<h3>Purpose/Objective(s)</h3> This study aims to report outcomes of intracoronary brachytherapy (ICBT) in treating drug-eluting stent (DES) in-stent restenosis (ISR) and to identify correlated, patient and treatment factors. <h3>Materials/Methods</h3> Patients undergoing ICBT for recurrent DES ISR from 2010 to 2021 were included in this single-institution retrospective PCI registry. Most lesions had ≥2 layers of pre-existing DES and were treated with balloon angioplasty, excimer laser coronary atherectomy (ELCA), or rotablator followed by ICBT at a dose of 18.4 Gy (vessel diameter ≤ 3.35mm) or 23 Gy (vessel diameter >3.35mm) delivered 2 mm from source center at the site of ISR. After March 2015, the standard dose was changed to 23 Gy or 25 Gy. All patients had operator-determined final target lesion stenosis <50%, and there were no in-hospital deaths. The primary outcome was 3-year target lesion failure rate (cardiac death, target lesion revascularization, or target vessel myocardial infarction). Secondary endpoints where target lesion failure (TLF) rate at one year, any cause mortality, and cardiac mortality. Cox regression analyses were used to determine covariates, correlated with TLF rate, with p<0.05 considered significant and 0.05<p<0.10 considered a trend. The covariates were pre-specified based on previous studies and limited in number to avoid over-modelling. Analysis was performed using IBM statistical software and SYSTAT version 13. <h3>Results</h3> There was a total of 330 patients with 345 treated lesions. The mean age was 66 years, and 70.3% were men. At 3 years, all-cause mortality was 22.6% and cardiac mortality was 13.5%. The mean time to ISR from last failed DES implantation was 284 days. 255 of the 285 patients with known number of stent layers had at least 2 stents at the target lesion. Most of the patients had advanced coronary disease; 62% had prior bypass surgery. Rate of TLF was 18% at 1 year and 46% at 3 years. More stent layers were found to be correlated with 3-yr TLF (p=0.045). Lower radiation dose was negatively correlated with 1-yr TLF (p=0.039) but not at 3 years. TLF was trended higher if the target lesion was a vein graft (p=0.097). Final diameter of stenosis of the target lesion was not correlated with TLF at 1 year or at 3 years. <h3>Conclusion</h3> ICBT for recurrent DES-ISR provided low recurrence rates at 1 year, which increased substantially by 3 years. Outcomes were significantly better with lower numbers of stent layers at the target lesion but not with greater final diameter of target lesion stenosis.
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