6.1-6.2: Introduction & The Roentgen
- In the orthovoltage era, the skin was the limiting organ to delivering radiation.
- The skin erythema dose (SED) was the unit used to define the amount of radiation needed to cause reddening of the skin.
- The SED was an unreliable unit because it depended on the skin type, radiation quality, extent of the skin exposed, dose fractionation, and early vs delayed skin reactions.
- The SED was abandoned when megavoltage beams with skin-sparing properties became the primary mode of treatment.
- An x-ray beam that passes through air sets electrons into motion, and the liberated electrons proceed to produce ionizations of their own.
- A simple ionization chamber is composed two ion collection plates in air between which a voltage is applied (ion-collection region), and as the beam passes through the ion chamber, the ionized electrons (positrons and negatrons) will move to their respective plates.
- The electrons which are liberated within the ion-collection region must expend all of their energies within that space.
- Electrons that are produced from inside the chamber may deposit their energies outside, but this loss is compensated by the electrons that are produced from the outside depositing their energies inside the chamber.
- This condition is known as electronic equilibrium where the ionization loss is compensated by the ionization gain.
- The number of collected charges of either sign can be measured by an electrometer and is the basis for determining the quantity exposure (X) which is the number of charges produced by photons (Q) per unit mass of air (m_air).
- Exposure is given by the following equation:
- The SI unit for exposure is C/kg, but is frequently expressed in terms of Roentgen (R) where:
- Exposure is given by the following equation:
6.3-6.5, & 6.7: Free-Air Ionization Chamber, Thimble Chamber, Farmer Chamber, & Special Chambers
- A free-air ionization chamber resembles a fish tank by which a x-ray beam passes through between a pair of parallel plates.
- The x-ray beam originates from a focal spot S outside of the chamber and is defined by the diaphragm D as the beam enters the chamber.
- The separation between the plates is large enough to prevent the liberated electrons within the ion-collecting region from escaping the chamber.
- The liberated electrons must expend all of their energy by ionization between the plates, and the condition of electron equilibrium must exist to precisely measure the exposure.
- However, when the x-ray beam energy increases to about 3 MeV, the range of the liberated electrons will increase enough such that to maintain electronic equilibrium, the separation between the plates will be too large for practicality.
- The following problems are associated with an increased plate separation: nonuniform electric field, and greater ion recombination, air attenuation, and photon scatter.
- However, when the x-ray beam energy increases to about 3 MeV, the range of the liberated electrons will increase enough such that to maintain electronic equilibrium, the separation between the plates will be too large for practicality.
- The thimble chamber is comprised of an inner air-cavity at the center of the chamber and an air-equivalent solid shell that surrounds the inner cavity.
- The inner surface (or the wall itself) of the air-shell forms one electrode and a rod located in the center of the air-cavity forms the second electrode.
- The rod will collect the ions that are produced in the air-cavity when a voltage is applied between the electrodes.
- When the thimble chamber is irradiated by a photon beam, the photons will pass through the air-shell which can be described as air that has been compressed to become a solid state with a higher density.
- The thickness of the air-shell must exceed the maximum range of the liberated electrons for electronic equilibrium to occur.
- The electrons that are liberated in the air-shell will enter into the cavity to further liberate more electrons which will become collected by the central electrode.
- Thus, by knowing the mass of air inside the air-cavity, we can calculate for the beam exposure (charge per unit mass) at the center of the air-cavity.
- The thimble chamber is millimeters to centimeters in size unlike the free-air chamber which is comparable to the size of a fish tank.
- The high density air-shell allows for the thimble chamber to be compact in size whereas a free-air chamber uses gaseous air resulting in increased physical space as electronic equilibrium is contingent on adequate separation between the collection plates.
- Furthermore, thimble chambers are frequently supplemented with a build-up cap that is used to help achieve electronic equilibrium in megavoltage beams. The build-up cap is omitted lower energy treatments such as in superficial and orthovoltage beams.
- The farmer chamber is a thimble chamber with another component.
- In addition to the electrodes in the ion-collecting rod and thimble wall, a third electrode is present which is the guard electrode that wraps around insulator surrounding the ion-collecting rod.
- The guard electrode has the same potential as the ion-collecting rod (no potential difference), and functions to define the ion-collecting region and to prevent the current from leaking from the ion-collecting rod.
- Another insulator wraps around the guard electrode to separate the guard electrode from the thimble wall.
