| Summary: | Objective and Impact Statement. This is the first longitudinal study investigating the effects of histotripsy on local tumor progression in an in vivo orthotopic, immunocompetent rat hepatocellular carcinoma (HCC) model. Introduction. Histotripsy is the first noninvasive, nonionizing, nonthermal, mechanical ablation technique using ultrasound to generate acoustic cavitation to liquefy the target tissue into acellular debris with millimeter accuracy. Previously, histotripsy has demonstrated in vivo ablation of noncancerous liver tissue. Methods. N1-S1 HCC tumors were generated in the livers of immunocompetent rats (n=6, control; n=15, treatment). Real-time ultrasound-guided histotripsy was applied to ablate either 100% tumor volume+up to 2 mm margin (n=9, complete treatment) or 50-75% tumor volume (n=6, partial treatment) by delivering 1-2 cycle histotripsy pulses at 100 Hz PRF (pulse repetition frequency) with p−≥30 MPa using a custom 1 MHz transducer. Rats were monitored weekly using MRI (magnetic resonance imaging) for 3 months or until tumors reached ~25 mm. Results. MRI revealed effective post-histotripsy reduction of tumor burden with near-complete resorption of the ablated tumor in 14/15 (93.3%) treated rats. Histopathology showed <5 mm shrunken, non-tumoral, fibrous tissue at the treatment site at 3 months. Rats with increased tumor burden (3/6 control and 1 partial treatment) were euthanized early by 2-4 weeks. In 3 other controls, histology revealed fibrous tissue at original tumor site at 3 months. There was no evidence of histotripsy-induced off-target tissue injury. Conclusion. Complete and partial histotripsy ablation resulted in effective tumor removal for 14/15 rats, with no evidence of local tumor progression or recurrence.
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