External Beam Radiotherapy

External beam radiotherapy uses an external x-ray or electron source from outside the patient, which target a tumour either superficial or deep within the patient. Only in brachytherapy, a form of EBR, is the radioactive source placed next to the tumour, e.g. for prostate cancers. Multiple radiation beams are used simultaneously to achieve uniform dose distribution with as little as possible irradiation of healthy tissue.

Machinery

External beam radiotherapy is possible because of three machines; an x-ray tube, a Cobalt-60 Teletherapy Unit and a Linear Accelerator (Linac).

X-Ray Tube

The x-ray tube was the first machine to be using in external beam radiotherapy. Although the tube is activated only for milliseconds in diagnostic radiography, the beam on time in therapy can be several minutes. This means a large heat output is created, which must be dissipated efficiently. Thus, a rotating anode is used to efficiently remove heat. X-ray tubes used in EBR are named according to the photon energies they produce:

• Superficial tubes produce 10-100keV photons
• Orthovoltage tubes produce 100-500keV photons

Cobalt-60 Teletherapy Unit

Cobalt-60 were the next machine to be introduced to therapy. Cobalt-60 is a radioactive material which decays to emit gamma rays. It has a half-life of 5.27 years an en energy of 1.2MeV. 1 gram of Co-60 has an activity of 44TBq (44 million million decays per second). However, when used in therapy, the activity is between 15-20TBq. The disadvantage of these teletherapy units is that they are always decaying and require a thick lead shielding to prevent leakage radiation.

Linacs

Linacs are the current gold standard for external beam radiotherapy. They have the benefit of a beam on time, meaning there is no radiation exposure after the beam is arrested, and they can produce energies up to 25MeV. For this reason, they are termed megavoltage sources. They can produce up to three photon beam energies and up to five or six electron energies.

Absorbed Dose

Absorbed dose is a measure of the energy (in joules) deposited per kilogram of mass, m. It is measured in joules per kilogram, or gray (Gy).

Photon Fluence

Photon fluence is the number of photons, N, which enter an imaginary sphere of cross-sectional area, A. It is measured in cm-2.

Why can ionising radiation make such damage to biological tissue when only small amounts are absorbed?

In terms of ionising radiation, it is not the total energy of all photons but instead the energies of individual photons which is of significance. For megavoltage photons, it requires only a few photons to cause damage to critical structures in cells, ultimately leading to cell death. This damage is induced through direct damage or indirect damage through free radicals (more common than direct damage). In radiation therapy, a large number of photons are required to make up the required energy, meaning many photons will be available to produce damage in cells. A lethal dose of 10Gy involves 1 x 10^3 photons. As this is approximately one tenth of the number of cells in our body, it is evident the effect damage to this number of cells would produce.

NB: We may consume 10,000kJ of energy per day. However, this energy is in the form of glucose, which is absorbed by cells, broken down and used to fuel bodily tissues. This is not a dangerous form of energy when compared to megavoltage photons used in radiotherapy.

Nature of Gamma Rays and X-rays

Gamma rays are isotropic, meaning they are of equal energy distribution when measured from multiple angles. Thus, they are monochromatic photons. The energy distribution across an x-ray beam is not equal, meaning they are non-isotropic. The energy spectra are thus heterogenous.

Dose Deposition

Dose deposition is a two step process; the first step is KERMA and the second step is absorbed dose. KERMA stands for kinetic energy released per unit mass. In this step, a photon transfers all or part of its energy to a secondary charged particle, i.e. an electron, through photoelectric absorption, Compton scattering or pair production.

• In photoelectric absorption, the photon gives up all of its energy to the anatomic electron and is absorbed.
• In Compton scattering, the photon interacts with an outer shell electron, transferring some energy to it. The photon then moves off in a different direction.
• In pair production, the photon interacts with an atomic nucleus and is transformed into an electron and positron.

In the second step, the secondary charged particle deposits energy through thermal heating, ionisation and excitation. This makes up the 'absorbed dose'.

Beam attenuation through air is determined by the inverse square law, i.e. the intensity is inversely proportional to the square of the distance. In a patient, however, attenuation contributes as well to reduction in beam intensity.

Dose Distribution

When a megavoltage photon enters the patient, they deposit dose at the surface, termed surface dose. At the patient's surface, the photon transfers energy to secondary charged particles, which then travel deeper into the patient until a dose maximum is reached at Zmax. This means the surface dose is less than the dose deeper inside the patient, termed the skin sparing effect. For superficial and orthovoltage tubes, the dose at the skin surface is the highest dose, however. Dose then decreases almost exponentially until the exit point of the patient where the exit dose is delivered.

When the dose distribution is graphed, there is an sharper decrease in dose just before the exit point. This is because of a lack of scatter contribution for points beyond the dose exit point.

NB: Depth of dose maximum increases with photon energy and field size.

Source to Skin vs Source to Axis Treatments

For source to skin treatments, the source to skin distance is constant for all treatment beams. For source to axis treatments, the treatment beams are centred around an isocentre, meaning the SSD for each treatment beam will be different. Source to axis treatments are the most common external beam radiotherapy method.

SSD + z = SAD

or

SAD - z = SSD

Percentage Depth Dose

Percentage depth dose is a way of describing how absorbed dose changes with increasing depth. The dose distribution is generalised so that Dmax exists at 100%, at the maximum depth Zmax.

• As photon energy increases, PDD increases because the beam is more penetrated and less easily attenuated.
• As field size increases, PDD increases because there is increased scatter contribution to points along the central axis.
• As SSD increases, PDD increases because there is less effect of z on the inverse square law. For example, if you increase the distance from 1cm to 2cm, this is doubling the distance and will result in a 75% decrease in x-ray intensity. However, if you increase the distance from 10 to 11cm, this is only an increase of 10% and will result in less effect on beam intensity. PDD is simply the ratio of two doses. Thus, if there is fixed separation, PDD will increase with increases seen in SSD.

Tissue Phantom Ratio

Tissue maximum ratio is the ratio of dose at a chosen point Q to a fixed reference dose DQref. Isocentric treatments are the most commonly used treatments clinically. However, they have varying source to skin distances, making PDDs inconvenient. Thus, tissue phantom ratio which is independent on SSD, can be used in relative dosimetry.

Off-Axis Ratios

Off-axis ratios provide the ratio of dose at a given point off-axis to the central axis, to the dose at the same depth on the central axis. Combining these two pieces of information gives volume dose matrix which can provide 2D and 3D information of dose distribution in the patient.

Isodose Distributions

Isodose distributions can be given for single beams as a family of isodose curves constructed from regular intervals of PDDs. They are expressed as a percentage of dose at a specific reference dose.