A Monte Carlo study of I-125 prostate brachytherapy with gold nanoparticles: dose enhancement with simultaneous rectal dose sparing via radiation shielding

Abstract

We investigate via Monte Carlo simulations a new 125I brachytherapy treatment technique for High-Risk prostate cancer patients via injection of Au nanoparticle (AuNP) directly into the prostate. The purpose of using the nanoparticles is to increase the therapeutic index via two synergistic effects: enhanced energy deposition within the prostate and simultaneous shielding of organs at risk from radiation escaping from the prostate. Both uniform and non-uniform concentrations of AuNP are studied. The latter are modeled considering the possibility of AuNP diffusion after the injection using brachy needles. We study two extreme cases of coaxial AuNP concentrations: centered on brachy needles and centered half-way between them. Assuming uniform distribution of 30mg/g of AuNP within the prostate, we obtain a dose enhancement larger than a factor of 2 to the prostate. Non-uniform concentration of AuNP ranging from 10mg/g and 66mg/g were studied. The higher the concentration in a given region of the prostate the greater is the enhancement therein. We obtain the highest dose enhancement when the brachytherapy needles are coincident with AuNP injection needles but, at the same time, the regions in the tail are colder (average dose ratio of 0.7). The best enhancement uniformity is obtained with the seeds in the tail of the AuNP distribution. In both uniform and non-uniform cases the urethra and rectum receive less than 1/3 dose compared to an analog treatment without AuNP. Remarkably, employing AuNP not only significantly increases dose to the target but also decreases dose to the neighboring rectum and even urethra, which is embedded within the prostate. These are mutually interdependent effects as more enhancement leads to more shielding and vice-versa. Caution must be paid since cold spot or hot spots may be created if the AuNP concentration versus seed position is not properly distributed respect to the seed locations.

Intercomparison of Monte Carlo calculated dose enhancement ratios for gold nanoparticles irradiated by X-rays: Assessing the uncertainty and correct methodology for extended beams

“…The first assumption, which is the basis of the correction for lack of CPE, neglects the so-called sink effect of the GNP [35], i.e. that electrons traversing the GNP will lose more energy than when traversing the same volume of water.…”

Intratumoral Injection of Low-Energy Photon-Emitting Gold Nanoparticles: A Microdosimetric Monte Carlo-Based Model

“…This attenuation effect at the macroscopic scale was explained in recent theoretical study from Brivio et al, which revealed a decrease of the dose to the rectum and to the urethra as a consequence to a combination of 125 I brachytherapy and Au NPs. 15 A detailed description of the "attenuation effect" undergone by the 103 Pd photons interacting with Au atoms inside of the injection volume, is available in the Supporting Information. Also, the isodose line profiles decrease more abruptly for the 103 Pd:AuNPSeed compared to the TheraSeed.…”

“…Overall, smaller volumes of energy deposition should not be seen as limitative: instead, the smaller volumes associated with the 103 Pd:AuNPSeed model, coupled to the sharper isodose fall-out simulation profiles, is an indication of enhanced treatment precision. Applied to the scale of the organs, this could help in avoiding excess dose deposition to the urethra and to the rectum, as suggested by the recent theoretical study of Brivio et al 15 The attenuation effect caused by the Au atoms present in the injection volume allows the dose to be deposited very locally. The photons emitted within the "cloud" of NPs interact with Au, a high-Z element, and transfer their energy much closer to the injection site compared with the insertion of conventional seeds.…”

Gold Nanoparticles in Radiotherapy and Recent Progress in Nanobrachytherapy

“…Brivio et al studied the effect of uniform and nonuniform distributions of GNPs in a prostate theoretical Monte Carlo model . Dose enhancement increased as a function of local GNPs concentration ( Figure ).…”