There are many critical tasks required to confirm the integrity of a radioactive source, and they are:
1. Source Calibration to determine that the source has the activity or strength assigned at the time of use,
2. Autoradiography to confirm that the spatial distribution of the radioactive material inside any
encapsulated source is correct and remains unaltered as the source is used,
3. Source identity,
4. Source transportation,
5. Source storage,
6. Source uniformity and symmetry,
7. To perform leak tests to confirm that the source is not releasing radioactive material, and
8. Source Inventory.

1. SOURCE CALIBRATION

Source Dosimetry System

The Nucletron Source Dosimetry System offers the physicist a simple and reliable method of calibrating the source strength after source exchange and for routine verification. Calibrates the activity of the source in the afterloader

The reference air Kerma-rate of a source is the Kerma rate in air, at a reference distance of 1 meter, corrected for air attenuation and scattering. The quantity is usually expressed in mGy/h at 1m or mGy/h at 1m. 10

The American Association of Physicists in Medicine (AAPM) recommends that the calibration of the sources should be made using reentrant ionization chambers that simplify significantly the calibration procedures.

Researches demonstrates that well-type chambers is appropriate system for the routine source calibration procedures in brachytherapy but other chambers such as Solid-state phantom setups can be used for calibration of sources. 11

Routine calibration of brachytherapy sources is usually carried out with a "re-entrant"-type chamber in which the walls of the chamber surround the source, approximating a 4p measurement geometry. This unit consists of an aluminium wall ion chamber filled with argon gas under high pressure. The well dimensions are height of 26cm and diameter of 7cm. The collection potential applied to the chamber is about 150V. 1

In order to guarantee and to maintain standards over a long period of time, it is necessary to ensure that the chamber response fulfills a number of requirements and that the calibration procedure can be reproduced in a convenient and easy manner. To achieve these objectives, several checks are performed before each calibration and at regular intervals, some of them being undertaken with special devices. 7

2. HOMOGENEITY OF SOURCE ACTIVITY AND SOURCE ARRANGEMENT

The distribution of radioactivity within the source and the position of discrete sources in source trains are checked. The relative activity and the spacing of seeds are checked for source trains.

Autoradiographic Check Device

The Autoradiographic Check Device enables you to verify the source position accuracy of a stepped source afterloader. The device can be connected to all microSelectron transfer tubes (GYN, needles and flexible implants). · Allows you to simulate the correct positioning of the source inside a catheter. · Enables validation of all 18 channels simultaneously. · Clearly shows source positions on the radiograph together with the lead markers at 20 mm intervals, without any additional X-ray exposure.

Autoradiograph:- Autoradiography and radiography are the most currently used techniques for provision of such information. They are of particular interest to check the location and the spacing of discrete sources in source trains, and thereby to confirm that the geometry of the source trains is in agreement with the information provided by the manufacturer.

They are also used to check the source positioning with respect to the applicator used. A specially designed device including a check film is provided by Nucletron to check easily before the treatment the source length, position, and in-channel identification of loaded sources used in microSelectron-LDR.

Autoradiography can also be used to check the uniformity of the activity within the source. This method is very convenient for ensuring a continuous distribution of activity along wire sources and the absence of 'dud' discrete sources in a source train. However, the evaluation of autoradiographs does not allow the detection of variations in activity of less than 10%, due to the very high dose gradient near the source.

Hence, the use of a linear activimeter has been preferred for this purpose at the Institut Gustave Roussy, (Duteix et al, 1982). The linear activimeter allows the measurement of the activity of the source along a small element of length, corresponding to the collimator aperture. The exploration of the entire active length is achieved by a slow and regular displacement of the source holder in front of the detector.

The variation of linear activity along the source can be followed on a digital meter or recorded on a plotter. A constancy check of the linear activimeter response is performed at regular intervals with a small caesium-137 source encapsulated in a special holder to ensure a reproducible position. 8

Verification of the programmed source sequence is accomplished using autoradiographs. Straight test applicators are fixed to the surface of a Kodak TL film in Ready Pack paper. An exposure time of 0.01 hour provides a good image for comparison with the expected source configuration. Simulator films with the special autoradiographic source inserts were also obtained to establish agreement between the simulated and the actual source position. Radiographs with these inserted marker were evaluated for all the applicators. This provides the necessary positional information of the sources relative tot he applicator surfaces. 5

3. SOURCE IDENTITY

Physical length, diameter, serial number, and color-coding of all sources should be checked. The source dimension should be checked by physical measurement or by radiography.

4. TRANSPORT OF SOURCES

For internal transport of sources should be moved only by staff who have received adequate training. Procedures should be written so that it is clear who is responsible for the sources at any time. While being transported a container of sources must not be left unattended in areas accessible to the public or staff not concerned with its use. Containers should be labelled correctly giving details of sources within. Care should be taken to remove labels from empty containers. 14

5. STORAGE AND CUSTODY OF SOURCES

The Emergency/Service Container offers a safe, shielded environment for source storage during routine maintenance or of an applicator and radioactive source in the event or return failure of an afterloader.

· Enables the source to be exchanged in a shielded environment.
· Allows the service engineer to perform regular maintenance of the applicator assembly.

Brachytherapy sources must be stored in a safe of having 3 to 4 TVL of lead shielding. There must be a register of stock of all sealed sources having a half-life greater than a few days. This register must contain a serial number, together with reports on leakage tests and repairs. When sources are not in use they must be stored in a lockable safe to which access is restricted. For manual-loading sources safes are often used which consists of drawers into whom a known number of sources can be placed. These safes are manufactured of lead and/or steel to provide the necessary radiation protection and the safes themselves are located in the specially designed area. With remote-afterloading systems the treatment units themselves act as a safe, as is the care with the Selectron where the design ensures that the maximum exposure rate on the surface dose not exceed 25mSv/h (2.5mR/h). 14

6. SOURCE UNIFORMITY AND SYMMETRY

The source symmetry can be ascertained by taking a simulator radiograph of the source but leaving it on the film for an appropriate length of time to obtain an autoradiograph. The superposition of the autoradiograph and transmission radiograph provides the required information on source symmetry relative to the physical ends of the sources. 1

7. LEAK TESTS

Good practice requires confirmation leak tests on receipt and before clinical use. For encapsulated sources in the permanent inventory (Cesium-137, Strontium-90) semiannual leak tests usually are a legal requirement.

The leak test must be able to detect the presence of 0.005mCi (185Bq) and is required of sources before the sources are delivered. 14

8. SOURCE INVENTORY

The source inventory is carried out in association with each application. The number of sources are checked carefully when they are removed from the patient at the end of the treatment or when, in some cases, a modification in source implant is made during the treatment. A check of the patient with a Geiger Muller counter at the end of the treatment confirms the removal of all sources. The removed sources are immediately stored in portable lead containers, before being transported to the storage room where they are counted again and stored in the appropriate safe. Different safes specially designed for each type of source are used. The several compartments within each safe allow the sources to be individually houses and then ensure a rapid and easy location of each source. According to their source strength, sources are stored in different safes (caesium-137 source trains) or arranged in compartments of the single safe by a given colour (iridium-192 wires). The source inventory, as well as each source transfer, is recorded in a book in which is also recorded the correction for radioactive decay.