3.10.6.4 - Use of High-Energy X-rays

Ionisation and absorbed dose

Radiotherapy uses ionising radiation to destroy tumour cells or prevent their continued division. High-energy X-rays transfer energy to charged particles in tissue; those particles ionise atoms and can damage DNA directly or through chemical reactions. The same processes can damage healthy tissue.

Absorbed dose is energy deposited per unit mass:

A tissue mass of $0.020\,\mathrm{kg}$ absorbing $0.040\,\mathrm{J}$ receives $2.0\,\mathrm{Gy}$. Dose is not photon energy, activity or exposure time, although each can affect it. For fixed mass, dose increases with absorbed energy; for fixed energy, doubling the mass halves the average dose.

A linear accelerator accelerates electrons onto a dense target to generate high-energy X-rays by bremsstrahlung. Compared with diagnostic beams, therapeutic megavoltage beams are more penetrating and can deliver useful dose at depth. Higher photon energy corresponds to shorter wavelength through $E=hc/\lambda$. A treatment beam contains a spectrum, not one photon energy.

High-energy beams can give a dose build-up beneath the surface, reducing skin dose relative to dose at greater depth. This does not mean the maximum dose automatically occurs at any chosen tumour depth, or that a photon deposits all its energy in the tumour. Treatment planning must account for the whole depth-dose distribution.

Locating the target and choosing penetration

Planning begins with the position, shape and depth of the tumour and nearby sensitive organs. CT, sometimes supplemented by MR, supplies anatomical information for a three-dimensional plan. Accurate patient positioning and immobilisation help the delivered beam follow that plan; motion or a setup error can underdose part of the target and overdose healthy tissue.

Photon energy is chosen so a useful dose reaches the target without an excessive superficial dose. Deeper targets generally require more penetrating beams. Lower-energy beams can be appropriate for superficial targets. Choosing the highest available energy for every case is not a valid rule: energy, field size, geometry and tissue properties jointly determine dose.

The treatment team also chooses beam directions, exposure and beam shape to meet the tumour requirement while limiting dose to healthy structures. Imaging guides this choice but is not itself a shield. Fractions spread a course over separate treatments and may allow healthy tissue to repair; they do not make any individual exposure non-ionising.

Shaping the beam by absorption

A collimator removes unwanted parts of an X-ray beam using dense absorbing material. A multi-leaf collimator has individually movable tungsten leaves that form an aperture matching the target's projected shape from that direction. Photons passing through the aperture continue in their original directions. The device does not bend a diverging beam into a parallel or converging one.