PTB News 1/2002 (116 kB) PTB News 1/2002, English edition, Issue March 2002The chances of lastingly opening constricted coronary vessels increase significantly when the vascular area affected is additionally irradiated with a radioactive source after having been mechanically dilated. For traceable dose measurements for this cardiovascular brachytherapy with b-radiation sources, the PTB has developed a secondary standard which is commercially manufactured under a licence and calibrated at the PTB.
In Germany, every year much more than 100000 persons contract a constriction of their coronary vessels. The therapy most frequently used is angioplasty by which the vascular obliteration is opened again using an inflatable balloon to be introduced into the artery through a cardiac catheter (balloon dilatation). However, more than half of the patients having undergone this therapy will contract another obliteration (restenosis) in the same location in the months to follow.
To avoid these restenoses, cardiovascular brachytherpy has been used for some years so that the restenosis rate could be reduced by up to 70 %. Directly after the angioplasty, the area of the coronary vessel injured by the dilatation is irradiated with a radioactive source inserted in the artery, in most cases a 90Sr/90Y beta emitter. Within the scope of various medical studies it is at present attempted to find an optimum dose for this radiation therapy. Particular difficulties are posed by the great changes in the dose rate within small spacings (typically by the factor 10 within 3 mm of tissue).
To enable clinics practising irradiation of the coronary vessels to ensure traceability of their dose measurements, the PTB has developed a secondary standard for cardiovascular brachytherapy using b-radiation sources. This standard consists of a 90Sr/90Y extended beam source about 15 mm in diameter with an activity of about 7,5 GBq, which is enclosed in a suitable protective casing. Owing to a precise mechanical system almost free from play, the source can be brought into four different beam positions (at distances between 1,7 mm and 40 mm from a reference area). Directly on the reference area and behind tissue-equivalent layers whose thicknesses are 0,1 mm to 8 mm, the water/absorbed dose rate is determined using a primary standard measuring device, an extrapolation chamber. In addition, the radial dose rate distribution is measured for each of the different layer thicknesses with the aid of a special ionization chamber of high spatial resolution. The clinical user thus has a b-radiation field in a tissue-equivalent medium, which is specified in three dimensions as regards the dose rate and shows similar properties as fields of clinical radiation sources. By calibration of his dosemeters in this radiation field, the user can ensure that his dose measurements are traceable to the national standards of the PTB.