8.3.5 - Inhomogeneity

Inhomogeneity impacts on the attenuation of primary radiation and the distribution of scattered radiation. For megavoltage energies, attenuation from the Compton Effect is dependent on electron density and most difficulties arise in areas of reduced density such as air cavities or the lung. Kilovoltage beams encounter problems with high Z materials due to increased attenuation through the photoelectric effect.

Inhomogeneity will always be asked in the exam. You must know this topic

For inhomogeneity, there are always several things to consider:

• What is the linear attenuation coefficient of the inhomogeneity
• What is the mass attenuation coefficient of the inhomogeneity
• What is the density of the inhomogeneity
• What happens at the interface of the inhomogeneity with tissue

For megavoltage beams, which rely on incoherent scattering for most of their attenuation, the mass attenuation of many tissues is similar. This means that the density of the inhomogeneity is more important in determining the amount of attenuation that occurs. Absorbed dose between inhomogeneities will be broadly similar, as dose is also density independent (J/kg).
For kilovoltage photons, the photoelectric effect is dominant. This means that the mass attenuation coefficient will be dependent on atomic number. Unlike megavoltage photons, the absorbed dose from kilovoltage beams will vary greatly depending on the atomic number of the inhomogeneities.

The other major factor to consider for inhomogeneity is the impact on scattered dose. For kilovoltage beams this is of minimal importance as generated electrons are deposited locally. Electrons generated by megavoltage beams have a range of up to 3 cm, and these scattered electrons will also have an impact on dose distribution, particularly at the interfaces between inhomogeneity.

• At a high/low density inhomogeneity, there will be increased production of electrons in the high density material relative to the low density material. This leads to an increase in dose on the low density side (due to increased numbers of electrons entering the tissue from the high density side). It also leads to a decrease in dose on the high density side (due to reduced numbers of scattered electrons in the low density material).
• Electrons may be backscattered if they encounter a dense material. In humans this occurs at soft tissue and bone interfaces, or in metal prostheses. This further increases dose in the soft tissue on the source side of the high density inhomogeneity.

## Common Inhomogeneities

### Air Cavity

There are several air cavities within the body, including the larger parts of the tracheobronchial tree, the oral cavity, the pharynx, the nasal cavity, and the paranasal sinuses. Air cavities cause difficulty as there is minimal attenuation of primary radiation, and therefore decreased dose generation. This causes electronic disequilibrium and loss of dose in regions close to the inhomogeneity. Beyond the air cavity, there is increased dose when compared to a homogeneous water phantom due to reduced attenuation within the air cavity.

### Lung

Lung is made up of soft tissue and has an equivalent atomic number similar to soft tissue. It contains alveolar air spaces and is therefore less dense than other soft tissues.
The reduced density leads to decreased attenuation of the primary radiation beam, which in turn leads to decreased production of scattered radiation. There is therefore a loss of electronic equilibrium within the lung unless the field is sufficiently large to create sufficient scattered radiation. The penumbra is also enlarged due to the decreased attenuation of scattered radiation.
Beyond the lung, there is an increase in dose compared with a pure water phantom as the beam has undergone less attenuation.

#### Lung-Bone Interface

Asked in the 2010 phase 1 examination.

##### Megavoltage Photons

Electrons set in motion by megavoltage photons have an increased range. This means that the differences in attenuation between lung and bone

• On the lung side
• Due to the increased density of bone relative to lung, there will be significantly more electrons generated per centimetre in bone than lung.
• At the interface, these electrons may escape the bone tissue and enter the lung
• This leads to an increased number of electrons relative to the lung tissue and a loss of electronic equilibrium
• This leads to an increased dose on the lung side of the interface
• On the bone side
• Due to the decreased density of lung relative to bone, there will significantly fewer electrons generated per centimetre in lung than bone
• There will be a loss of electronic equilibrium on the bone side of the interface due to decreased scatter from the lung
• This will cause a decreased dose on the bone side of the interface

### Bone

Bone has a higher atomic number than soft tissues and the electron density is also higher. Bone also contains small islands of soft tissue (marrow) which must also be considered.
Bone mineral causes increased attenuation per centimeter for both kilovoltage and megavoltage beams. For kilovoltage beams the attenuation is due to both increased density and the photoelectric effect, leading to increased production of scattered radiation that increases dose to bone mineral by 2 to 4 times. For megavoltage beams the attenuation only occurs to the increased density of bone - there is less attenuation per gram of bone than per gram of water. Absorbed dose within bone is actually less than in soft tissue for low megavoltage energies (about 96-98%). Pair production increases at higher photon energy levels and bone mineral dose begins to increase again.

Importantly:

• The linear attenuation coefficient of bone is higher than water (due to increased density of bone)
• The mass attenuation coefficient of bone is lower than water (there is less attenuation within bone per gram of tissue)

This causes a decreased dose to bone mineral compared with water

Soft tissue within bone (bone marrow) also receives much higher dose than would be expected from kilovoltage x-rays (2 – 5 times higher dose). Unlike the bone mineral, it also receives slightly higher doses than expected from megavoltage beams due to the increased electron density per gram (by 3-5% more).
At the junction of bone and soft tissue, there is an increase in dose due to backscatter of electrons from the bone surface. This leads to a higher dose for a few millimetres on the transmission side of the bone (up to 8%). This is also seen for high megavoltage energy beams (10 MV and over) due to increased electron generation through pair production. For energies between 1 and 10 MV the increased dose is not significant. On the transmission (distant side) of the inhomogeneity (ie. bone/soft tissue) there is either a reduced dose (for low MV beams and below) due to absorption of electrons within the bone structure, or increased dose for beams over 10 MV due to increased pair production of electrons in the bone.

### Prosthesis

Most prostheses are made from a metallic number with a relatively high atomic number. This can lead to increased dose at the junction of the prosthesis and soft tissue as well as causing increased attenuation due to the photoelectric effect / increased density.
The artefact on CT imaging produced by metallic prostheses may make delineation of treatment volumes as well as calculation of dose distributions unreliable.

## Correcting for Inhomogeneity

The tissue-air ratio method and isodose shift method described for correcting dose in the presence of surface contour irregularity can also be used for inhomogeneity corrections. The TAR method alters dose by converting the inhomogeneity to a water equivalent material, therefore making the point beyond the inhomogeneity effectively closer or further away from the surface.
The Power Law or Batho method takes into account the electron density of the inhomogeneity, but does not take into account interactions other than incoherent scattering.
The equivalent tissue-air ratio method is the most complex and considers the effects of the inhomogeneity on both scattered and primary radiation.
Modern treatment planning systems use dose kernels to adjust for inhomogeneities. Monte Carlo simulation (modelling of individual photons and their interactions) offers the best attempt at predicting dose distribution but is limited by computing power.