Photon Treatment: Beam Quality Flashcards
Explain the concept of “beam quality.” What is one simple way of defining “beam quality”?
Beam quality describes the energy spectrum of an X-ray (or Gamma-ray) photon beam. Some photon beams, for example, Co-60, are nearly monoenergetic [1.25 megaelectron volt (MeV) average]. Other beams, for example, a 6 MV linac beam, are poly-energetic. MeV implies one energy whereas the 6 MV implies a spectrum, with 6 MeV as the maximum energy. The beam quality from each manufacturer and machine is slightly different for beams with the same nominal energy. For therapy photon beams, the detailed beam quality is measured by a curve called the percent depth dose (PDD) curve. The PDD value at 10 cm depth is often used to report the quality of the beam in one number. For diagnostic beams, the kVp or half-value layer is used to define the beam quality instead of PDD.
Explain the concept of “beam hardening.” After a beam passes through a filter, how do the maximum energy, average energy, half-value layer (HVL), and dose rate change?
Beam hardening occurs when the beam passes through any matter, which acts as a filter, absorbing low-energy photons. The transmitted beam will have an increased average energy, decreased dose rate while the maximum energy is unchanged. For a diagnostic beam, this increase in energy results in an increase in HVL. In the figure, the black spectrum is the initial beam and the gray is the filtered beam.
Explain the relationship of tube voltage, tube current, and filament current to the relative output of a kV diagnostic X-ray tube.
The output in a kV X-ray tube is proportional to the tube current, the square of the tube voltage, and exponentially with the filament current.
Why is Cerrobend employed for custom blocking? What is the transmission through a block?
Cerrobend is an alloy of bismuth, lead, tin, and cadmium. It is useful as a high density (9.4 g/cm3) and high-Z material with a low melting point to quickly create a block in a desired shape. The goal is to achieve
How many half-value layers (HVLs) of the Cerrobend are necessary to limit the transmission to 10% and 1%?
3.322 HVLs are necessary to limit the dose to 10% transmission. 6.644 HVLs are necessary for
1% transmission.
Does a virtual wedge affect the beam quality?
A virtual wedge does not produce variable beam hardening as the gradient profile is created by slowly opening one collimator jaw of the linear accelerator, rather than transmission through a physical wedge. Any scatter from the jaw is reduced by shielding in the head of the machine and does not contribute significantly to the dose experienced by the patient.
Describe the spatial distribution of photons generated by bremsstrahlung. How is the distribution affected by the energy of the incident electron?
For kV incident electron energies, the X-rays are produced isotropically (in all directions). With
megavolt (MV) incident electron energies, the X-ray distribution becomes progressively more aligned with the direction of the incident electrons. Therefore, a kV X-ray tube may use a reflection target where useful X-rays are at 90° to the electron beam, whereas an MV linear accelerator uses a transmission target.
What is a Thoraeus filter?
A Thoraeus filter is commonly used with diagnostic X-ray tubes. The filter consists of tin, copper, and aluminum layers to remove low-energy photons and characteristic X-rays from the beam. These low- energy photons do not contribute to image quality but increase patient dose.
Why is the order of layers in the Thoraeus filter important?
The order of filter layers is important because tin contributes the most to filtration of the characteristic X-rays of the tungsten target, which fall between 58 and 69 keV. Tin’s characteristic X-rays are of very low energy and can be filtered with copper. Copper’s characteristic X-rays are filtered by a thin film of aluminum, leading to a hard beam capable of producing sharp images.
What are bremsstrahlung interactions and why do they happen?
Bremsstrahlung interactions occur when a high-energy electron pass near the nucleus of an atom. The electron is deflected by the nucleus due to its Coulomb force. The electron undergoes sudden acceleration in a different direction, losing all or part of its energy in the process, which is converted into a photon.
A single electron may undergo multiple bremsstrahlung interactions before finally coming to rest.
What is the most probable energy of a 10 MV photon beam?
High-energy photon beams are created by bremsstrahlung interactions and therefore contain a spectrum of energies. The most probable energy is approximately one third that of the maximum energy, which by convention is used to name the beam. Therefore, for a 10 MV beam the maximum energy is 10 MeV and the most probable energy is approximately 3.33 MeV. This is not the same as the average energy, which is more difficult to calculate.
Describe the source of and difference between inherent and added filtration.
Inherent filtration in a linear accelerator is typically caused by the tungsten target itself—as electrons interact with the target, a spectrum of photons are created via bremsstrahlung interactions. The low-energy photons may interact and be absorbed by the remaining tungsten before emerging as a part of the spectrum. This effect increases as the thickness of the target increases. Added filtration is intentionally placed in the path of a beam with the goal of increasing the average beam energy (beam hardening) or decreasing the intensity of a beam.
How is beam penumbra defined?
Beam penumbra (umbra is Latin for “shadow”) is the gradual reduction of beam intensity at the edge of a photon field. It is typically measured as the width between the 80% and 20% isodose lines, although 90% to 10% isodose lines are also used.
What are the three main sources of beam penumbra in therapy photon beams?
The three main sources of beam penumbra are transmission penumbra, geometric penumbra, and internal penumbra. Transmission penumbra occurs as the beam passes through the edge of the jaw, block, or multileaf collimator (MLC). Geometric penumbra occurs because the source is not a point-source. Internal penumbra is due to scatter within the patient.
Is there a difference between the first and second half-value layer (HVL) for a cobalt unit? What about a diagnostic X-ray tube?
Cobalt-60 generates a near-monoenergetic beam (1.17 MeV, 1.32 MeV combined for an average of
1.25 MeV) and therefore the HVL1 = HVL2 = HVL3. However, diagnostic tubes use bremsstrahlung X-ray generation to produce a spectrum of energies. After the beam passes through the first HVL, due to the concept of “beam hardening,” the second HVL is now greater than the first.
