Perfluorobutane Nanodroplets for the Selective Sensing and Range Verification of Carbon-Ion Radiotherapy: In Vitro Evaluation on Cells

Range verification procedures are crucial in carbon-ion radiotherapy (CIRT) to avoid undesired exposure while harnessing the superior features in dose-depth distribution that allow for better biological effectiveness than other adopted radiotherapies. Recently, we have proposed an innovative approach to dosimetry and uncertainties measurement in CIRT by upgrading phase-change ultrasound contrast agents to function as injectable tracers for ionizing particle beams. We developed a stable formulation of superheated nanodroplets (NDs) made of a liquid perfluorobutane core encapsulated within a cross-linked poly(vinyl alcohol) shell. The core vaporization is triggered by the high-linear energy transfer (LET) carbon ions at the Bragg peak, generating echogenic microbubbles, whereby the resulting echo-contrast relates to beam range with submillimeter precision. So far, in vitro experiments revealed the dependence of the NDs’ response to carbon-ion radiation on their size, concentration, as well as particle fluence, which is associated with CIRT dose and beam energy. In this study, we further investigate the biological effect of carbon-ion radiation, before and after the Bragg peak, in 2D cell layer system and correlate this effect with the NDs’ radiation response. We evaluated the interaction of nanodroplets of various sizes with the V-79 cell line, a well-established radiobiological model. Tissue-mimicking phantoms containing NDs, along with untreated cells and cells treated with size-sorted NDs, were exposed to C-ion doses of 4 and 8 Gy (130 mm, E = 257.5 MeV/u) at 37 °C. Results indicated that nanodroplets show affinity for the cells and good biocompatibility after incubation. Ultrasound image processing of the vaporization range of nanodroplets, triggered by the carbon-ion radiation, reveals a strong correlation with the cell mortality at the Bragg peak, as assessed by Crystal Violet staining. Our findings highlight the potential of nanodroplets as an injectable and noninvasive dosimeter from the perspective of real-time range verification, offering the ability to optimize treatment plans.