Radiation Sources and Devices in Brachytherapy

Author: Dr. Pál Zaránd

 
The radiation sources in bracytherapy are also classified by the isotope’s type, half-life, the application’s aim, repeatability or by the devices using the isotopes. In this chapter, we discuss only the closed sources, not talking about the former, reusable sources developed to manual treatment, or the ²²⁶Ra, which is no longer used. The table below summarizes the parameters of some sources used in brachytherapy. The data only informatory, because depending on the material of the case, fluorescent X-ray is also formed, but the unnecessary electrons and low energy photons are absorbed by the case.

Permanent brachytherapy or seed implantation

The sources left in the patient, called seeds, exactly about 0.8-1 mm diameter, 4-5 mm long rods, and their build are diverse, depending by the isotope type, and the application. For example, the ¹²⁵Iis placed on a carrier in a thin Ti case, because of the low energy. The ¹⁰³Pd 103Pd source isn’t seen in the X-ray, that’s why a lead marker put in the centre of the seed, etc. The seeds can inserted to the target place with a special tool, through the puncture channel.

Manual afterloading

The essence of this method is in its name. Inactive devices, lead wires, hollow needles, templates punctured in the patient properly to the treatment plan, to decrease the staff’s radiation exposure. After the setting of this complex setup, which proved appropriate in inactive phase, the sources are placed manually into it. The usually ¹⁹²Ir wire could be made from Ir-Pt alloy, what can be cut to proper size with special, radiation protected tools. The other usual process is to put 3 mm size ¹⁹²Ir sources with 1 cm distance into a plastic tube, and the necessary length is reached by cutting the inactive part to the appropriate size.

Afterloading

In this method, first the inactive applicators placed into the patient’s body and after the fitting setup controlled by imaging, the required dose rate reached with moving only one point source.
The main parts of the afterloading machines are: The source-moving device, the channel selector, source container, source-leading tubes and applicators and the computerized controlling device. The installed system, in addition to locking mechanisms, is completed by security devices. The inactive (dummy) source is also a part of the modern afterloading machine’s security system, which is used to verify all sources moving before the irradiation. Moving the point source, arbitrary source can be created, only the stepping times have to planned by an irradiation planning program. The common source is ¹⁹²Ir and ⁶⁰Co (less common). The Iridium source’s initial activity is usually 370GBq, that of the cobalt source is 37 GBq. Greater specific activity can be reached with iridium, and therefore it can used with many types of applicators including needles. The disadvantage of this type is that in an institution with larger patient throughput a source replacement required in every 3 months. Because of the cobalt source’s lower specific activity, the charge’s size is bigger, that’s why it is not suitable to prickling, but it is suitable for intracavital therapies (esophagus, rectum, gynecological treatment) The frequency of the charge’s replacement is only specified by the quality of the fixation on the mover cables.

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