Boiling Histotripsy Ex Vivo in Human Liver Metastases of Various Origin

Given the high incidence rate of secondary liver tumors and limitations of the existing treatment options, boiling histotripsy (BH) may provide an incisionless, non-ionizing, ultrasound-guided approach for the mechanical ablation of liver metastases using pulsed high-intensity focused ultrasound waves with shock fronts. This pilot ex vivo study investigated the feasibility of BH for mechanical disintegration of human liver metastases with respect to tumor origin, stiffness, and prior anti-tumor treatment.

Eighteen human liver metastases were collected via surgical resection or rapid autopsy from anonymized patients aged 39-85 y (median: 68) with confirmed colorectal, gastric, breast, or skin (cutaneous melanoma) cancers, with or without prior targeted therapy or radiation therapy. The Young's moduli of the tumors and adjacent liver tissue were measured using shear wave elastography. BH was then applied to nodes of a volumetric grid (1-4 layers, 3 × 3 - 5 × 5 points per layer, and 1-mm spacing) at 25-200 pulses per point (1-ms pulses and 1% duty cycle) using a 9-ring 2-MHz annular array under real-time ultrasound guidance. Histological analysis of the treatment outcomes was performed with H&E and Masson's trichrome stains; scanning and transmission electron microscopy were employed for ultrastructural analysis of the fragmented tissue.

Liver metastases were significantly stiffer than the adjacent liver parenchyma (44 ± 17 kPa vs. 9.3 ± 3.9 kPa; p < 9 × 10^-5) regardless of tumor origin; these findings are consistent with clinically relevant Young's modulus values reported in prior in vivo studies. In metastases with spontaneous or prior therapy-induced necrosis, all investigated BH protocols successfully disintegrated the planned volumes; the highest treatment rate of 54 mm³/min was achieved using 75 pulses per point. In metastases with therapy-induced fibrosis, complete disintegration of target volumes was achieved only at 200 pulses per point (treatment rate: 14 mm³/min), whereas lower pulse numbers resulted in the selective fragmentation of non-fibrotic tumor components. Electron microscopy confirmed mechanical disintegration of the tumor tissue down to the subcellular level.

BH enables non-invasive mechanical ablation of human liver metastases across multiple primary origins. The treatment process can be accelerated by reducing pulse numbers for non-fibrotic or necrotic tumors; however, fibrotic tumors require higher number of BH pulses for complete disintegration. These findings support the use of BH as a potential modality for managing liver metastases, whether applied as a standalone procedure, a neoadjuvant strategy, or following anti-tumor therapy to ensure complete removal of the residual tumor burden.