33 Endovascular Brachytherapy

Endovascular Brachytherapy 643

The clinical target length (CTL) must secondly include every injured part of the vessel. The injury caused by the intervention may often extend beyond the position of the device (as demonstrated on the angiogram), which serves as the basis for the definition of the IL. As radiotherapy is given to prevent the process of neointimal proliferation taking place in all the injured parts of the vessel, all this injury must be included. Because in an individual case, the precise definition of the margin to be added to the interventional length for additional injury is often difficult to determine, it may be arbitrarily chosen (e.g. 1 - 2 mm on each side). Finally, for the definition of the Planning Target Length (PTL), a safety margin must be added, which depends on the geometrical uncertainties of the radiation treatment which are patient and treatment device related. These uncertainties may be due to patient movements (e.g. heart beats) or to catheter positioning uncertainties and margins may be chosen in the range of a few mm on each side. In summary the Planning Target Length is obtained by adding the following safety margins (sm): PTL = IL + sm (overall) sm = sm inj (additional injury) + sm geom (geometrical uncertainties) In coronary arteries a minimum overall sm of 5 mm each side is recommended, in femoral arteries 10 mm. Design of devices for intravascular brachytherapy In the design of devices for intravascular brachytherapy, the dimensions of sources and catheters have to be small to permit treatment of thin arteries and for bending around tight curves. If a single source is used, it must be very precisely moved during treatment to achieve the projected dose distribution. An individual matching of the irradiated length to the target length is possible (stepping source technology). Wires or serial sources can be used without movement of the source during treatment, but these devices become stiffer and more difficult to position, especially in vessels which bend. The wire must be flexible enough to follow smoothly the curves of the guiding catheters. An adaptation in length in these cases is only possible by manufacturing wires or serial sources of different lengths. 7.1.1 Femoropopliteal arteries The brachytherapy device consists of a remote-controlled afterloader unit (e.g. microSelectron-HDR afterloader) which delivers gamma radiation with an Iridium-192 source safely and reproducibly. Such devices are already well known and widely available for HDR brachytherapy of cancer. The positioning of the source is computer controlled and initiated and controlled from outside the treatment room. The device has a safe to store the source when it is not in use. The afterloader is operated by programming the different source positions and dwelling times with the support of integrated libraries. These libraries can be modified by a Treatment Planning System attached to the afterloader unit. The source has a nominal activity of 370 GBq and is sealed in a stainless steel capsule with an Active Source Length of 3.5 mm and an outer diameter of 1.1 mm, which is welded to an ultraflexible part of a flexible cable driven from the afterloader. Due to the high activity of the source, treatment must be performed in rooms with specially shielded walls as are usual in HDR brachytherapy units. 7 Technique 7.1