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Chapter 7. Determining Radiation Intensity. The Importance of Standardized Radiation Measurement. Standard units allow radiation oncologists to predict biological effects and therapeutic outcomes.
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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 x-ray 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 x-rays. • 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. • X-ray, gamma rays: 1 • Electrons: 1 • Neutrons & protons < 10 MeV: 10 • Natural alpha particles: 20 • Heavy recoil nuclei: 20