Edge Restenosis After Implantation of High Activity 32P Radioactive β-Emitting Stents
Background—A high restenosis rate has been reported at the edges (“edge restenosis”) of 32P radioactive stents with an initial activity level of 3 to 12 μCi. This edge effect might be due to balloon injury and to a low dose of radiation at the stent margins. The aim of this study was to evaluate whether the implantation of 32P radioactive stents with a higher activity level (12 to 21 μCi) combined with a nonaggressive stent implantation strategy could solve the problem of edge restenosis.
Methods and Results—We compared the results of lesions treated with single radioactive BX stents with an activity of 12 to 21 μCi (group 2, n=54 lesions) with the results of lesions treated by single radioactive BX stents with an initial activity level of 3 to 12 μCi (group 1, n=42 lesions). There were no procedural events. At the 6-month follow-up, no myocardial infarctions, deaths, or stent thromboses had occurred. Intrastent binary restenosis was 0% in group 1 versus 4% in group 2 (n=2, both at the ostium of the right coronary artery, P=NS). Intrastent neointimal hyperplasia was significantly lower in group 2 than in group 1. The intralesion (intrastent plus peri-stent) restenosis rate was 38% in group 1 versus 30% in group 2 (P=NS).
Conclusions—Single 32P radioactive stents with an initial activity level of 12 to 21 μCi reduced intrastent neointimal hyperplasia compared with stents of 3 to 12 μCi, but they did not solve the problem of edge restenosis, even if a nonaggressive stent implantation strategy was used.
In patients with coronary artery disease treated with 32P radioactive β-emitting stents with an initial activity of 3 to 12 μCi, no intrastent restenosis was observed at the 6-month angiographic follow-up.1 However, the intralesion restenosis rate was >40% due to restenosis at the stent edges (“edge restenosis”).1 This edge effect might be due to a low dose of radiation at the stent margins2 combined with systematic balloon injury in the reference segments, especially when the balloon was oversized and inflated at high pressures (aggressive stent implantation strategy).
The purpose of this study was to evaluate whether 32P radioactive stents with higher activity levels (12 to 21 μCi) combined with a nonaggressive stent implantation strategy could solve the problem of edge restenosis.
The characteristics of the radioactive BX 15-mm long stent (Isostent) mounted on a 20-mm-long, compliant balloon and of the isotope (32P) used in this study have been previously described.1 A calculation of the dose of radiation delivered by each stent implanted was performed as recommended3 using the method proposed by Janicki et al.2 Inclusion criteria for enrollment in this study have been previously reported.1 The angiographic results of the lesions treated at our center from October 1998 to April 1999 by single radioactive BX stents with an initial activity of 12 to 21 μCi (group 2: 54 lesions, 40 patients) were compared with the results of the lesions previously treated with single radioactive BX stents with an activity of 3 to 12 μCi (group 1: 42 lesions, 40 patients). The additional lesions treated with >1 stent (group 1: 22 lesions, 17 patients; group 2: 22 lesions, 19 patients) were excluded.
The technique used to implant the radioactive stents in group 1 has been previously reported.1 In group 2, a nonaggressive stent implantation technique was used. Lesion predilatation was performed with a nonoversized balloon, and the balloon used to deploy the stent was inflated at 8 to 10 atm; postdilatation was performed using a shorter balloon inflated at high pressure inside the stent to avoid mechanically damaging the reference segments outside the stent.
After stenting, patients received long-term treatment with aspirin (325 mg daily) plus ticlopidine (250 mg twice daily) or clopidogrel (75 mg daily) for ≥3 months. Angiographic and intravascular ultrasound (IVUS) analyses were performed as previously described.1 Intrastent and intralesion restenosis, death, myocardial infarction, stent thrombosis, and target lesion revascularization were defined as previously reported.1 4 5 6
Statistical analysis was performed using the StatView statistical package (StatView 5, SAS Institute). Continuous, normally distributed data were expressed as mean±SD. Comparisons of continuous variables between groups were performed using ANOVA techniques. Subgroup comparisons of categorical variables were performed by the Fisher exact test or the χ2 test. Differences were considered statistically significant at P<0.05.
