RapidBrachyIVBT: A dosimetry software for patient-specific intravascular brachytherapy dose calculations on optical coherence tomography images

Background

Coronary artery disease is the most common form of cardiovascular disease. It is caused by excess plaque along the arterial wall, blocking blood flow to the heart (stenosis). A percutaneous coronary intervention widens the arterial wall with the inflation of a balloon inside the lesion area and leaves behind a metal stent to prevent re-narrowing of the artery (restenosis). However, in-stent restenosis may occur due to damage to the arterial wall tissue, triggering neointimal hyperplasia, producing fibrotic and calcified plaques and narrowing the artery again. Drug-eluting stents, which slowly release medication to inhibit neointimal hyperplasia, are used to prevent in-stent restenosis but fail up to 20% of cases. Coronary intravascular brachytherapy (IVBT), which uses $\beta$-emitting radionuclides to prevent in-stent restenosis, is used in these failed cases to prevent in-stent restenosis. However, current clinical dosimetry for IVBT is water-based, and heterogeneities such as the guidewire of the IVBT device, fibrotic and calcified plaques and stents are not considered.

Purpose

This study aimed to develop a Monte Carlo-based dose calculation software, accounting for patient-specific geometry from Optical Coherence Tomography (OCT) images.

Methods

RapidBrachyIVBT, a Monte Carlo dose calculation software based on the Geant4 toolkit v. 10.02.p02, was developed and integrated into RapidBrachyMCTPS, a treatment planning system for brachytherapy applications. The only commercially available IVBT delivery system, the Novoste Beta-Cath 3.5F, with a ${}^{90}{\mathrm{Sr}}^{90}Y$ source train, was modeled with 30, 40, and 60 mm source train lengths. The software was validated with published TG-149 parameters compared to Monte Carlo simulations in water. The dose calculation engine was tested with OCT images from a patient undergoing coronary IVBT for recurrent in-stent restenosis at Brigham and Women's Hospital in Boston, Massachusetts. Considering the heterogeneities, the images were segmented and used to calculate the absorbed dose to water and the absorbed dose to medium. The prescribed dose was normalized to 23 Gy at 2.0 mm from the source center, which is the target volume in IVBT.

Results

The dose rate values in water obtained using RapidBrachyIVBT aligned with TG-149 consensus values, showing agreement within a range of 0.03% to 1.7%. Considering the heterogeneities present in the patient's OCT images, the absorbed dose in the entire artery segment was up to 77.5% lower, while within the target volume, it was up to 56.6% lower, compared to the dose calculated in a homogeneous water phantom.

Conclusion

RapidBrachyIVBT, a Monte Carlo dose calculation software for IVBT, was developed and successfully integrated into RapidBrachyMCTPS, a treatment planning system for brachytherapy applications, where accurate attenuation of the absorbed dose by heterogeneities is considered.