Irregular surface contour or tissue inhomogeneities may both alter the distribution of dose from what is expected.
Electron beams present unique problems when they strike a surface at an angle other than 90 degrees.
- Angles of over 60o show shifting of isodose curves in relation to the surface contour (similar to photons)
- Angles of under 60o lead to elevation of zmax towards the surface and decreased depth dose along the central beam axis.
The shifting of zmax to the surface occurs because of electron scatter. Electrons scatter laterally, and if the beam is on an oblique angle then some of these laterally scattered electrons will instead be scattered towards the sloping surface. These electrons deposit their dose in more superficial parts of the medium and increase surface dose.
The loss of dose along the central axis is due to a similar problem, namely that the electronic equilibrium that normally occurs between electrons scattered in and out of the beam is disturbed. This leads to less dose at a particular point on the central dose axis.
Aside from an oblique angle, the contour of a patient can also present rapid changes in contour, such as around the nose or the ear. This occurs as the lateral scatter from regions closer to the source tend to bulge into the more distant from the source. For very irregular contours (such as a contour that is parallel to the beam axis) there is loss of electron equilibrium at the interface and corresponding cold spot in the medium. Hot spots and cold spots can also form deep to this parallel section of contour, the hot spot due to scatter into the more distant tissue and a cold spot due to attenuation of the beam in the closer tissues.
Irregular contours can also be compensated for by the use of bolus. This fills the gaps in the patient contour and creates a uniform dose distribution. Irregular contours are to be avoided as the dose distribution predicted by treatment planning systems is also unreliable. If there is a single sharp step present, then tapering bolus should be used to smooth the contour and prevent the development of hot and cold spots.
Bone, lung and air cavities within the treatment volume can significantly alter electron dose distribution.
For large, uniform inhomogeneities correction may be performed by using the coefficient of equivalent thickness (CET) method. The CET corrects the thickness of the inhomogeneity by converting it to a thickness of water, based on the density of electrons in the tissue.
The CET method is very close to in vivo measurements of dose beyond bone. For dense cortical bone the CET is approximately 1.65. For spongy bone the CET approaches unity. The increased attenuation in bone leads to loss of dose in structures beyond bone.
Bone limits the utility of of electrons in treating deeper structures (ie. brain, oral cavity) due to this attenuation.
Perhaps the most important feature of bone inhomogeneity is that it causes backscattering of electrons. Backscattering occurs due to the higher atomic number and density of bone, causing a larger than expected number of electrons to deposit dose on the soft tissue side of the inhomogeneity.
Unfortunately, the use of the CET method in lung is complicated. This is because the CET for lung varies with depth. An average CET of 0.5 has been suggested but accuracy is a problem. Alternatively, using the mass density of lung on CT (about 0.20 – 0.25 of water) is possible.
The reduced attenuation in lung leads to an increase in range of electrons, of about three times. This leads to an increase in dose (relative to a homogenous water phantom) at depth, both to lung tissue and to structures beyond the lung. There is a loss of electronic equilibrium at the interface, with reduced dose on the soft tissue side and increased dose on the lung side.
The effect of a small inhomogeneity is predominately due to changes in scattering of electrons.
If a small dense inhomogeneity is present, it scatters electrons laterally due to its greater mass scattering power. This leads to a cold spot beyond the inhomogeneity and hot spots lateral to the inhomogeneity.
If a small, non-dense inhomogeneity is present, then electrons passing through will have minimal scattering interactions. There will be a loss of electronic equilibrium in the tissue on each side of the cavity. Beyond the inhomogeneity, there will be a hot spot due to scatter from the tissue lateral to the inhomogeneity combined with the unscattered electrons that have passed through the cavity.
The edge of an inhomogeneity can cause difficulties with dose distribution. This includes the air-tissue interface on the surface, as discussed in the contour section above. Increased scatter from the denser material into the less dense material leads to a cold spot beneath the dense material and a hot spot in the adjoining less dense material. The effect is most pronounced at the edge itself, but blurs at increasing depth.