Use of radiation to treat cancer
Intensity-modulated Radiation Therapy (IMRT)
Fluence
- Fluence is defined as the number of photons passing through a unit cross-sectional area.
- Maximum fluence is achieved when the beam is perpendicular to the surface as there will be no angular component.
- Fluence can be calculated by integrating the flux density over a given time period.
- A fluence map is created which plots out on a grid the proposed dose for each part of the tumour. Fluence map optimisation is an issue which arises, as it is not certain that each grid space contains only one type of tissue[29]. This results in some healthy tissue inevitably being treated with radiation, which could damage the cells and cause side effects, or potentially secondary cancer.
Multi-Leaf Collimator
- Turns radiation beam into a grid of beamlets, all of which can be independently controlled - turned off and on, and the intensity monitored. This means that irregular shaped tumours can be more accurately targeted[30][31].
- Leaves (usually made from Tungsten) are positioned to conform a radiation beam to the shape of the tumour using information gathered from detailed 3D imaging such as CT or MRI scans.
- Why is Tungsten used? In order to effectively absorb radiation, which is the purpose of the leaves, Tungsten is used due to its high atomic number, high density and high melting point, which result in an excellent ability to absorb radiation. The high melting point and low thermal expansion coefficient also mean that the metal remains largely the same shape before and during heating, which is critical for shaping radiation beams so precisely[32].