AUTHORS: Dana Wang (2), Victor A. Lopez (1), Jessica Gannon (1), Sheryl Coutermarsh-Ott (3), Jeremy A. Brown (4), Adam Maxwell (1), Joanne Tuohy (5), Eli Vlaisavljevich (1)

(1) Biomedical Engineering and Mechanics, Virginia Tech (2) Virginia Tech Carilion School of Medicine, Virginia Tech (3) Biological Sciences and Pathobiology, Virginia-Maryland College of Veterinary Medicine (4) School of Biomedical Engineering, Dalhousie University (5) Department of Small Animal Clinical Sciences, Virginia-Maryland College of Veterinary Medicine

BACKGROUND: Oral tumors are frequently diagnosed in both humans and dogs and have potential for malignancy. Current standard of care treatments like surgery, radiation, and chemotherapy are invasive, often leading to facial disfigurements and other adverse effects that decrease quality of life. There is a pressing need for less invasive treatments that preserve oral function and appearance.

Histotripsy is a non-invasive, non-thermal, and non-ionizing image-guided focused ultrasound technique. It utilizes acoustic cavitation to break down tissue into acellular debris by producing high-pressure ultrasound pulses that converge at the transducer focus to generate a cavitation bubble cloud for mechanical tissue ablation. While current clinical histotripsy systems are large, cart-based devices operating between 500kHz to 1MHz for deep tissue treatments like the liver, this study aims to develop miniaturized high-frequency histotripsy transducers for targeting smaller, superficial structures like oral tumors.

METHODS: CT images of canines with oral tumors were segmented and analyzed in MATLAB to identify accessible acoustic windows with histotripsy treatment. A ray-sphere intersection approach was used to guide transducer designs based on predicted acoustic windows. 2MHz and 6.3MHz single-element transducers were developed and tested on excised canine oral tumors, producing bubble cloud diameters of approximately 2mm and 0.25mm, respectively. Excised canine oral tumors were degassed, embedded in 1% agarose gel, and underwent treatment with both transducers using a pulse repetition frequency of 1kHz and 2,000 pulses per point. Real-time ultrasound imaging was used for treatment monitoring and post-treatment ablation was quantified histologically with hematoxylin and eosin stain.

RESULTS: CT acoustic window analysis produced a matrix showing where ultrasound pulses could reach the tumor unimpeded by the skull. This visual representation illustrated the spatial relationship between the acoustic window, skull, and oral tumor. During ex-vivo treatments, real-time ultrasound imaging indicated that both 2MHz and 6.3MHz transducers generated well-confined bubble clouds. After treatment, hypoechoic regions observed within the tissue suggested tumor ablation and liquefaction. Histological analysis confirmed complete, delineated ablation and decellularization of the tissue within the treated area. The 2MHz transducer demonstrated faster ablation rates, approximately 7.4 times quicker than the 6.3MHz transducer.

CONCLUSIONS: Preliminary acoustic window analysis informed optimal transducer positioning for targeting oral tumors effectively with a smaller transducer system. Both the 2MHz and 6.3MHz transducers successfully ablated excised canine oral tumors. Overall, this study shows the feasibility of ablating oral tumors with histotripsy for the first time and provides insight on optimizing histotripsy devices for oral tumor treatment.