PO0310

Purpose Drug-eluting stents are the first-line therapy for in-stent restenosis. However, intravascular brachytherapy (IVBT) is used to treat patients whose drug-eluting stents fail. Current clinical dosimetry for IVBT is water-based, i.e., the absorbed dose in the target volume is calculated by assuming that the patient's artery, calcified plaques, metallic stents, and off-centred guidewire from the IVBT delivery system are all water with unit mass density. We have previously developed a Monte Carlo-based dosimetry software, RapidBrachyIVBT, to account for these heterogeneities and allow for dose calculations on optical coherence tomography images (OCT). This study examines the impact of off-centred guidewires on dose inhomogeneity during irradiation, considering scenarios with multiple guidewires, often overlooked in previous studies. Multiple guidewires are used in cases where the source train passes through a bifurcation of blood vessels. Materials and Methods RapidBrachyIVBT, a Monte Carlo dosimetry software based on the Geant4 toolkit, including the Novoste Beta-Cath 3.5F IVBT device with a 90 Sr 90 Y source train, was used. OCT images from a patient undergoing coronary IVBT for recurrent in-stent restenosis treated at Brigham and Women's Hospital (Boston, Massachusetts) were used to calculate the absorbed dose considering all heterogeneities compared to the dose calculated in water. The patient artery was segmented as water (lumen), fibrotic plaque (around the lumen), calcified plaque (behind the fibrotic plaque), smooth muscle (tunica media) and cobalt-alloy (stents). The source position was assumed to be at the origin of the image, where the OCT imaging device was placed. The guidewire positions were assumed to be at the exact locations used during imaging. Simulations were performed on the Digital Research Alliance of Canada Cedar computing cluster with 200 million decay events to yield less than 1% uncertainty on absorbed dose in the target volume, 2 mm from the source center. The absorbed dose was scored in rectangular voxels of 0.1 × 0.1 × 1.0 mm 3 along a 42 mm length, which includes the stents, source train and an additional 2 mm margin. The prescribed dose was 23 Gy to the target volume. The dose homogeneity index, the maximum to minimum dose ratio in the target volume, was calculated in both water and patient cases. Results The dose difference between the water and patient-specific cases was up to 56.2%, 55.8%, and 64.6% in the target volume with one, two, and three guidewires, respectively. The mean dose at the target volume was reduced by 3.4% and an additional 4.1% when adding the second and third guidewire, respectively. The dose homogeneity index was 1.29 in water and 2.96, 2.94, and 3.33 for the respective patient-specific cases. Each guidewire added a cold spot around the IVBT source at the target volume. Conclusions The dose at the target volume in IVBT is significantly reduced when three guidewires are present. Limiting the number of guidewires present during irradiation would reduce cold spots and dose inhomogeneity at the target volume.

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