Our joint study with the MSU University Clinic, Sechenov University, and Davydovsky City Hospital is the first to demonstrate the feasibility of non-invasive mechanical fractionation of liver metastases ex vivo using boiling histotripsy under ultrasound guidance. The results have been published in a top-rated journal Ultrasound in Medicine and Biology (Q1, IF 2.9) in the paper titled “Boiling histotripsy ex vivo in human liver metastases of various origin“.
Liver metastasis (secondary malignant neoplasms) is one of the most common causes of death of patients with cancer. Liver metastases occur in 25% of all cancer patients and in 50% of patients with colorectal cancer. Currently, the standard of care is surgical resection; however, it is available to less than 20% of patients. Systemic therapy is often limited in efficacy and associated with significant toxicity, while existing thermal ablation techniques are unreliable under ultrasound guidance and are affected by heat diffusion and perfusion near large vessels.
The concept of boiling histotripsy (BH) is based on focusing periodic sequences of high-amplitude millisecond-long ultrasound pulses from a transducer located outside the patient’s body, through the skin and healthy tissue onto the target area. Fractionation occurs not through heating but mechanically – due to the impact of nonlinear acoustic waves with shock fronts on the tissue. This approach allows the remote disintegration of unwanted tissue into a structureless suspension without damaging surrounding healthy structures, and the entire process can be monitored using conventional diagnostic ultrasound.
In our study, we used 18 specimens of liver metastases from five patients with various primary tumors (gastric, colon, breast cancer, and skin melanoma). Some patients had previously received courses of anti-tumor therapy. Before BH treatment, we measured the stiffness of the metastases using shear wave elastography, as tissue susceptibility to mechanical fractionation depends on its elastic properties. Then, under real-time ultrasound guidance, volumetric fractionation was performed in each specimen and analyzed at the macroscopic level (with gross bisection), microscopic level (with histological analysis with various stains), and ultrastructural level (with transmission and scanning electron microscopy).
ultrasound image during mechanical fractionation (histotripsy) of a liver metastasis;
histological specimen of a liver metastasis after volumetric fractionation within the tumor tissue; magnified view of the fractionation boundary on the histological specimen (left – intact tumor tissue, right – fractionated tissue);
electron microscopy image of a tumor cell nucleus damaged after ultrasound exposure.
The results showed:
- Metastases are significantly stiffer than healthy liver tissue, which is consistent with clinical in vivo data and confirms the relevance of our ex vivo model.
- The key factor determining resistance to BH was found to be not so much the overall stiffness but the internal structure of the tumor.
- In metastases with necrotic areas (spontaneous or after prior anti-tumor therapy), fractionation was rapid and efficient, whereas fibrotic metastases (after prior radiation therapy) required significantly more pulses, as dense collagenous tissue is more resistant to mechanical disruption.
- Electron microscopy confirmed that metastases are disintegrated down to the level of individual cellular organelles and collagen fibers.
(left) metastasis with spontaneous necrosis without prior treatment – complete homogenization of the target volume;
(center and right) metastases with significant fibrosis (blue areas) induced by prior radiation therapy – (center) selective homogenization of non-fibrotic tumor components at 100 pulses/point, (right) complete homogenization of the target volume including fibrosis at 200 pulses/point.
Our findings are the first to demonstrate that boiling histotripsy can be effective for various types of liver metastases, with tumor structure being the critical factor. Necrotic tumors can be fractionated faster with fewer pulses, while fibrotic tumors require higher doses for complete fractionation, although lower doses selectively destroy non-fibrotic components.
These results enable the personalized treatment planning: depending on the therapy-induced changes in the tumor, BH parameters can be adapted to achieve maximum efficacy in minimal time. Successful clinical translation of this method in the future will allow non-invasive treatment of liver metastases in a larger number of patients, reducing risks through non-invasive approach, as well as procedure costs through ultrasound guidance instead of MRI.
For more details – see the full text of the paper.
