Radiosensitive Vaporizable Exoskeletal Droplets for Contrast-Enhanced X-ray Acoustic Computed Tomography (XACT)

Introduction: X-ray acoustic computed tomography (XACT) is a novel technique used to monitor the pathway of radiation delivered to patients during radiation therapy. The technique employs ultrasound to map the acoustic signals generated from localized rises in temperature to recreate a dose distribution in real time [1]. To increase the signal-to-noise ratio, superheated phase-change droplets have been proposed as contrast agents for XACT [2]. Upon exposure to photons, the droplets vaporize into echogenic microbubbles, increasing the emitted acoustic signal and allowing subsequent contrast-enhanced ultrasound imaging. Due to the high level of superheat required for photon-induced vaporization, previous work of radiosensitive droplets have required heating beyond body temperature (65 °C) or acoustic modulation to achieve vaporization [2], [3]. Furthermore, these droplets were made of shorter chain perfluorocarbon (n-C4F10), which have low colloidal stability (high fugacity) at body temperature.
Vaporizable exoskeletal droplets (VEDs) potentially offer a unique solution to the high levels of superheat required. These droplets comprise a solid hydrocarbon (HC) particle encapsulating a colloidally stable liquid FC (n-C5F12) (Figure 1a). Upon exposure to heat, the HC melts and intermixes with the FC, interrupting the intermolecular cohesion of the FC molecules, causing vaporization [4]. By selection of an appropriate HC, VEDs can be tuned to vaporize near body temperature. In this study, we designed novel, radiosensitive VEDs enhanced with gold nanoparticles (GNPs). We hypothesized that the high-Z nature of the GNPs would enhance absorption of photons delivered during radiation therapy, cause localized heating and melting of the HC, and vaporize the droplets.

Materials and

Methods: Organic gold nanoparticles, (GNPs) 1.8 nm in diameter and labeled with FITC were obtained from Nanopartz Inc (Loveland, CO). The VEDs, with and without GNPs, were fabricated using n-C20H42 as the HC and n-C5F12 as the FC, following a shaking approach [4]. The resulting droplets were measured with a Multisizer 3 Coulter Counter. Fluorescent images were obtained to confirm the presence of GNPs within the VEDs. The VEDs were heat treated following a procedure from Shakya et al [5]. The thermal vaporization was determined through optical heating experiments, where the VEDs were heated from room temperature to 45 °C, using a microscope heating stage. The temperature at which individual droplets vaporized was recorded and the data were fit to a cumulative normal distribution curve. The mean temperature of the normal distribution was used as the thermal vaporization temperature.
To test the radiosensitivity of the droplets, the droplets were monitored using a small, inverted microscope while radiation was delivered using a clinical linear accelerator (LINAC) (Figure 2). Prior to treatment, the droplets were heated to near their thermal vaporization temperature and allowed to equilibrate for 10 min. This limited the thermal vaporization during radiation exposure. The samples were irradiated with a 5 x 5 cm field size using a 10X flattening filter free (FFF) beam at a dose rate of 2400 MU/min. The bubbles formed during the irradiation were recorded and plotted against the cumulative dose delivered. This procedure was performed at temperatures 31-34 °C for all VED samples.

Results, Conclusions, and Discussions: The size distribution for the VEDs is shown in Figure 3a-b, showing the polydispersity of the droplet population. No changes in the size distribution due to the presence of the GNPs was seen. A representative image of a VED with FITC-labeled GNPs is shown in Figure 1b, confirming the presence of the GNPs within the droplet. Thermal vaporization data with fitted cumulative normal distribution curves are shown in Figure 3c. Vaporization temperatures were 31.8 ± 0.9 °C and 33.8 ± 0.8 °C for VEDs with and without GNPs (n = 9), respectively.
Figure 4 shows vaporization due to radiation treatment at different temperatures, between 31-34 °C. Dose thresholds that produced 50% vaporization were 6827, 5003, 4567, and 3668 cGy for 31, 32, 33, and 34 °C, respectively. For 10% vaporization, the measured thresholds were 2341, 930, 754, and 413 cGy (Figure 5). This result indicates a decreasing radiation threshold for vaporization as temperature of the sample increases. The vaporization of the droplets in response to radiation treatment supported our hypothesis, and we conclude we have successfully created radiosensitive droplets.

Acknowledgements (Optional): [1] P. Samant, L. Trevisi, X. Ji, and L. Xiang, “X-ray induced acoustic computed tomography,” Photoacoustics, vol. 19, p. 100177, Sep. 2020, doi: 10.1016/j.pacs.2020.100177.
[2] Y. Toumia et al., “Ultrasound-assisted investigation of photon triggered vaporization of poly(vinylalcohol) phase-change nanodroplets: A preliminary concept study with dosimetry perspective,” Physica Medica, vol. 89, pp. 232–242, Sep. 2021, doi: 10.1016/j.ejmp.2021.08.006.
[3] S. V. Heymans et al., “Modulating ultrasound contrast generation from injectable nanodroplets for proton range verification by varying the degree of superheat,” Medical Physics, vol. 48, no. 4, pp. 1983–1995, Apr. 2021, doi: 10.1002/mp.14778.
[4] G. Shakya, S. E. Hoff, S. Wang, H. Heinz, X. Ding, and M. A. Borden, “Vaporizable endoskeletal droplets via tunable interfacial melting transitions,” Sci. Adv., vol. 6, no. 14, p. eaaz7188, Apr. 2020, doi: 10.1126/sciadv.aaz7188.
[5] G. Shakya, A. K. Fajrial, X. Ding, and M. A. Borden, “Effect of Thermal History and Hydrocarbon Core Size on Perfluorocarbon Endoskeletal Droplet Vaporization,” Langmuir, vol. 38, no. 8, pp. 2634–2641, Mar. 2022, doi: 10.1021/acs.langmuir.1c03350.