33 Endovascular Brachytherapy

Endovascular Brachytherapy 653

vessel wall varies with the size of the artery, 1 mm depth into the vessel wall is recommended for coronary arteries and 2 mm for femoropopliteal arteries. The reference dose at the reference depth in the central plane representing the reference lumen diameter is then calculated taking the radius of the reference vessel lumen after angioplasty and adding 1 mm for coronary arteries and 2 mm for femoropopliteal arteries. In order to assess the dose fall-off in the target, the dose at the luminal surface should also be given. For a non centred device, the maximum and minimum doses for the reference depth dose and the luminal dose should be reported in addition. If IVUS is used, more points may be added to describe the dose in terms of topography and pathology (plaque) more precisely. A well reproducible structure in the vessel wall to use as a reference point is the external elastic membrane, which can be reliably shown on IVUS. IVUS facilitates reporting of dose distribution not only in the central plane, but also in other planes in the target, to give a better idea of the dose distribution of dose throughout the vessel wall. For the longitudinal dose distribution, there is again significant variability. It is therefore necessary to follow a given terminology to define the different lengths relevant for unambiguous dosimetry, as it has been recommended recently (5). The Active Source Length (ASL) is defined as the length of the radioactive source or source train or the active dwell positions of a stepping source used to achieve the prescribed dose in the PTL. For the different isotopes and devices, the ASL is always longer than the length of the Reference Isodose (“Reference Isodose Length” (RIL)), depending on the dose fall-off, the arrangement of sources, and the vessel diameter. The Reference Isodose Length indicates the segment of the vessel wall enclosed by the 90% isodose at the reference depth. In the ideal case the RIL equals the PTL. Usually, the RIL is somewhere between the ASL and the PTL, as the ASL cannot be continuously adapted to the prescribed PTL. In the available devices, different Active Source Lengths are available which must be chosen when selecting the adequate catheter, which will ensure that the RIL covers at least the PTL.

Fig 32.11: Definition of the Reference Isodose Length (RIL) based on dose distribution along the axis of the source arrangement. Different isodose lines are indicated: 100%, 90%, 50%. The Reference Depth Dose is given at the Reference Depth in the Central Plane, which represents the 100% isodose. The Reference Isodose Length is the Length enclosed by the 90%-isodose (Potter, et.al. (5)). A radiation dosimetry survey must be done before and after the procedure by measuring radiation levels: in the room, at the patient, and at the removed catheter and for the water in the Novoste system (for contamination).