- The stem effect occurs if the chamber stem (chamber body) and cable are unguarded and are exposed in the field.
- The amount of stem is related to the beam energy and quality, and corrections can be made based by making measurements with the chamber oriented in different positions.
- Fully guarded chambers will have a negligible stem effect.
- Extrapolation chambers are specially designed to make measurements for high energy photon beams at or near the surface where the build-up effect takes place.
- The dose rapidly builds up to d_max in a short distance in the build-up region, and thus, a thin chamber is required as dose gradients are more sensitive.
- Furthermore, the chamber must also be of a size that minimally perturbs the dose distribution.
- The electrodes of this chamber face the direction of the beam.
- The beam enters in through the upper electrode, and the ions are collected by the lower electrode (surrounded by a guard ring) that is connected to an electrometer.
- The separation between these electrodes can be varied by micrometer screws, and in making measurements at regular intervals of separation (ionization per unit volume as a function of electrode spacing), the incident dose can be extrapolated for at zero electrode spacing.
- Plane parallel chambers are similar to extrapolation chambers except they have a fixed plate separation of about 2 mm.
- Layers of phantom material are added onto the top of the chamber to study the variation of dose with depth at shallow depths.
- The small electrode spacing also minimizes cavity perturbation for the radiation beam in question, and is more useful in the dosimetry of electron beams due to a small air cavity.
6.8-6.11: Ion Collection, Chamber Polarity Effects, Environmental Conditions, & Measurement of Exposure
- When the voltage difference between the electrodes in an ion chamber is low, the tendency for ion recombination is increased because of a weaker electric field, and measured ionization current will not represent a complete ion collection.
- As the voltage is increased, the ionization current will increase linearly initially and then taper off until the saturation value is achieved which is a condition where ion recombination is negligible.
- If the voltage is increased beyond the saturation region, the ions become accelerated enough to produce ionization with gas molecules in the chamber which will result in an over-measurement.
- Despite using ion chambers in the saturation region, nonetheless, a certain amount of ionization loss by recombination is expected due to chamber design and/or high ionization intensity, e.g., in the case of pulsed beams.
- These ionization losses may need to be accepted and have an ion recombination correction applied (P_ion) during the calculation of exposure.
- The collection efficiency (related to P_ion) can be determined by taking the ratio, either by calculation or measurement, of the number of ions collected divided by the number of ions actually produced.
- A collection efficiency of at least 99% or a 1% loss of charge by recombination is ideal.
- The polarity of the collecting voltage, i.e., collecting positrons vs negatron, can affect the measurement.
- Given saturation conditions, polarity effects may take place if the collecting electrode is too thick such that Compton electrons in the rod are ejected during ion collection.
- The electrometer will either over-measure (negatron collection) or under-measure (positron collection) the collector current depending on the chamber polarity.
- On the contrary, as the ions move toward the ion-collecting plate, they may also become stopped due to the thickness of the rod, and will result in an inaccurate measurement if these lost ions are not balanced by electrons ejected by recoil as the incident ions come to a stop.
- These polarity effects can be reduced by a constructing a thin collector electrode.
- Given saturation conditions, polarity effects may take place if the collecting electrode is too thick such that Compton electrons in the rod are ejected during ion collection.
- The extracameral current collected from outside the chamber from irradiation of the cable/electrometer may contribute to the polarity effects.
- Taking the mean current between the two chamber polarity measurements will result in the true ionization current. The difference between the ionization current between polarity should be less than 0.5%.
- The ion chamber reading can be affected by air temperature and pressure. The density of air is dependent on temperature and air, and results in the following relationships in relation to the chamber measurement:
- As temperature increases, the density will decrease resulting in reduced chamber measurement.
- As pressure increases, the density will increase resulting in an increased chamber measurement.
- The NIST (National Institute of Standards and Technology) and ADCLs (Accredited Dose Calibration Laboratories) provide chamber calibration factors for reference environmental conditions of temperature and pressure to be:
- The temperature and pressure correction, P_T,P, at different conditions is given by the equation:
- The temperature and pressure correction, P_T,P, at different conditions is given by the equation:
- The exposure in air, if the chamber were absent in the beam path, measured under equilibrium conditions, and corrected for polarity effects, can be represented by the equation:
- M = chamber reading
- N_x = chamber exposure calibration factor (R/C)
- P_T,P = temperature and pressure correction
- P_ST = stem leakage correction
- P_ion = ion recombination factor
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