INTRAARTERIAL (90)Y BRACHYTHERAPY - PRELIMINARY DOSIMETRIC STUDY USING A SPECIALLY MODIFIED ANGIOPLASTY BALLOON

Purpose: Irradiation has been shown to be effective in preventing rest enosis after dilatation in human peripheral arteries, We have develope d a dedicated system for coronary intraarterial irradiation using a Y- 90 pure beta-emitting source inside a specially modified angioplasty b alloon. This paper presents a preliminary dosimetric evaluation of thi s system. Methods and Materials: Specially fabricated titanium-covered and activated yttrium wires (outer diameter 0.32 mm) were used for th ese studies, Dosimetry was performed using small thermoluminescent dos imeters (TLDs) placed on the surface of the 2-cm long angioplasty ball oons, inflated with contrast medium to a diameter of 2.5, 3, 3.5, and 4 mm. Radioactive Y-90 wires were left in the inner balloon catheter a nd the surface dose rate was measured and extrapolated to 72 h after a ctivation to allow a comparison between the values obtained. After obs erving the poor centering of the source within the standard angioplast y balloon, a new centering balloon was developed. A conventional ballo on was subdivided into four evenly spaced interconnecting chambers, th us assuring adequate centering of the inner catheter. Thermoluminescen t dosimetric measurements were performed with a 3.5 mm centering ballo on to evaluate the homogeneity of the surface doses compared to those measured with the conventional balloon. Results: Thermoluminescent dos imetric measurements using the standard balloons filled with contrast medium were plotted semilogarithmically as a function of distance from the balloon surface. The logarithms of the measured doses fit a strai ght line as a function of depth. The doses at 1 mm and 3 mm are approx imately 50 and 10% of the surface dose, respectively. Due to the poor centering of the source in the conventional balloons, the dispersion a nd standard deviations (SDs) of the measured surface doses increased p roportionally to the balloon diameter (SDs are 1.89, 5.52, 5.79, and 6 .46 Gy for 2.5, 3, 3.5, and 4 mm balloon diameters, respectively). For the 3.5 mm centering and conventional balloons the respective mean, m inimum, and maximum surface doses were 8.41 Gy (min. 7.26; max. 9.46) and 7.89 Gy (min. 2.18; max. 16.06) and their standard deviations were 0.66 and 5.79 Gy, respectively. Conclusions: Conventional angioplasty balloons cannot ensure a homogeneous dose delivery to an arterial wal l with an intralumenal Y-90 beta source. Preliminary dosimetric result s using a modified centering balloon show that it permits improved sur face dose distribution (axial and circumferential homogeneity), making it suitable for clinical applications,