Acoustic Modulation Enables Proton Detection With Nanodroplets at Body Temperature

A Preliminary Investigation of Radiation-Sensitive Ultrasound Contrast Agents for Photon Dosimetry

Radiotherapy treatment plans have become highly conformal, posing additional constraints on the accuracy of treatment delivery. Here, we explore the use of radiation-sensitive ultrasound contrast agents (superheated phase-change nanodroplets) as dosimetric radiation sensors. In a series of experiments, we irradiated perfluorobutane nanodroplets dispersed in gel phantoms at various temperatures and assessed the radiation-induced nanodroplet vaporization events using offline or online ultrasound imaging. At 25 °C and 37 °C, the nanodroplet response was only present at higher photon energies (≥10 MV) and limited to <2 vaporization events per cm2 per Gy. A strong response (~2000 vaporizations per cm2 per Gy) was observed at 65 °C, suggesting radiation-induced nucleation of the droplet core at a sufficiently high degree of superheat. These results emphasize the need for alternative nanodroplet formulations, with a more volatile perfluorocarbon core, to enable in vivo photon dosimetry. The current nanodroplet formulation carries potential as an innovative gel dosimeter if an appropriate gel matrix can be found to ensure reproducibility. Eventually, the proposed technology might unlock unprecedented temporal and spatial resolution in image-based dosimetry, thanks to the combination of high-frame-rate ultrasound imaging and the detection of individual vaporization events, thereby addressing some of the burning challenges of new radiotherapy innovations.

Analytic prediction of droplet vaporization events to estimate the precision of ultrasound-based proton range verification

BACKGROUND

The safety and efficacy of proton therapy is currently hampered by range uncertainties. The combination of ultrasound imaging with injectable radiation-sensitive superheated nanodroplets was recently proposed for in vivo range verification. The proton range can be estimated from the distribution of nanodroplet vaporization events, which is stochastically related to the stopping distribution of protons, as nanodroplets are vaporized by protons reaching their maximal LET at the end of their range.

PURPOSE

Here, we aim to estimate the range estimation precision of this technique. As for any stochastic measurement, the precision will increase with the sample size, that is, the number of detected vaporizations. Thus, we first develop and validate a model to predict the number of vaporizations, which is then applied to estimate the range verification precision for a set of conditions (droplet size, droplet concentration, and proton beam parameters).

METHODS

Starting from the thermal spike theory, we derived a model that predicts the expected number of droplet vaporizations in an irradiated sample as a function of the droplet size, concentration, and number of protons. The model was validated by irradiating phantoms consisting of size-sorted perfluorobutane droplets dispersed in an aqueous matrix. The number of protons was counted with an ionization chamber, and the droplet vaporizations were recorded and counted individually using high frame rate ultrasound imaging. After validation, the range estimate precision was determined for different conditions using a Monte Carlo algorithm.

RESULTS

A good agreement between theory and experiments was observed for the number of vaporizations, especially for large (5.8 ± 2.2 µm) and medium (3.5 ± 1.1 µm) sized droplets. The number of events was lower than expected in phantoms with small droplets (2.0 ± 0.7 µm), but still within the same order of magnitude. The inter-phantom variability was considerably larger (up to 30x) than predicted by the model. The validated model was then combined with Monte Carlo simulations, which predicted a theoretical range retrieval precision improving with the square-root of the number of vaporizations, and degrading at high beam energies due to range straggling. For single pencil beams with energies between 70 and 240 MeV, a range verification precision below 1% of the range required perfluorocarbon concentrations in the order of 0.3-2.4 µM.

CONCLUSION

We proposed and experimentally validated a model to provide a quick estimate of the number of vaporizations for a given set of conditions (droplet size, droplet concentration, and proton beam parameters). From this model, promising range verification performances were predicted for realistic perfluorocarbon concentrations. These findings are an incentive to move towards preclinical studies, which are critical to assess the achievable droplet distribution in and around the tumor, and hence the in vivo range verification precision.

Photoacoustic Vaporization of Endoskeletal Droplets Loaded with Zinc Naphthalocyanine

Vaporizable endoskeletal droplets are solid hydrocarbons in liquid fluorocarbon droplets in which melting of the hydrocarbon phase leads to the vaporization of the fluorocarbon phase. In prior work, vaporization of the endoskeletal droplets was achieved thermally by heating the surrounding aqueous medium. In this work, we introduce a near-infrared (NIR) optically absorbing naphthalocyanine dye (zinc 2,11,20,29-tetra-tert-butyl-2,3-naphthalocynanine) into the solid hydrocarbon (eicosane, n-C₂₀H₄₂) core of liquid fluorocarbon (C₅F₁₂) drops suspended in an aqueous medium. Droplets with a uniform diameter of 11.7 ± 0.7 μm were formed using a flow-focusing microfluidic device. The solid hydrocarbon formed a crumpled spherical structure within the liquid fluorocarbon droplet. The photoactivation behavior of these dye-containing endoskeletal droplets was investigated using NIR laser irradiation. When exposed to a pulsed laser of 720 nm wavelength, the dye-containing droplets vaporized at an average laser fluence of 65 mJ/cm², whereas blank droplets without the dye did not vaporize at any fluence up to 100 mJ/cm². Furthermore, dye-loaded droplets with a smaller, polydisperse size distribution were prepared using a simple shaking method and studied in a flow phantom for their photoacoustic signal and ultrasound contrast imaging. These results demonstrate that dye-containing endoskeletal droplets can be made to vaporize by externally applied optical energy. Such droplets may be useful for a variety of photoacoustic applications for sensing, imaging, and therapy.