Chapter 11 Brachytherapy
Introduction
The use of discrete gamma ray sources to irradiate tissues is usually referred to as brachytherapy and falls into three distinct applications. Interstitial therapy is where the sources are implanted directly into the diseased tissues. Intracavitary therapy is where the sources are arranged in a suitable applicator to irradiate the walls of a body cavity from inside, in effect the sources are placed in the heart of the tumour in both of these cases. The use of surface applicators is where an external surface of the patient is treated by locally applied sources arranged on an appropriately shaped applicator. While the use of gamma-emitting surface applicators has been largely replaced by the use of electron beams and will not be covered in this chapter, limited use is made of beta plaques and these will be briefly described.
• A short period of time which may be measured in minutes as in the case of temporary implants
• A long period, over the effective life-time of the radionuclide during the decay phase, in the case of permanent implants.
In brachytherapy, most radionuclides are photons emitters, however, beta or even neutron-emitting sources are used for some applications. The present chapter describes the physical aspects of sealed sources and how they are used in brachytherapy.
Indications for brachytherapy
The extent of the neoplasm must be known precisely as treatment is often given to a relatively small volume and ‘geographic miss’ is otherwise likely.
The site should be accessible for both inserting and, where appropriate, removing sources and allowing satisfactory geometric positioning of those sources.
Advantages of brachytherapy
The probability of local tumour control increases with increasing radiation dose, however, so does the probability of normal tissue damage. Brachytherapy allows the delivery of a highly localized radiation dose to a small tumour volume, increasing the chance of local control. There is a sharp fall off of radiation dose in the surrounding normal tissue, therefore, the risks of complication are reduced.
The overall duration of brachytherapy is relatively short, and can vary from a minute or two to several days depending on dose rate, prescribed dose and treatment distance from the radiation source. Constant low dose rate irradiation (below 1.0 Gy/hr) takes advantage of the different rates of repair and repopulation of normal and malignant tissue to produce differential cell killing. Hypoxic cells are relatively resistant to radiation treatment. Reoxygenation may occur during low dose rate radiotherapy with initially resistant hypoxic cells becoming well aerated and sensitive. Often in brachytherapy treatments, the dose distribution within tumour volume is not homogeneous. Treatment is often prescribed to the minimum dose received around the periphery of the treated volume. Areas close to the radiation sources in the centre of the tumour volume often receive up to twice this dose. Hypoxic cells are situated in avascular, sometimes necrotic, areas in the centre of tumours and the higher doses received here help to compensate for the relative radioresistance of these hypoxic cells. Irregular shaped tumours can be treated by judicious positioning of radiation sources and critical surrounding normal tissues can be avoided. At higher dose rate (above 12.0 Gy/hr) the radiobiological issues considerations are similar to those of external beam treatments.
Disadvantages of brachytherapy
Many of the sources used in brachytherapy emit gamma rays and nursing and medical staff may be exposed inadvertently to low doses of radiation from the patient. Staff exposure can be minimized by afterloading techniques or the use of low energy radionuclides (see below).
Large tumours are usually unsuitable for brachytherapy. However, brachytherapy may be employed as a boost treatment following reduction in size by external beam radiotherapy and/or chemotherapy.
The radiation dose falls off rapidly from the sources, therefore, in order to treat the required tissue volume adequately, accurate geometric positioning is critically important. The spatial arrangement of sources used varies depending on the type of source applicator, the anatomical position of the tumour and the surrounding dose limiting normal tissue. Accurate positioning of sources or applicators requires special skill and training and this is not universally available.
Surrounding structures, such as lymph nodes that may contain overt or microscopic cancer, will not be irradiated by the implant or intracavitary treatment.
Radionuclides in brachytherapy
Gamma emitters
Radium, which has a half-life of 1600 years, and its alpha emitting gaseous daughter product radon, were used for many years as the major source of gamma rays for brachytherapy. The major source of gamma rays is the gaseous daughter product radon. When they were used, radium tubes and needles had to be gas tight and frequently checked for leaks (for radium the mean photon energy is 0.78 MeV). The gamma rays used are highly penetrating and very thick lead shields are required to provide adequate radiation protection. Other radionuclides with more suitable properties are now available and as a result this material is no longer in use.
The ideal radionuclide should have the following properties:
Radioactivity
At low gamma ray energies photoelectric effect can cause the absorbed dose in bone to be higher than that of soft tissue. Emission energy should therefore be high enough to minimize this but be low enough to reduce the level of scattered radiation and satisfy radiation safety requirements and sheilding cost constraints. A useful working range lies between 0.2 and 0.4 MeV. Where present unwanted charged particle emissions should be easily screened, radionuclides should also have no gaseous disintegration products and have a high specific activity.
