In vivo MR guided boiling histotripsy in a mouse tumor model evaluated by MRI and histopathology

Insights from in vivo preclinical cancer studies with histotripsy

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technique that mechanically fractionates target tissue into acellular homogenate via controlled acoustic cavitation. Histotripsy has been evaluated for various preclinical applications requiring noninvasive tissue removal including cancer, brain surgery, blood clot and hematoma liquefaction, and correction of neonatal congenital heart defects. Promising preclinical results including local tumor suppression, improved survival outcomes, local and systemic anti-tumor immune responses, and histotripsy-induced abscopal effects have been reported in various animal tumor models. Histotripsy is also being investigated in veterinary patients with spontaneously arising tumors. Research is underway to combine histotripsy with immunotherapy and chemotherapy to improve therapeutic outcomes. In addition to preclinical cancer research, human clinical trials are ongoing for the treatment of liver tumors and renal tumors. Histotripsy has been recently approved by the FDA for noninvasive treatment of liver tumors. This review highlights key learnings from in vivo shock-scattering histotripsy, intrinsic threshold histotripsy, and boiling histotripsy cancer studies treating cancers of different anatomic locations and discusses the major considerations in planning in vivo histotripsy studies regarding instrumentation, tumor model, study design, treatment dose, and post-treatment tumor monitoring.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Mechanical high-intensity focused ultrasound creates unique tumor debris enhancing dendritic cell-induced T cell activation

Introduction

In situ tumor ablation releases a unique repertoire of antigens from a heterogeneous population of tumor cells. High-intensity focused ultrasound (HIFU) is a completely noninvasive ablation therapy that can be used to ablate tumors either by heating (thermal (T)-HIFU) or by mechanical disruption (mechanical (M)-HIFU). How different HIFU ablation techniques compare with respect to their antigen release profile, their activation of responder T cells, and their ability to synergize with immune stimuli remains to be elucidated.

Methods and results

Here, we compare the immunomodulatory effects of T-HIFU and M-HIFU ablation with or without the TLR9 agonist CpG in the ovalbumin-expressing lymphoma model EG7. M-HIFU ablation alone, but much less so T-HIFU, significantly increased dendritic cell (DC) activation in draining lymph nodes (LNs). Administration of CpG following T- or M-HIFU ablation increased DC activation in draining LNs to a similar extend. Interestingly, ex vivo co-cultures of draining LN suspensions from HIFU plus CpG treated mice with CD8⁺ OT-I T cells demonstrate that LN cells from M-HIFU treated mice most potently induced OT-I proliferation. To delineate the mechanism for the enhanced anti-tumor immune response induced by M-HIFU, we characterized the RNA, DNA and protein content of tumor debris generated by both HIFU methods. M-HIFU induced a uniquely altered RNA, DNA and protein profile, all showing clear signs of fragmentation, whereas T-HIFU did not. Moreover, western blot analysis showed decreased levels of the immunosuppressive cytokines IL-10 and TGF-β in M-HIFU generated tumor debris compared to untreated tumor tissue or T-HIFU.

Conclusion

Collectively, these results imply that M-HIFU induces a unique context of the ablated tumor material, enhancing DC-mediated T cell responses when combined with CpG.

Quantitative Assessment of Boiling Histotripsy Progression Based on Color Doppler Measurements

Boiling histotripsy (BH) is a mechanical tissue liquefaction method that uses sequences of millisecond-long high intensity focused ultrasound (HIFU) pulses with shock fronts. The BH treatment generates bubbles that move within the sonicated volume due to acoustic radiation force. Since the velocity of the bubbles and tissue debris is expected to depend on the lesion size and liquefaction completeness, it could provide a quantitative metric of the treatment progression. In this study, the motion of bubble remnants and tissue debris immediately following BH pulses was investigated using high-pulse repetition frequency (PRF) plane-wave color Doppler ultrasound in ex vivo myocardium tissue. A 256-element 1.5 MHz spiral HIFU array with a coaxially integrated ultrasound imaging probe (ATL P4-2) produced 10 ms BH pulses to form volumetric lesions with electronic beam steering. Prior to performing volumetric BH treatments, the motion of intact myocardium tissue and anticoagulated bovine blood following isolated BH pulses was assessed as two limiting cases. In the liquid blood the velocity of BH-induced streaming at the focus reached over 200 cm/s, whereas the intact tissue was observed to move toward the HIFU array consistent with elastic rebound of tissue. Over the course of volumetric BH treatments tissue motion at the focus locations was dependent on the axial size of the forming lesion relative to the corresponding size of the HIFU focal area. For axially small lesions, the maximum velocity after the BH pulse was directed toward the HIFU transducer and monotonically increased over time from about 20-100 cm/s as liquefaction progressed, then saturated when tissue was fully liquefied. For larger lesions obtained by merging multiple smaller lesions in the axial direction, the high-speed streaming away from the HIFU transducer was observed at the point of full liquefaction. Based on these observations, the maximum directional velocity and its location along the HIFU propagation axis were proposed and evaluated as candidate metrics of BH treatment completeness.

