Impact of Histotripsy on Development of Intrahepatic Metastases in a Rodent Liver Tumor Model

SIMPLE SUMMARY: Histotripsy is a novel technique that mechanically disrupts tumors, through precisely controlled acoustic cavitation. There is insufficient evidence regarding the effects of histotripsy on the risk of recurrence and metastases, following tumor debulking. The aim of this study is to evaluate the effect of partial histotripsy tumor ablation (~50–75% tumor volume targeted) on untargeted tumor progression, survival outcomes, risk of metastases and immune infiltration, in an orthotopic, immunocompetent, metastatic rodent hepatocellular carcinoma (HCC) model. Even with partial ablation, complete local tumor regression was observed in 81% of treatment rats, with no recurrence or metastasis. In contrast, 100% of the untreated control animals showed local tumor progression and intrahepatic metastases. Histotripsy-treated animals had statistically significant improved survival outcomes compared to controls (p-value < 0.0001). Histotripsy-treated animals had increased immune infiltration, as compared to controls, which may have contributed to the eventual regression of the untargeted tumor region in partial histotripsy-treated tumors. ABSTRACT: Histotripsy has been used for tumor ablation, through controlled, non-invasive acoustic cavitation. This is the first study to evaluate the impact of partial histotripsy ablation on immune infiltration, survival outcomes, and metastasis development, in an in vivo orthotopic, immunocompetent rat HCC model (McA-RH7777). At 7–9 days post-tumor inoculation, the tumor grew to 5–10 mm, and ~50–75% tumor volume was treated by ultrasound-guided histotripsy, by delivering 1–2 cycle histotripsy pulses at 100 Hz PRF (focal peak negative pressure P– >30 MPa), using a custom 1 MHz transducer. Complete local tumor regression was observed on MRI in 9/11 histotripsy-treated rats, with no local recurrence or metastasis up to the 12-week study end point, and only a <1 mm residual scar tissue observed on histology. In comparison, 100% of untreated control animals demonstrated local tumor progression, developed intrahepatic metastases, and were euthanized at 1–3 weeks. Survival outcomes in histotripsy-treated animals were significantly improved compared to controls (p-value < 0.0001). There was evidence of potentially epithelial-to-mesenchymal transition (EMT) in control tumor and tissue healing in histotripsy-treated tumors. At 2- and 7-days post-histotripsy, increased immune infiltration of CD11b(+), CD8(+) and NK cells was observed, as compared to controls, which may have contributed to the eventual regression of the untargeted tumor region in histotripsy-treated tumors.