| Ch 8 | Page 2 / 25 | |
| Cancer Radiotherapy |
Physical foundations of radiotherapy | |
Several types of ionising radiations are used
Electromagnetic radiation:
- X photons emitted during the rearrangement of electrons: X-Rays tubes, accelerators;
- γ photons emitted during nuclear disintegration: 60Cobalt source,192 Ir wires, 137Cs wires
Their main physical characteristics are:
- no mass: they are propagated in a straight line;
- no charge: their interaction with matter is random with important leakage after crossing any depth of matter
Particule radiation: neutrons
These particles are artificially produced by cyclotrons: their route is straight throughout matter. They interact by pulling protons out of crossed tissue. At a similar dose, their relative biological efficiency (RBE) is approximately 3 times higher than photons.
β - Radiation
β - radiation particles are emitted by certain radioactive nuclei. They comprise electrons which interact with matter by moving the electrons within human tissue by electrostatic repulsion. Their route is more or less winding depending on their original energy. Their biological efficiency is very similar to that of X and γ photons.
Accelerated electrons
Produced by accelerators, they possess the same physical characteristics as ß Radiation. Their energy is chosen according to the depth at which the tumour to be treated is situated, but they do not penetrate and offer the major advantage of sparing tissue situated deeper than the tumour itself.
α Radiation
α particles are heavy, positively charged helium nuclei. These particles are spontaneously produced by instable nuclei and behave within matter by interacting with electrons and protons. Their route is very short; only a few millimetres in water. Their biological efficiency is 5 to 10 times higher than X or γ Photons, however their short penetration prevents their clinical use.
Produced by cyclotrons or synchrotrons, they loose their energy by colliding with electrons and nuclei. The in-depth dose distribution is very different from that of photons and is concentrated within a very narrow peak (Bragg peak). Thus, this type of irradiation is well adapted for deep small sized tumours situated close to radiosensitive healthy tissue. Nowadays, the main indications are choroidal melanoma, tumours at the base of the skull and tumours close to the spinal cord (chondroma, chondrosarcoma). The biological efficiency is less than that of neutrons.
Light ions
They can be produced by synchrotrons, have a similar penetration to protons but a biological efficiency comparable to that of neutrons. The main ion used in a few specialised centres is Carbon. They constitute an interesting research domain.
When an ionising radiation beam penetrates human tissue, part of the radiation is absorbed (this is the useful part of the beam), another part is deviated from its path (depending on many factors) and the third part continues its path.
Diffusion (propagation outside the beam’s path) explains why the regions situated outside the irradiation beam can receive a parasitical dose of radiation.
Electromagnetic radiation interactions with tissues
Photons transmit their energy to molecules by different fundamental interaction mechanisms; these phenomena lead to ionisations or electronic excitations, then to the emission of secondary photons of lesser energy when the molecules return to their stable stage. These secondary photons are, themselves, the origin of new interactions with excitations and ionisations in neighbouring molecules.
Electron interactions with tissues
Most of this interaction is not mechanical, as in photons, but is electrostatic with the electrons of crossed tissue. The electrons very rapidly loose their speed and then their energy. At the end of the path, their energy loss per unit of crossed depth is much higher than at the beginning of the path, thus giving electrons an advantage for the protection of deep and superficial tissues.
The absorbed dose represents the quantity of energy absorbed per unit of matter. It is totally different from the emitted energy.
It is measured in Grays (in honour of the great British physicist Hal Gray - 1905-1965 - who worked in Cavendish Laboratory - Cambridge, UK): 1 Gy represents the deposition of 1 Joule per kg of matter. In previous denominations, 1 Gy is equivalent to 100 Rad.
A dose of 5 Gy in one shot to the whole body of a man is a lethal dose for approximately 50% of subjects (DL 50).
70 Gy is the dose which is generally prescribed (in several fractions) for head and neck cancers exclusively treated by radiotherapy.
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