The role of anti-tumor immunity of focused ultrasound for the malignancies: depended on the different ablation categories

Improving anti-tumor immunity has promising outcomes in eradicating malignant tumors. Tumor cells can escape from immune surveillance and killing; therefore, various strategies are continuously developing to inhibit immune escape. Focused ultrasound (FUS) has recently emerged to play an important role in immune modulation. After FUS therapy, various tumor antigens and related signals are released. The non-thermal effect of FUS strengthens the blood and lymph circulation, increases cell permeability, and helps in crossing the physical barrier like the blood-brain barrier and blood-tumor barrier. However, the different ablation of FUS is proposed to have a different anti-tumor immune effect. Therefore, we categorized the FUS ablation into thermal and non-thermal ablation and summarized possible anti-tumor immunity mechanisms.

A Review of Imaging Methods to Assess Ultrasound-Mediated Ablation

Ultrasound ablation techniques are minimally invasive alternatives to surgical resection and have rapidly increased in use. The response of tissue to HIFU ablation differs based on the relative contributions of thermal and mechanical effects, which can be varied to achieve optimal ablation parameters for a given tissue type and location. In tumor ablation, similar to surgical resection, it is desirable to include a safety margin of ablated tissue around the entirety of the tumor. A factor in optimizing ablative techniques is minimizing the recurrence rate, which can be due to incomplete ablation of the target tissue. Further, combining focal ablation with immunotherapy is likely to be key for effective treatment of metastatic cancer, and therefore characterizing the impact of ablation on the tumor microenvironment will be important. Thus, visualization and quantification of the extent of ablation is an integral component of ablative procedures. The aim of this review article is to describe the radiological findings after ultrasound ablation across multiple imaging modalities. This review presents readers with a general overview of the current and emerging imaging methods to assess the efficacy of ultrasound ablative treatments.

Interventional Radiology Image-Guided Locoregional Therapies (LRTs) and Immunotherapy for the Treatment of HCC

Image-guided locoregional therapies (LRTs) are a crucial asset in the treatment of hepatocellular carcinoma (HCC), which has proven to be characterized by an impaired antitumor immune status. LRTs not only directly destroy tumor cells but also have an immunomodulating role, altering the tumor microenvironment with potential systemic effects. Nevertheless, the immune activation against HCC induced by LRTs is not strong enough on its own to generate a systemic significant antitumor response, and it is incapable of preventing tumor recurrence. Currently, there is great interest in the possibility of combining LRTs with immunotherapy for HCC, as this combination may result in a mutually beneficial and synergistic relationship. On the one hand, immunotherapy could amplify and prolong the antitumoral immune response of LRTs, reducing recurrence cases and improving outcome. On the other hand, LTRs counteract the typical immunosuppressive HCC microenvironment and status and could therefore enhance the efficacy of immunotherapy. Here, after reviewing the current therapeutic options for HCC, we focus on LRTs, describing for each of them the technique and data on its effect on the immune system. Then, we describe the current status of immunotherapy and finally report the recently published and ongoing clinical studies testing this combination.

