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Brachytherapy (Greek: brachys = close) is a method of radiation therapy where one or several small radiation sources - almost exclusively radionuclide sources - are placed either very close to, or in contact with, or directly into the tumour. Brachytherapy dates back to the year 1901. Already shortly after the detection of radioactivity by Becquerel in 1896, radioactive sources were inserted into a tumour and it was observed that radiation made the tumour shrink [1]. In the early 20th century, new application techniques were developed by Danlos at the Institut Curie in Paris and by Robert Abbe at the Memorial Hospital in New York [2][3] which used radium. After the initial interest in brachytherapy in Europe and in the USA, its use declined in the middle of the 20th century. The reason for this was the radiation exposure of the medical personnel due to the manual handling of the sources.

The development of remote-controlled afterloading systems and of new brachytherapy sources in the 1950s and 1960s clearly improved the source handling as well as the radiation protection. This, as well as new developments in the field of three-dimensional imaging methods, automated treatment planning, and the enhancement of the afterloading systems have turned brachytherapy into a secure and efficient form of treatment for many cancer types [1]. It is often used to treat cervical cancer, prostate cancer, breast cancer and skin cancer.

Brachytherapy can be used alone or in combination with other forms of therapy, for example surgery, external radiation therapy and chemotherapy. Studies prove that the cure rates of cancer only due to brachytherapy are comparable to those due to a surgery or external radiation therapy [4]. The risk of side effects is, however, clearly lower in brachytherapy [7].

When using this treatment method, the radiation source can be very diverse: radioactive beta, gamma and X-ray emitter and, recently, also miniaturized X-ray tubes. Since the first treatment with brachytherapy the radionuclides given in Table 1 have established themselves for clinical use:

Table 1: Properties of the radionuclides mainly used in clinical brachytherapy [5][6].

In general, radioactive brachytherapy sources are constructed as small cylindrical sources with a length of up to 0.5 cm. The radioactive material is usually enclosed by a titanium shell. The interior structure of the sources is manufacturer- and type-specific. The sources are classified into the two categories "high dose rate (HDR) sources" and "low dose rate (LDR) sources".

Sources which provide a dose rate of more than 12 Gy/h at a depth of 1 cm in water are designated as "high dose rate sources" (HDR sources). In this field, the radionuclide ¹⁹²Ir has established itself as source material. Lately, ⁶⁰Co is experiencing a renaissance. Due to its longer half-life time, its use offers economic advantages. For ⁶⁰Co sources, however, much higher radiation protection precautions must be made than when using ¹⁹²Ir sources. Sources which provide less than 2 Gy/h at a depth of 1 cm in water are designated as "low dose rate sources" (LDR sources), so-called LDR seeds. They are implanted in the tumour permanently or temporally. ¹²⁵I and ¹⁰³Pd seeds are mainly used.

A considerable advantage of brachytherapy is that due to the fact that the sources are being placed in the tumour or next to it, a high dose can be deposited in the tumour and, at the same time, healthy tissue which is located further away from the tumour can be spared. In the case of radiation penetrating from outside (external radiation therapy), also healthy tissue is inevitably exposed.

The objective of radiation therapy is to generate in the tumour a sufficiently high - and as homogenous as possible - dose distribution. In brachytherapy, this is achieved by using several sources simultaneously and/or by modifying the position of the source during a patient's irradiation period.

The dose in brachytherapy is the absorbed dose to water, D_W, in water at a distance of 1 cm from the center of activity of the radiator perpendicular to the source axis. Their value results from the value of the Reference Air Kerma Rate (RAKR) multiplied by the dose conversion coefficient Λ, also called dose rate constant. The Reference Air Kerma Rate is defined as the air kerma rate free in air at a distance of 100 cm from the center of activity of the radiator [9].

The value of the dose conversion coefficient is type-specific or design-specific, i.e. equal for seeds of the same design. Until recently, the value could only be calculated by means of radiation transport calculations based on Monte Carlo simulations. These values can be found in freely accessible databases for all frequently used seed types. Recent research has also enabled these values to be experimentally determined for some HDR and LDR seeds. The results obtained so far show a good agreement between calculation and measurement.

In order to achieve the greatest possible therapeutic success and at the same time to minimize the risk for the patient, a traceable and quality-assured dosimetry is of great importance. The basis for this is the primary realization of the unit of the dose and its dissemination by calibration of secondary standards or brachytherapy sources. HDR and LDR sources are currently calibrated at PTB in terms of the Reference Air Kerma Rate.

In the case of the calibration of HDR sources, the air kerma rate is measured by means of an ionization chamber which is traceable to the primary standards of PTB for the air kerma free-in-air (WG 6.25 Dosimetry for diagnostic radiology). The measurements are carried out in the collimated radiation field of the source. For this purpose, a special measurement unit was set up (see Figure 2). In the centre of a lead box (dimension: 30 cm x 30 cm x 40 cm; wall thickness: 5 cm), which is equipped with a DensiMed^® diaphgram with an aperture of optionally 5.5 cm or 11 cm, the source to be calibrated is put in place using an afterloading system. The positioning of the calibrated sphere ionization chamber on the central axis of the radiation field defined by the diaphragm is carried out by means of a commercial industrial robot, with an uncertainty of less than 0.1 mm. To eliminate uncertainties which occur due to the positioning of the source by the afterloading system (approximately 0.3 mm), several measurements are performed at various distances between the radiation source and the ionization chamber, and the positioning of the source is determined by a calculation.

For more than 15 years, ¹⁹²Ir HDR brachytherapy sources have been calibrated at PTB in the measurand RAKR, with a relative standard measurement uncertainty of 1.25 % (k = 1).

For the realization of the reference air kerma rate (RAKR) of LDR sources, PTB operates an ionization chamber as a primary standard in the form of a large-volume parallel-plate extrapolation chamber (so-called GROVEX) (see Figure 3). The front electrode (high-voltage electrode) and the rear measurement electrode are 12 µm thin graphitated polyethylene foils [12]. The source is located at a distance of 30 cm from the front high-voltage electrode. In front of the high-voltage electrode, there is installed a lead diaphragm to limit the radiation field of the source which enters into the chamber measurement volume. The distance between the electrodes can be freely varied to be able to measure the ionization current at various distances. The RAKR is determined from the slope of the straight which is adapted to the measurement points.

The realization of the unit of the absorbed dose rate to water for brachytherapy using LDR sources - and also for using HDR sources - was realized within the scope of a European research project and established at PTB [13]. It is realized by means of ionometry using a large-volume coplanar plate extrapolation chamber. Its set-up and operation resemble that of the primary standard for the reference air kerma rate; however, the extrapolation chamber is located in a phantom made of water-equivalent material. Thus, the measurement method differs from that of the RAKR primary standard. It was developed on the basis of the radiation transport theory and - following an old-established idea - aims at the differences of current values at various plate distances [14]. The evaluation procedure uses quantities whose values were determined by means of Monte Carlo simulation. As these values are based on the ratios of calculated air kerma values within the extrapolation chamber, this method is very stable against uncertainties of the atomic interaction cross sections and the spectra [14]. The developed method was adopted by the Italian national metrology institute ENEA which also took part in the research project [15].

With the direct realization of the unit of absorbed dose rate to water, a measurement uncertainty of 2.6 % (k = 2) was achieved which - compared with the uncertainty of 10 % (k = 2) for the determination of the absorbed dose rate to water via the reference air kerma rate - is a clear reduction in the uncertainty. In contrast to HDR brachytherapy, a direct calibration of sources in the unit of the absorbed dose rate to water is feasible and is aimed at for LDR brachytherapy.

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