Histotripsy Dose Impacts Treated Tumor Immune Infiltration and Survival Outcomes in a Murine B16F10 Melanoma Model

<b>Background/Objectives:</b> Preclinical animal studies and clinical case reports have shown evidence of histotripsy being capable of inducing anti-tumor immune responses strong enough to inhibit tumor growth of off-target tumors. Previous studies exploring histotripsy immune stimulation have used a single therapy dose. This study aims to explore how histotripsy dose affects local tumor immune stimulation in a murine melanoma model. <b>Methods:</b> C57BL/6J mice bearing subcutaneous B16F10 tumors were treated with histotripsy using an ultrasound-guided 8-element 1 MHz transducer operating at a 100 Hz pulse repetition frequency (PRF) and >30 MPa peak-negative pressure. The histotripsy dose was defined by the number of pulses (8, 20, 40, or 100) per focal location (ppl). Tissue damage and residual tissue structure were measured histologically and scored by a trained pathologist. The longitudinal effect of histotripsy dosing was assessed using tumor growth and survival. Acute immune stimulation was measured at days 2 and 7 post-treatment via immunofluorescence staining of the treated tumor. <b>Results:</b> Histotripsy doses at 20, 40, and 100 ppl achieved significant tumor necrosis within the target region (>75%), with residual structure decreasing as the dose increased. Overall, the greatest tumor control was observed in mice that received the 40 ppl dose compared to untreated mice. This correlates with the 40 ppl dose also having the largest increase in CD45+ immune cells and CD8⁺ T cells 7 days post-treatment compared to untreated mice. <b>Conclusions:</b> The effect of histotripsy dosing on immune infiltration and tumor growth highlights the significant impact of histotripsy dose on clinical effects.