03/24/2026
By Benjamin MacConnell

The Kennedy College of Science, Department of Physics and Applied Physics, invites you to attend a Master’s thesis defense by Benjamin MacConnell on “Spatiotemporal Dependence of Dose Enhancement due to Gold Nanoparticles Diffusing from Brachytherapy Spacers”

Candidate Name: Benjamin MacConnell
Degree: Master’s
Defense Date: Monday, April 6, 2026
Time: 12:30 - 2 p.m.
Location: Olney Hall, Room 136D, North Campus

Thesis/Dissertation Title: Spatiotemporal Dependence of Dose Enhancement due to Gold Nanoparticles Diffusing from Brachytherapy Spacers

Committee:

Members:

  • Advisor: Romy Guthier, Ph.D., Department of Physics and Applied Physics, UMass Lowell
  • Marian Jandel, Ph.D., Department of Physics and Applied Physics, UMass Lowell
  • Erno Sajo, Ph.D., Department of Physics and Applied Physics, UMass Lowell
  • Kwok-Fan Chow, Ph.D., Department of Chemistry, UMass Lowell

Brief Abstract:
The spatiotemporal evolution of AuNP concentration diffusing from loaded brachytherapy spacers was simulated using a second-order finite difference model in two geometries, spacer-seed pairs (SS) and spacer-spacer-seed triplets (SSS). Four pairs and triplets were placed 5 mm apart in opposite configurations to adjacent pairs or triplets with periodically mirrored boundaries in three dimensions. The spacers were loaded with 30 mg gold/g of spacer, material, and three assumed release rates were used. A first order approximation of nanoparticle uptake due to endocytosis was used to simulate nanoparticle removal from intercellular space. The Dose Enhancement Factor (DEF) was found from the concentration of AuNPs using linear interpolation of previously published results. The dose distribution from the Iodine-125 seeds was calculated using the TG-43 formalism. The volume averaged DEF (vDEF) was calculated by dividing the dose with and without AuNPs and averaging over all voxel volumes. Results showed the dose enhancement of the added AuNPs allows us to decrease initial air kerma strength (S_k^0) of the Iodine-125 by 10-13% and 7-11% and still achieve the same D90 or D50, respectively, as when normal brachytherapy spacers. In the SS geometry, as AuNP size increases D90 increases and D50 follows a quadratic trend with a minimum at Hydrodynamic diameter of 80 nm. In the SSS geometry, as AuNP size increases D90 increases from 2nm to 30 nm and 70 to 100 nm then decreases from 30nm to 70 nm. D50 follows a quadratic trend with a minimum at 40 nm. For both the vDEF follows a quadratic with a minimum at 50 nm. Slower release rates underperform in all three metrics. The nanoparticle loaded brachytherapy spacers result in a more homogeneous dose distribution inside the tumor while lowering the air kerma strength needed in each seed to reach dosimetric values like D90 and D50, thus increasing the normal tissue sparing.