Immune Modulation Plus Tumor Ablation: Adjuvants and Antibodies to Prime and Boost Anti-Tumor Immunity In Situ

In situ tumor ablation techniques, like radiotherapy, cryo- and heat-based thermal ablation are successfully applied in oncology for local destruction of tumor masses. Although diverse in technology and mechanism of inducing cell death, ablative techniques share one key feature: they generate tumor debris which remains in situ. This tumor debris functions as an unbiased source of tumor antigens available to the immune system and has led to the concept of in situ cancer vaccination. Most studies, however, report generally modest tumor-directed immune responses following local tumor ablation as stand-alone treatment. Tumors have evolved mechanisms to create an immunosuppressive tumor microenvironment (TME), parts of which may admix with the antigen depot. Provision of immune stimuli, as well as approaches that counteract the immunosuppressive TME, have shown to be key to boost ablation-induced anti-tumor immunity. Recent advances in protein engineering have yielded novel multifunctional antibody formats. These multifunctional antibodies can provide a combination of distinct effector functions or allow for delivery of immunomodulators specifically to the relevant locations, thereby mitigating potential toxic side effects. This review provides an update on immune activation strategies that have been tested to act in concert with tumor debris to achieve in situ cancer vaccination. We further provide a rationale for multifunctional antibody formats to be applied together with in situ ablation to boost anti-tumor immunity for local and systemic tumor control.

Focused Ultrasound for Immunomodulation of the Tumor Microenvironment

Focused ultrasound (FUS) has recently emerged as a modulator of the tumor microenvironment, paving the way for FUS to become a safe yet formidable cancer treatment option. Several mechanisms have been proposed for the role of FUS in facilitating immune responses and overcoming drug delivery barriers. However, with the wide variety of FUS parameters used in diverse tumor types, it is challenging to pinpoint FUS specifications that may elicit the desired antitumor response. To clarify FUS bioeffects, we summarize four mechanisms of action, including thermal ablation, hyperthermia/thermal stress, mechanical perturbation, and histotripsy, each inducing unique vascular and immunological effects. Notable tumor responses to FUS include enhanced vascular permeability, increased T cell infiltration, and tumor growth suppression. In this review, we have categorized and reviewed recent methods of using therapeutic ultrasound to elicit an antitumor immune response with examples that reveal specific solutions and challenges in this new research area.

High-intensity focused ultrasound treatment as an alternative regimen for myxofibrosarcoma

Myxofibrosarcoma is a common soft-tissue sarcoma in elderly patients, characterized by an infiltrative growth pattern and a high risk for persistent local recurrence. A 35-years-old woman was diagnosed with myxofibrosarcoma on the right upper arm and the tumor is surgically resected. The tumor relapsed 7 months later. Then the patient received five cycles of low power cumulative high-intensity focused ultrasound (HIFU) treatments, which completely ablated the tumor without complications. Now the patient is disease free with a high quality of life more than 30 months. This case indicates HIFU ablation might be a novel, promising therapy for recurrent myxofibrosarcoma.

Immune modulation resulting from MR-guided high intensity focused ultrasound in a model of murine breast cancer

High intensity focused ultrasound (HIFU) rapidly and non-invasively destroys tumor tissue. Here, we sought to assess the immunomodulatory effects of MR-guided HIFU and its combination with the innate immune agonist CpG and checkpoint inhibitor anti-PD-1. Mice with multi-focal breast cancer underwent ablation with a parameter set designed to achieve mechanical disruption with minimal thermal dose or a protocol in which tumor temperature reached 65 °C. Mice received either HIFU alone or were primed with the toll-like receptor 9 agonist CpG and the checkpoint modulator anti-PD-1. Both mechanical HIFU and thermal ablation induced a potent inflammatory response with increased expression of Nlrp3, Jun, Mefv, Il6 and Il1β and alterations in macrophage polarization compared to control. Furthermore, HIFU upregulated multiple innate immune receptors and immune pathways, including Nod1, Nlrp3, Aim2, Ctsb, Tlr1/2/4/7/8/9, Oas2, and RhoA. The inflammatory response was largely sterile and consistent with wound-healing. Priming with CpG attenuated Il6 and Nlrp3 expression, further upregulated expression of Nod2, Oas2, RhoA, Pycard, Tlr1/2 and Il12, and enhanced T-cell number and activation while polarizing macrophages to an anti-tumor phenotype. The tumor-specific antigen, cytokines and cell debris liberated by HIFU enhance response to innate immune agonists.

