Objective : Histotripsy is a non-invasive, non-ionizing, non-thermal ultrasonic ablation procedure that mechanically fractionates tissue through the precise control of acoustic cavitation guided by real-time ultrasound imaging (US). Current liver cancer ablation methods are primarily thermal or arterial-based therapies, which can result in inconsistent tissue ablation and collateral damage. Our previous studies have demonstrated the potential of histotripsy for highly precise and effective tumor ablation in in vivo subcutaneous HCC rodent models. This study evaluates the feasibility and tumor volume reduction effects of histotripsy for liver cancer ablation in an orthotopic, immune-competent in vivo rat HCC model.
Methods : Orthotopic liver tumors were generated by injecting 2-4 million N1S1 cells (rat HCC cell-line) into the left liver lobes of 21 Sprague Dawley rats. The rats were categorized into two groups: Controls (n=6) received no treatment and treatment rats (n=15) received histotripsy. The treatment animals received either: 1) complete tumor treatment with an additional 2 mm margin (n=9) and partial, 50-75% tumor treatment with intentionally untreated residual tumor (n=6). Once mean tumor diameter reached at least 5mm, histotripsy was performed using a custom built 1 MHz histotripsy transducer attached to a motorized positioning system and guided by an US imaging system. 1-2 cycle histotripsy pulses at 100 Hz PRF (p >30 MPa) were delivered to the focus which traversed the user-defined target volume. Visualization of the histotripsy-generated cavitation cloud at the targeted location provided real-time treatment feedback. T2-weighted MR images were obtained pre and post treatment to assess tumor ablation. Treatment and control groups were monitored weekly using MRI for 3 months or until tumors reached ~2.5cm. Post euthanasia, tumor and treated tissue were harvested for histology.
Results : Local tumor shrinkage was observed in 14/15 treatment rats (93.3%). Within 7-10 weeks post-histotripsy, MRI in 14/15 treated tumors (both partial and complete treatment) demonstrated near complete resorption of the ablated tumor with only a small residual T2 hypointense nodular area within the treatment bed, in comparison to the pre-treatment hyperintense signal within the tumor (Figure 1). Corresponding, gross morphology of the treated liver harvested at 3 months in all 14 rats showed shrunken fibrous residual tissue at the ablation site with no evidence of tumor (Figure 2a). In 1 treatment rat and 3/6 control rats, there was a large tumor burden at 3 weeks which required euthanization. In the remaining 3/6 control rats, there was nodular and diffuse tumor within the liver seen on gross evaluation at 3 months (Figure 2b). There were no complications post-histotripsy, and no evidence of parenchymal changes such as bile duct injury, vascular injury or parenchymal scarring in the adjacent tissue or along the ultrasound path. 93.3% of the cases had no evidence of local tumor progression at the ablation site or elsewhere in the liver.
Conclusions : This study demonstrates the potential of histotripsy for non-invasive HCC tumor ablation with a high safety profile and relatively low risk of local tumor progression in an animal model. Histotripsy achieved complete tumor disappearance in 93.3% of rats, even including the partial treatment cases. Research is ongoing to further evaluate the possible abscopal effect on metastatic tumor post-histotripsy.