of magnitude more energy than would be
predicted by L
3/2003 Rev 1
II.2.9a – slide 10 of 25
Site-Relevant Approach to
Microdosimetry
 Different approach was needed to take into account
the previously discussed limitations on L
 The approach must account for the energy spent by
radiation in a defined site volume
 Approach was developed by Rossi (1955) and
defined new stochastic quantities in terms of which
the energy dissipated in microscopic sites by
individual ionizing events could be stated
3/2003 Rev 1
II.2.9a – slide 11 of 25
Harald Rossi, Father of
Microdosimetry
3/2003 Rev 1
II.2.9a – slide 12 of 25
Stochastic Quantities
 Energy Deposited, i
 Energy Imparted, 
 Specific Energy Imparted, z
 Lineal Energy, y
3/2003 Rev 1
II.2.9a – slide 13 of 25
Energy Deposit, i
This is the energy deposited in a single
interaction, i:
i = Tin – Tout + Qm
3/2003 Rev 1
II.2.9a – slide 14 of 25
Energy Imparted, 
This quantity is defined as:
 =  i
i
 is to be expressed in joules or eV
3/2003 Rev 1
II.2.9a – slide 15 of 25
Specific Energy Imparted, z
This is the quotient of  by m, where  is the
energy imparted by ionizing radiation to matter
of mass m:
z = /m
z is to be expressed in J/kg or grays
3/2003 Rev 1
II.2.9a – slide 16 of 25
Lineal Energy, y
_
This is the quotient of  by l, where  is the
energy imparted to the matter in a volume
by a single energy-deposition event, and l is
the mean chord length in that volume:

Y = _
l
3/2003 Rev 1
II.2.9a – slide 17 of 25
Absorbed Dose
 The absorbed dose, D, is given by:
_
_
 D = lim (/m ) = d  /dm
m0
_
 Here,  is the expectation value of the
stochastic quantity energy imparted
3/2003 Rev 1
II.2.9a – slide 18 of 25
Absorbed Dose vs Kerma
 Under charged particle equilibrium in the
material at the point of interest:
 D = K – B, where B represents
bremsstrahlung losses.
 If bremsstrahlung losses are negligible,
D = K
3/2003 Rev 1
II.2.9a – slide 19 of 25
Boron Neutron Capture Therapy
(BNCT) Microdosimetry
3/2003 Rev 1
II.2.9a – slide 20 of 25
Prototype Silicon-Based
Microdosimeter
3/2003 Rev 1
II.2.9a – slide 21 of 25
Simulation of Alpha Particle
Interacting in Silicon Microdosimeter
3/2003 Rev 1
II.2.9a – slide 22 of 25
Summary
 Microdosimetry theory and principles were
discussed
 Students learned about microdosimetry
theory and principles, track descriptive
approach, limitations on the concept of linear
energy transfer (LET), site relevant approach,
and various stochastic quantities created to
support microdosimetry
3/2003 Rev 1
II.2.9a – slide 23 of 25
Where to Get More Information
 Knoll, G.T., Radiation Detection and
Measurement, 3rd Edition, Wiley, New York (2000)
 Attix, F.H., Introduction to Radiological Physics
and Radiation Dosimetry, Wiley, New York (1986)
 International Atomic Energy Agency,
Determination of Absorbed Dose in Photon and
Electron Beams, 2nd Edition, Technical Reports
Series No. 277, IAEA, Vienna (1997)
3/2003 Rev 1
II.2.9a – slide 24 of 25
Where to Get More Information
 International Commission on Radiation Units and
Measurements, Quantities and Units in Radiation
Protection Dosimetry, Report No. 51, ICRU,
Bethesda (1993)
 International Commission on Radiation Units and
Measurements, Fundamental Quantities and Units
for Ionizing Radiation, Report No. 60, ICRU,
Bethesda (1998)
3/2003 Rev 1
II.2.9a – slide 25 of 25
Where to Get More Information
 Hine, G. J. and Brownell, G. L., (Ed. ), Radiation
Dosimetry, Academic Press (New York, 1956)
 Bevelacqua, Joseph J., Contemporary Health
Physics, John Wiley & Sons, Inc. (New York, 1995)
 International Commission on Radiological
Protection, Data for Protection Against Ionizing
Radiation from External Sources: Supplement to
ICRP Publication 15. A Report of ICRP Committee 3,
ICRP Publication 21, Pergamon Press (Oxford, 1973)
3/2003 Rev 1
II.2.9a – slide 26 of 25
Where to Get More Information
 Cember, H., Introduction to Health Physics, 3rd
Edition, McGraw-Hill, New York (2000)
 Firestone, R.B., Baglin, C.M., Frank-Chu, S.Y., Eds.,
Table of Isotopes (8th Edition, 1999 update), Wiley,
New York (1999)
 International Atomic Energy Agency, The Safe Use
of Radiation Sources, Training Course Series No. 6,
IAEA, Vienna (1995)
3/2003 Rev 1
II.2.9a – slide 27 of 25