Perspectives on Recent Progress in Focused Ultrasound Immunotherapy

Immunotherapy holds tremendous promise as a strategy for eradicating solid tumors. However, poor T cell infiltration and persistence within most solid tumor microenvironments, as well as mechanisms of adaptive resistance, continue to severely limit the accessibility of most immunotherapies to a broad patient population. This limitation perpetuates the demand for allied therapeutic strategies. Among such strategies is focused ultrasound (FUS), a non-invasive, non-ionizing technique for precisely targeted acoustic energy deposition into tissues. FUS has gained remarkable attention over recent years as a modality for elicitation of immune mechanisms in cancer and other pathologies. In 2017, we published a comprehensive review paper detailing existing evidence for immune modulation and therapy with FUS, as well as impending challenges and opportunities of consideration for the field. Over the last two years, a multitude of clinical trials have come online to explore safety, feasibility, and efficacy of FUS for cancers of the brain and periphery - including the first clinical trial to combine FUS with immunotherapy. Moreover, the last two years have seen a surge in FUS immunotherapy presentations at therapeutic ultrasound scientific meetings. Given the burst of activity in this field, we submit that an update on FUS immunotherapy progress is timely. In this review, we offer an updated overview and perspectives on scientific and clinical development in the FUS immunotherapy domain.

Assessment of Enhanced Thermal Effect Due to Gold Nanoparticles during MR-Guided High-Intensity Focused Ultrasound (HIFU) Procedures Using a Mouse-Tumor Model

An in vivo study was conducted using a mouse tumor model, to assess the utility of using gold nanoparticles (gNPs) during HIFU procedures to locally enhance heating at low powers. Tumors were grown using melanoma tumor cells (B16/F10) subcutaneously on the right flanks of mice (C57Bl/6J). Physiologically relevant concentrations (0 and 0.125%) of gNPs were directly injected into the tumors. Sonications at acoustic powers of 10 and 30 W were performed for a duration of 16 s inside a magnetic-resonance system. Temperature increases and lesion volumes were calculated and compared for procedures with and without gNPs. Histopathology study was conducted using a cleaved caspase 3 antibody and hematoxylin and eosin staining after removing the tumors from the mice. For an acoustic power of 30 W, end-of-sonication temperature increases of 25.4 ± 3.8 °C (0% gNP) and 42.2 ± 4.6 °C (0.125% gNP) were measured. Using cleaved caspase 3 antibody, it was observed that more than 1% of nuclei are affected in the case of 0.125% and 30 W but only 0.01% of nuclei are affected in the 0% case. For 30 W and a gNP concentration of 0.125%, a lesion volume of 0.33 ± 0.22 mm³ was obtained, while no lesion was observed without gNP's.

Boiling Histotripsy Ablation of Renal Cell Carcinoma in the Eker Rat Promotes a Systemic Inflammatory Response

Boiling histotripsy (BH) is an experimental focused ultrasound technique that produces non-thermal mechanical ablation. We evaluated the feasibility, short-term histologic effects and the resulting acute inflammatory response to BH ablation of renal cell carcinoma (RCC) in the Eker rat. Genotyped Eker rats were monitored for de novo RCCs with serial ultrasound (US) imaging. When tumors were ≥8 mm, rats underwent ultrasound-guided extracorporeal ablation of the tumor with BH, a pulsed focused US technique that produces non-thermal mechanical ablation of targeted tissues, or a sham US procedure. Treatments targeted approximately 50% of the largest RCC with a margin of normal kidney. BH treated rats were euthanized at 1 (n = 4) or 48 (n = 4) h, and sham patients (n = 4) at 48 h. Circulating plasma cytokine levels were assessed with multiplex assays before and at 0.25, 1, 4, 24 and 48 h following treatment. Kidneys were collected and processed for histologic assessment, immunohistochemistry and intrarenal cytokine concentration measurements. For statistical analysis Student's t-test was used. US-guided BH treatment was successful in all animals, producing hypoechoic regions within the targeted volume consistent with BH treatment effect. Grossly, regions of homogenized tissue were apparent with evidence of focal intra-parenchymal hemorrhage. Histologically, BH produced a sharply demarcated region of homogenized tumor and non-tumor tissue containing acellular debris. BH treatment was associated with significantly increased relative concentration of plasma TNF versus sham treatment (p < 0.05) and transient elevations in high-mobility group box 1 (HMGB1), IL-10 and IL-6 consistent with acute inflammatory response to trauma. Intrarenal cytokine concentrations followed the same trend. At 48 h, enhanced infiltration of CD8+ T cells was observed by immunohistochemistry in both the treated and un-treated contralateral RCC/kidneys in BH-treated animals versus sham treatment. BH treatment was well tolerated with transient gross hematuria and a perinephric hematoma developing in one subject each. The study demonstrates the feasibility of BH ablation of de novo RCC and suggests activation of the acute inflammatory cascade following treatment that appears to stimulate CD8+ T cell infiltration of both treated and untreated tumors. Longer duration chronic studies are ongoing to characterize the longevity and robustness of this response.

