Determining Radiation Intensity 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.

1 roentgen= 1R=2.58*10-4 coulomb/kg(of dry air) 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.

[0.873cGy/R x (μ/ρ)medium/ (μ/ρ)air] 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 = Absorbed Dose x Quality Factor 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 1