Transcript Chapter 7 - RadTherapy

Chapter 7
Determining Radiation Intensity
The Importance of Standardized
Radiation Measurement
• Standard units allow radiation oncologists to
predict biological effects and therapeutic
outcomes.
• Skin erythema dose: earliest radiation unit, the
amount of radiation necessary to barely redden
skin of a light skinned person.
• 1electrostatic unit of charge (esu): unit of xray exposure that describes an ionization
chamber of 1cc volume in which the calibration
of radiation beams was based on the ionization
of gas.
Roentgen
• Roentgen: an exposure of x or gamma radiation such
that the associated corpuscular emission (ions:
electrons, positive atoms or molecules) per 0.001293
gram of dry air produces in air, ions carrying 1
electrostatic unit of charge of either sign.
 Designed to make the calibration of x-ray machines consistent
throughout the world.
1 roentgen= 1R=2.58*10-4 coulomb/kg(of dry air)
• Describes the amount of radiation present by measuring
its ionizing effects on air.
 Not an energy measurement of the photons, or energy absorbed
by matter.
Limitations in using Roentgen
• Assumes all corpuscular emissions are collected
 Dosimeter must be capable of collecting all secondary
electrons produced by photons.
 The highest energy that can be measured in
roentgens is 3 MeV.
• Only defined for x-ray & gamma rays.
 Particle beams of electrons, neutrons, etc. cannot be
measured.
• Measuring instruments must be similar to air in
composition so that secondary emissions are
similar to those in air.
Electrometers
• Electrometers: measure the charge or
current collected by the ion chamber and
displays results in coulombs, radiation
units, or radiation rate.
Calibrated dosimeter
• Calibrated dosimeter: one that has been calibrated by
an Accredited Dosimetry Calibration Laboratory (ADCL)
which has accurately determined the chamber calibration
factor.
 The number of Roentgens per minute is determined for a given
set of treatment factors: distance, filter, kVp, mAs.
R = Reading * Temp in °K * 760 mmHg * Calibration factor
Min
Exposed time * 295 °K * Pressure in mmHg
• Calibration factor: used to obtain the accurate
conversion of collected charge or other units to
Roentgen for a specific radiation beam quality.
 Specifies the relationship between the dosimeter’s reading and
the Roentgen.
Standard Temp. & Pressure
• Temperature and Pressure are necessary
because for most ionization chambers, both
affect the sensitivity of the detector.
• Sensitivity: dependant on the density of gas in
the chamber (number of molecules available to
be ionized).
 Standard Temperature: 295 °K
 Standard Pressure: 760 mmHg (1 torr)
• Conversions:
 °K = °C + 273
 °F = 9/5°C + 32
 °C = (°F – 32) * 5/9
Beam Output
• Factors that effect beam output in conventional
x-ray units:
 Filtration: output drops with added filter
• HVL: the thickness required for a particular material to cut the
beam’s intensity in half.
• Attenuation: the removal of energy from the beam.
 Distance: intensity is inversely proportional to the
square of the distance from the source.
• I2 = I1(D1/D2)2
 Tube current: output increases linearly with mA.
• mAs1/I1 = mAs2/I2
 kVp: as kVp increases, output increases.
• OP2 = OP1 x (kVp2/kVp1)2
Kerma
• Kerma: Kinetic Energy Released in Matter
 When photons interact with material in a phantom or
in a patient, atoms in the material are ionized, and the
electrons are set into motion with various kinetic
energy.
 Measured in joules/kg.
 The energy of electrons set in motion is not a
measure of energy deposition in tissue or of the
biological effect.
 Used to relate the energy released in matter to
energy absorbed in matter.
Radiation Absorbed Dose
• Rad: energy absorption of ionizing radiation by materials.
• 100 ergs of energy absorbed per gram of matter.
• The absorbed dose per unit mass of air exposed to
exposure X(R):
 Dair = X(R) x 0.873 cGy/R
• Absorbed Dose vs. Exposure
 Applicable to all ionizing radiations, not just x-rays and gamma
rays.
 Applicable even in areas where electronic equilibrium does not
exist.
 Directly related to radiation effects because it is the deposition of
energy by ionizing radiation, not the mere ionization of air
molecules.
Fmedium Factor
• Fmedium Factor: relates the dose in air to the dose in tissue.
 The roentgen to rad conversion factor.
[0.873cGy/R x (μ/ρ)medium/ (μ/ρ)air]
• Mass attenuation coefficient (μ/ρ): probability of interaction per
unit mass length when μ/ρ << 1, (cm2/g).
 Decreasing the density of the material will cause much less
attenuation.
 Every material has a unique value of (μ/ρ) for a given photon
energy.
• The ratio of doses in two different materials is the ratio of their
mass energy absorption coefficients.
 The dose in a particular medium is equal to the dose in air
times the ratio of the mass energy absorption coefficient of
the medium to that of air.
• Dmed = Dair x [(μ/ρ)medium/ (μ/ρ)air]
• Dmed = X(R) x [0.873cGy/R x (μ/ρ)medium/ (μ/ρ)air]
• Dmed = X(R) x fmed
Fmedium Factor
• At lower photon energies and materials having
high atomic numbers the fmed will be higher due
to the photoelectric absorption at lower photon
energies and higher Z.
• At energies >100keV, the fmed falls with higher Z
materials due to Compton interactions.
• At high energies >2MeV the fmed rises due to
pair production.
• The fmed is fairly constant for soft tissues
 (lower Z).
Bragg-Gray Cavity Theory
• Bragg-Gray cavity theory: a model of
radiation interaction that assumes that the
ionization chamber acts like a tiny cavity
inside a uniform phantom. The cavity
must be small enough to leave the
spectrum of the beam unchanged.
Dose Equivalent
• Dose Equivalent: relates different types of radiation to xrays.
• The amount of a standard x-ray radiation that has the
same biological effect an another non-standard radiation.
 Heavy particle beams have greater biological effectiveness than
x-rays.
Dose Equivalent = Absorbed Dose x Quality Factor
• Quality Factor: compares the biological effectiveness of
a particle radiation to a standard x-ray radiation.
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X-ray, gamma rays:
Electrons:
Neutrons & protons < 10 MeV:
Natural alpha particles:
Heavy recoil nuclei:
1
1
10
20
20