Clinical and Procedural Characteristics
Patient and procedural characteristics are shown in Table 1⇓. Most of the treated lesions (>77%) were de novo lesions. Because of the less aggressive stent implantation strategy, the final balloon size and balloon-to-artery ratio were smaller, although the difference was not statistically significative, in group 2 than in group 1.
At the 6-month follow-up, no myocardial infarctions, deaths, or stent thromboses had occurred. Target lesion revascularization, which was performed in all lesions with angiographic restenosis even if the patients were asymptomatic and had no objective evidence of ischemia, was 38% in group 1 and 30% in group 2.
Quantitative Angiographic and IVUS Analysis
Table 2⇓ summarizes the quantitative angiographic results. There were no differences between the groups, except for lesion length, which was shorter in group 2 than in group 1. Because of the less aggressive stent implantation strategy, there was a trend for a smaller final minimum lumen diameter and acute gain in group 2 than in group 1. In both groups, intralesion restenosis was ≥30%; it mainly occurred as a focal restenosis at the edge of the stent (33% in group 1 versus 26% in group 2, P=NS). Two examples of proximal edge restenosis that occurred in group 2 are shown in Figure 1⇓. Intrastent restenosis was absent in group 1 and occurred in 2 lesions (4%) in group 2; these 2 lesions were both at the ostium of the right coronary artery, as shown in Figure 2⇓. A total occlusion at follow-up, which was not associated with clinical events, was observed in 2 patients (5%) in group 1.
Intrastent plaque volume, as calculated by quantitative IVUS measurements, was significantly lower (P<0.01) in group 2 (4.4±5.6 mm3, n=32 lesions) than in group 1 (15.1±14.1 mm3, n=33 lesions). As shown in Figure 3⇓, late lumen loss in the first 3 mm from the proximal and distal margins of the 12 to 21 μCi radioactive stents with edge restenosis was mainly due to remodeling (shrinkage of the vessel), and only in the first 1 mm from the proximal edge of the stent it was due to a similar amount of remodeling and intimal hyperplasia.
The results of this study indicate that single, 15-mm-long, 32P radioactive β-emitting BX stents with an initial activity level of 12 to 21 μCi are more effective than 3 to 12 μCi stents in reducing intrastent neointimal hyperplasia, as measured by IVUS; the potential mechanisms by which this effect occurs have been previously described.1 No myocardial infarctions, death, or stent thrombosis were observed at the 6-month follow-up. However, although a nonaggressive stent implantation strategy was used, the problem of edge restenosis was not solved.
Mechanism of Edge Restenosis
In this study, the edge restenosis in 12 to 21 μCi radioactive stents that were implanted using a nonaggressive strategy was mainly due to remodeling. This result differs from our prior observations1 in 3 to 12 μCi radioactive stents implanted using an aggressive strategy, in which edge restenosis was mainly due to tissue growth. Thus, by increasing the initial stent activity level and limiting the balloon-induced injury outside the stent, there was a reduction of edge restenosis related to plaque growth but not of that related to negative remodeling.
To reduce the problem of edge restenosis, 2 different modifications of the existing 32P radioactive BX stent are under investigation: (1) the hot-ends stent and (2) the cold-ends stent. The hot-ends stent, which has a higher activity level at its proximal and distal ends, might diminish the problem of edge restenosis related to tissue growth and/or remodeling by extending the area of irradiation beyond the balloon-injured area outside the stent. However, if merely subtherapeutic levels of radiation are sufficient to induce proliferation/remodeling in uninjured tissue,7 increasing the activity at the stent ends would only relocate the restenotic zone further from the stent. Lengthening the stent with a nonradioactive cold-ends stent might also diminish the edge effect related to negative remodeling, which was demonstrated in this study to be the principal mechanism of edge restenosis in radioactive stents with an activity of 12 to 21 μCi that were implanted using a nonaggressive strategy.
Single 32P radioactive β-emitting stents with an initial activity of 12 to 21 μCi were more effective than 3 to 12 μCi stents in reducing intrastent neointimal hyperplasia, as measured by IVUS, but they did not solve the problem of edge restenosis, even if a nonaggressive stent implantation strategy was used. Edge restenosis in 12 to 21 μCi radioactive stents was mainly due to remodeling.
- Received December 31, 1999.
- Revision received April 5, 2000.
- Accepted April 7, 2000.
- Copyright © 2000 by American Heart Association
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