A new delivery system to resolve dosimetric issues in intravascular brachytherapy

Extreme coronary radiation doses from intravascular brachytherapy

PURPOSE

To evaluate coronary artery integrity after very high radiation doses from intravascular brachytherapy (IVBT) in the setting of source asymmetry.

METHODS

Ten patients treated for right coronary artery (RCA) in-stent restenosis (ISR) between 2017 and 2021 and for whom follow-up angiograms were available were identified from departmental records. Procedural angiograms, taken to document source position, were used to estimate vascular wall doses. The 2.5 mm proximal source marker was used to estimate the distance from source center to the media and adventitia. Distances were converted to dose (Gy) using the manufacturers' dose fall-off table, measured in water. Follow-up films were scrutinized for any sign of late vascular damage.

RESULTS

The average minimal distance from catheter center to the adjacent media and the adventitia was 0.9 mm (±0.2) mm and 1.4 mm (±0.2), respectively. The average maximum media and adventitial doses adjacent to the source were 75 Gy (±26) and 39 Gy (±14), respectively. Follow-up angiograms were available from 0.6 years to 3.9 years following IVBT (median: 1.6 years). No IVBT-treated vascular segment showed signs of degeneration, dissection or aneurysm.

CONCLUSION

IVBT vascular wall doses are frequently far higher than prescribed. The lack of complications in this unselected group of patients gives a modicum of reassurance that raising the prescription dose is unlikely to lead to a sudden appearance of complications.

Image-guided intravascular brachytherapy dose escalation

PURPOSE

Coronary stents reduce IVBT radiation dose with a single layer by 10-30%. However, the impact of multiple stent layers and stent expansion remains unexplored. Individualized dose adjustments considering variations in stent layers and expansion could improve radiation delivery effectiveness.

METHODS

EGSnrc was used to compute the delivered vessel wall dose in various IVBT scenarios. Stent effects were modeled for the stent density of 25%, 50%, and 75% with 1, 2, and 3 layers respectively. Doses were calculated at 1.75 to 5.00 mm away from the source center, normalized to 100% at 2 mm.

RESULTS

Dose fall-off increased with increasing stent density. With a single layer, the dose at 2 mm from source fell from 100% of prescription to 92%, 83% and 73% at 25%, 50% and 75% density, respectively. The computed dose to points with increasing radial distance from the source decreased progressively with increasing stent layers. With three layers, at 75% stent density, the dose at 2 mm from source center fell to 38%.

CONCLUSIONS

A schema for image-guided IVBT dose adjustment is described. While it would be an improvement over current standard of care, myriad factors remain to be addressed in a comprehensive effort to optimize IVBT.

Dose perturbation and inhomogeneity of multi-arrays of 125I seed-loaded stent for treatment of portal vein tumor thrombosis

PURPOSE

To investigate the dosimetry of ¹²⁵I seed-loaded stent system currently used for an adjuvant treatment of portal vein tumor thrombosis (PVTT).

METHODS

The stent system consisted of an inner metallic stent and outer seed-loaded capsules. Four arrays of ¹²⁵I seeds were attached longitudinally to the outer surface of the stent at 90° separation. 145 Gy was prescribed at 5 mm from the axes of seed-arrays. For the geometries of the 4-array, and potential 6- and 8-array configurations, treatment planning system (TPS) and Monte Carlo (MC) calculations were performed to evaluate 3D dose distributions and dosimetric impact of the metallic stent.

RESULTS

The MC simulations indicated the metallic stent reduced a dose to the prescription points by over 10%, compared to the water-based TPS results. The total activity calculated by the water-based TPS to deliver the prescription dose should compensate for this amount of reduction. The MC- and TPS-calculated doses normalized to the prescription points for the current configuration were in agreements within 4.3% on a cylindrical surface along 5 mm from the axes of seed-arrays. The longitudinal underdosage worsened as approaching the edge of arrays, and ranged from 2.8% to 25.5%. The angular underdosage between neighboring arrays was 2.1%-8.9%.

CONCLUSIONS

With this compensation and a special care of near-edge underdosage, the current 4-array system can provide adequate dose coverage for treatment of PVTT. Further dosimetric homogeneity can be achieved using 6-or 8-array configurations.