Measurement of Absorbed Dose in Radiation Oncology

1. MEASUREMENT OF
ABSORBED DOSE
PRESENTER- Dr ABHISHEK MEWARA
MODERATOR- Mr. RATTAN SINGH
DATE- 25.1.2024
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2. OVERVIEW
1. FLUENCE
2. KERMA
3. EXPOSURE
4. ABSORBED DOSE
5. RELATIONSHIP BETWEEN KERMA AND ABSORBED DOSE
6. STOPPING POWER
7. BRAGG GRAY CAVITY THEORY
8. CHARGE MEASUREMENTS
9. UNITS AND CONVERSIONS
10. METHODS OF ABSORBED DOSE MEASUREMENTS
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3. • Radiation measurements / dosimetry deals with the methods
for quantitative determination of energy, deposited in a
medium by directly and indirectly ionizing radiation.
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4. FLUENCE
• Fluence is the total number of particles that cross a sphere of unit
cross section that surrounds a point source of ionizing radiation.
• Consists of particle fluence, energy fluence, particle fluence rate,
and energy fluence rate.
1. PARTICLE FLUENCE-
 It is the number of particles dN incident on a sphere of cross
sectional area dA.
=> dN/dA.
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5. 2. ENERGY FLUENCE-
Amount of radiant energy (dE) incident on a sphere of cross sectional
area (dA).
=> dE/dA.
 Unit= J/m²
3. PARTICLE FLUENCE RATE-
It is a quotient of increment of the fluence in a fixed time interval.
4. ENERGY FLUENCE RATE-
It is the quotient of increment of the energy fluence in a fixed time interval.
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6. KERMA
• Kinetic Energy Released per unit MAss.
• Kerma is the sum of kinetic energies of electrons and positrons
released by photons in a medium, per unit mass.
• The energy of photons is imparted to matter in 2 stages-
1. photon interactions
2. atomic excitations and ionizations
• It’s unit is J/Kg = Gy, i.e. same as dose.
• It has two types-
1. Collision kerma
2. Radiative kerma, aka bremsstrahlung reactions 6

7. EXPOSURE
• It is defined as dQ/dM, where dQ is the total charge of the
ions of one sign produced in air when all electrons liberated
by photons in air of mass dM are completely stopped in air.
• It is used for photon beams only.
• It is proportional to photon energy fluence.
• It is maximum at the surface and decreases exponentially
with depth.
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8. ABSORBED DOSE
• Applicable to both directly and indirectly ionizing radiation.
• It is defined as mean energy E imparted by radiation to
matter of mass M in a finite volume V.
• Its unit is J/Kg = Gy (i.e. same as Kerma)
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9. RELATIONSHIP BETWEEN KERMA
AND ABSORBED DOSE
• Kerma is maximum at the surface and decreases with depth.
• Hence the absorbed dose, which is directly related to kerma,
reduces at the same rate as kerma, with the depth.
• β is the ratio of absorbed dose to collision kerma.
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10. • β = 1 when the absorbed dose is equal to kerma at the state
of equilibrium.
• In the built up region, β < 1 because kerma is maximum at
surface.
• β > 1 in regions of transient electronic equilibrium, where
kerma decreases more than the absorbed dose at a
particular depth.
• For cobalt 60, β = 1.005.
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12. STOPPING POWER
• It is of two types-
1. linear stopping power
2. mass stopping power
• Linear stopping power is the rate of energy loss per unit
length of the charged particle.
• Mass stopping power is linear stopping power divided by the
density of the absorbing medium.
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13. • The stopping power is classified differently into 2 more
types-
1. Collision / ionizing stopping power
- resulting from the interaction of charged particles with
atomic orbital electrons.
2. Radiative stopping power
- resulting from interaction of charged particles with atomic
nuclei.
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14. BRAGG GRAY CAVITY THEORY
• The principle of Bragg–Gray (B-G) cavity theory is that the
ionization produced in a gas-filled cavity placed in a medium
is related to the energy absorbed in the medium surrounding
the cavity.
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15. • Fulfilment of this theory depends on the-
1. cavity size
2. range of electrons in the cavity medium
3. the cavity medium itself
4. electron energy
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16. CHARGE MEASUREMENTS
• Fully corrected charge reading M is given by-
M = Mraw X Pion X Pt.p X Pelec X Ppol
• Where-
Mraw = raw chamber reading in Coulomb
Pion = ion recombination correction
Pt.p = air temperature and pressure correction
Pelec = electrometer calibration factor
Ppol = polarity correction
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17. UNITS AND CONVERSIONS
• 1 Rad = .01 Gray
• 1 Rem = .01 Sievert
• 1 Roentgen = 2.58 x 10-4 C/Kg
• 1 Curie = 3.7 x 1010 Becquerel
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18. METHODS OF ABSORBED DOSE
MEASUREMENT
1. CALORIMETRY
2. CHEMICAL DOSIMETRY
3. SOLID STATE METHODS
4. SILICON DIODES
5. RADIOGRAPHIC FILMS
6. RADIOCHROMIC FILMS
7. MOSFET
8. DIAMOND DOSIMETERS
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19. 1. CALORIMETRY
• Energy absorbed in a medium from a radiation appears as
heat energy.
• This causes increase in temperature of the absorbing medium
and can be related to the absorbed dose.
• This rise in temperature is measured by thermistors.
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20. 2. CHEMICAL DOSIMETERS
• Energy absorbed from the ionizing radiation may produce
chemical change.
• This chemical change can be used to measure the absorbed
dose.
• E.g. Frick’s dosimeter, which uses ferric ions for calculations.
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21. 3.SOLID STATE METHODS
• Most widely used system is thermoluminescent dosimeters / TLD’S.
THERMOLUMINESCENT DOSIMETERS-
• Crystalline materials exhibit thermoluminescence.
• When such crystals are irradiated, minute fraction of absorbed
dose is stored in the crystal lattice.
• If this material is heated later, then this stored energy responds as
light, which can be used to measure the absorbed dose.
• They are most commonly used for personal dose monitoring.
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22. 22
LiF is the
most
commonly
used
chemical
in TLD.

23. GLOW CURVE-
• On plotting the thermoluminescence against temperature we
get a glow curve.
• As the temperature increases the release of trapped electrons
increases, which results in glow or light emission.
• The light emission increases initially to maximum, then falls
to a minimum.
• Most of the phosphors contain traps at different energy
levels, causing a number of glow peaks at those levels.
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25. 4. SILICONE DIODES
5. RADIOGRAPHIC FILMS-
• They contain silver bromide.
6. RADIOCHROMIC FILMS
7. MOSFET-
• Metal Oxide Semiconductor Field Effect Transistor.
8. DIAMOND DOSIMETERS-
• Diamond changes the resistance on radiation exposure.
• This change in resistance can be used to measure the absorbed dose.
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26. REFERENCE- KHAN’S TEXTBOOK
OF RADIATION PHYSICS.
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27. THANK YOU,
AND HAVE A NICE
DAY.
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