Half-life
For temporary implants the half-life should be long enough so that radioactive decay does not need to be taken into account when calculating treatment times. Although most centres do not now keep a permanent stock of radionuclides, where these are kept a very long half-life is desirable to avoid frequent replacement of stock.
Material properties
Sources should be small to enable easy delivery to (and retrival from) afterloading carriers which may have tightly curved loops. Some material can still be obtained in the form of flexible radioactive wire, e.g. Iridium-192, which can be cut into appropriate lengths with minimal risk of contamination. It is also highly desirable that the radionuclide should be non-toxic, insoluble, not prone to break up or easy dispersal if the source container is broken and should be able to undergo frequent sterilization without suffering damage.
Although none of the currently available radionuclides satisfies all of the above criteria, those in common use satisfy several. Radionuclides are therefore chosen for different uses where they are suitable. Developments in the production of radionuclides in nuclear reactors, which began following World War II, account for one source of these materials, the other being naturally occurring materials. All sources have a recommended working life beyond which the manufacturer will not guarantee the integrity of the source.
Half-life and specification of source strength
Radioactive decay is characterized by a period called the half-life of decay, T½. In simple terms, this is the time taken for the activity of a source A0 at time t = 0 to decay to half of its value or 0.5A0. During the next period, T½, the activity will decay to half of that again or 0.25A0 and so on. The process is an exponential one and can be modelled using the equation:
11.1where A0 is the activity at time t = 0 and A the activity at time t. The factor λ is the decay constant and λ = ln2/ T½
Absorbed dose
The absorbed radiation dose rate, Dr, in tissue can be calculated from the reference air kerma rate. Kerma rate (Kr) is an acronym for kinetic energy, (dEr), released per unit mass (dm) and is defined as:
11.2The special unit for kerma is the gray (Gy) and the relationship between kerma and dose rate, Dr is:
11.3The dose rate Dr is less than kerma rate Kr by a factor (1−g), this takes account of the energy lost by processes like bremsstrahlung and the production of delta rays (high energy electrons) produced by the electrons as they interact with matter and which are not absorbed in the volume dm. The factor g (<1) is the fraction of the energy lost in the bremsstrahlung process and the production of delta rays.
Gamma emitters
Caesium-137
As a fission product derived from spent uranium fuel rods used in nuclear reactors, Caesium-137 has no gaseous daughter products and largely replaced radium as the nuclide of choice in the 1960s. It has a very useful half-life of 30 years and a somewhat less penetrating 0.662 MeV (mean) gamma ray than radium which was used some decades ago, which emitted gamma rays of 0.780 MeV (mean). It was favoured for gynaecological insertions and was extensively used in low dose rate (LDR) and medium dose rate (MDR) afterloading systems from the late seventies but has now been largely replaced by high dose rate (HDR) afterloading systems using iridium-192.
It is an alkaline metal but, as a compound of chloride or sulfate, it is chemically stable, however, these salts are soluble and, therefore, for clinical sources it is mixed with other materials to reduce the risk of being absorbed by tissue should the source capsule break. When used with LDR afterloading systems, it was most commonly in the form of spherical pellets. This was achieved by mixing the caesium with glass to form beads which could be encapsulated by spherical stainless steel shells. These pellets could then be used in the form of a source train. Caesium was also incorporated into zirconium phosphate for needles and tubes used for manual interstitial and intracavitary brachytherapy. Sources are doubly encapsulated and have a recommended working life of 10 years, during which their activity falls by approximately 20%.
Iridium-192
Iridium-192 with a mean gamma energy of 0.370 MeV and half-life of 74 days is now being widely used, taking advantage of its high specific activity and the properties of a flexible wire, in which form it has many advantages over traditional radium or caesium needles. It has been in use since the late 1950s, first in the form of seeds, by Henschke, and then a few years later in the form of wire and hair-pins at the Institute Gustave Roussy in Paris. Coils of thin wire (0.3 mm diameter) can be cut to convenient lengths and inserted into flexible nylon tubes or rigid hollow afterloading needles similar to hypodermic needles, which have been previously implanted into tumours (see Figure 11.1A and B). The active iridium–platinum core is 0.1 mm in diameter and contained within a sheath of platinum 0.1 mm thick. This sheath is adequate to filter out most of the beta-rays produced as a result of the decay process. Beta emmissions are predominantly at energies of 0.530 MeV and 0.670 MeV. Thicker wires, 0.6 mm in diameter, in the form of hairpins (see Figure 11.1C and D) can also be inserted directly into tumours through suitable introducers. In the USA, iridium is available in the form of seeds sealed in thin nylon coated ribbon. Although iridium in the form mentioned here is generally used for low dose rate treatments, it can also be used as a high activity source for HDR systems (see Figure 11.2A and B).
Full access? Get Clinical Tree
