RTY2003 – Radiation Therapy Physics – Measurement of Absorbed Dose
1 electron Volt (1eV) is the energy gained by an electron as it traverses a potential difference of one volt.
As discussed in a previous chapter, the typical x-ray energy used for imaging is around 140kvP, which is around 40keV, and the x-rays used in linear accelerators for radiotherapy is around 6000keV or 6MeV.
Roentgen
1R is defined as the electric charge produced by such radiation in a specified volume of air divided by the mass of that air. In 1928 it was the first international measurement quantity for ionizing radiation to be defined for radiation protection. Although relatively easy to measure, it had the disadvantage that it was only a measure of air ionization and not a direct measure of radiation absorption in other materials. As science of radiation and dosimetry developed, it was realized that the ionizing effect, and hence damage, was linked to energy absorbed by irradiated materials. New radiometric units for radiation protection were defined from 1953 onwards, which took this into account.
The SI unit for exposure to ionising radiation is C/kg. This has since replaced the Roentgen. The work done / energy produced per ion pair produced is 33.97eV.
Hence one ion pair = 33.97eV/C = 1J/C
We hence have a link between ionisation charge per kg and energy absorbed dose.
The principle of ionisation chambers is that incoming radiation will ionise the encapsulated air, and capacitors are used to measure the amount of charge generated by the ionising radiation passing through.
Principle of Thimble chamber
Consider an air shell consisting of air cavity exist at the centre of a spherical volume. If the sphere of air is irradiated uniformly with photon beams also if that the distance between the outer sphere and inner cavity is equal to the maximum range electrons generate in air.
If the number of electrons entering and leaving the cavity is the same, then electronic equilibrium exists. If we are able to measure the ionisation charge produced in the cavity by the electrons liberated in the air surrounding the cavity, charge per unit mass or the beam exposure at the center of the cavity can be calculated if the volume or mass of air inside the cavity are known.
The most common thimble chamber design is 0.6cc air volume.
Typically for an ionisation chamber, a low current will be passed between electrodes, typically in magnitudes of nano amperes.
Electronic equilibrium is present when, due to ionisation events within and outside this volume element, the same number of electrons with the same energy distribution enter and exit this volume element.
We are dealing with MV radiation and the electron range is in mm, so we have the same number of electrons entering as there are leaving the air volume of the ionisation chamber. We can achieve this with build up caps or by placing the chamber in a phantom, such as water.
KERMA
Kerma is an acronym for “kinetic energy released per unit mass”, defined as the sum of the initial kinetic energies of all the charged particles liberated by uncharged ionising radiation.
A new quantity Kerma was defined which can measure air ionisation and is the modern metrological successor to the roentgen, and from this the absorbed dose can be calculated using known coefficients for specific target materials.
Calibration Theory
Bragg Gray cavity theory relates the radiation dose in a cavity volume of material g to the dose that would exist in a surrounding medium m in the absence of the cavity volume.
However the cavity and gas must meet certain conditions before being assumed to be under the cavity theory:
Cavity must be small and does not perturb the beam – cavity does not change the number, energy or direction of the charged particles that would exist in m in the absence of the cavity.
The absorbed dose in the cavity is deposited entirely by charged particles crossing it.
The mean energy to produce one ion pair is 33.97 J/C. Dose to water can be obtained by multiplying the ratio of stopping power in water to the stopping power in air.
The stopping power of the material is numerically equal to the loss of energy per unit path length:
Mass stopping power divides stopping power value by density.
The ratio air to water relates the difference in energy loss in the two medium. This quantity is energy dependent.
Calibration protocols
The primary standard calibration laboratory uses a cross calibration with a calorimeter to match against their ionising chambers. Many factors can affect the reading of the calibrator, and hence there are correction factors to ensure accuracy.
These are called the KQ corrections, and there are 3 notable ones:
Correction for temperature and pressure of air volume in ionising chamber as this affects the mass of air in the volume
Polarity of ionisation chamber correction: chamber efficiency slightly different depending on charge + or – to central electrode
Ion recombination correction as not all charges are collected at electrodes even at 400V