Coalescence of residual histotripsy cavitation nuclei using low-gain regions of the therapy beam during electronic focal steering

Following collapse of a histotripsy cloud, residual microbubbles may persist for seconds, distributed throughout the focus. Their presence can attenuate and scatter subsequent pulses, hindering treatment speed and homogeneity. Previous studies have demonstrated use of separate low-amplitude (~1 MPa) pulses interleaved with histotripsy pulses to drive bubble coalescence (BC), significantly improving treatment speed without sacrificing homogeneity. We propose that by using electronic focal steering (EFS) to direct the therapy focus throughout specially-designed EFS sequences, it is possible to use low-gain regions of the therapy beam to accomplish BC during EFS without any additional acoustic sequence. First, to establish proof of principle for an isolated focus, a 50-foci EFS sequence was constructed with the first position isolated near the geometric focus and remaining positions distributed post-focally. EFS sequences were evaluated in tissue-mimicking phantoms with gas concentrations of 20% and 100% with respect to saturation. Results using an isolated focus demonstrated that at 20% gas concentration, 49 EFS pulses were sufficient to achieve BC in all samples for pulse repetition frequency (PRF)  ⩽  800 Hz and 84.1%  ±  3.0% of samples at 5 kHz PRF. For phantoms prepared with 100% gas concentration, BC was achieved by 49 EFS pulses in 39.2%  ±  4.7% of samples at 50 Hz PRF and 63.4%  ±  15.3% of samples at 5 kHz. To show feasibility of using the EFS-BC method to ablate a large volume quickly, a 1000-foci EFS sequence covering a volume of approximately 27 ml was tested. Results indicate that the BC effect was similarly present. A treatment rate of 27  ±  6 ml min^-1 was achieved, which is signficantly faster than standard histotripsy and ultrasound thermal ablation. This study demonstrates that histotripsy with EFS can achieve BC without employing a separate acoustic sequence which has the potential to accelerate large-volume ablation while minimizing energy deposition.

Focused ultrasound: tumour ablation and its potential to enhance immunological therapy to cancer

Various kinds of image-guided techniques have been successfully applied in the last years for the treatment of tumours, as alternative to surgical resection. High intensity focused ultrasound (HIFU) is a novel, totally non-invasive, image-guided technique that allows for achieving tissue destruction with the application of focused ultrasound at high intensity. This technique has been successfully applied for the treatment of a large variety of diseases, including oncological and non-oncological diseases. One of the most fascinating aspects of image-guided ablations, and particularly of HIFU, is the reported possibility of determining a sort of stimulation of the immune system, with an unexpected "systemic" response to treatments designed to be "local". In the present article the mechanisms of action of HIFU are described, and the main clinical applications of this technique are reported, with a particular focus on the immune-stimulation process that might originate from tumour ablations.

Reconstruction of nonlinear ultrasound field of an annular therapeutic array from acoustic holograms of its individual elements

Acoustic holography method has been shown to provide accurate reconstruction of 3D ultrasound fields generated by various medical transducers including multi-element arrays as well as to set a boundary condition for nonlinear field modeling at high pressure levels. Here an approach of measuring holograms of individual array elements for modeling of an entire array field is proposed and tested for a 3 MHz 16-element annular array (48 mm diameter and 35 mm radius of curvature). The array is a part of a high intensity focused ultrasound system with magnetic resonance guidance used for developing thermal and mechanical methods of tissue ablation in mouse tumors. The holograms measured separately for each array element were combined together to obtain a boundary condition for the array with all operating elements. Modeling results were compared to low-amplitude beam scans and good agreement was demonstrated. Then, nonlinear field simulations were performed at increasing power outputs based on the 3D Westervelt equation. It was shown that the transducer is capable to produce focal waveforms with 140 MPa shock amplitude at 110 W acoustic power and thus is well suited for evaluating shock-based ablation therapies in small animals.

Focused Ultrasound Immunotherapy for Central Nervous System Pathologies: Challenges and Opportunities

Immunotherapy is rapidly emerging as the cornerstone for the treatment of several forms of metastatic cancer, as well as for a host of other pathologies. Meanwhile, several new high-profile studies have uncovered remarkable linkages between the central nervous and immune systems. With these recent developments, harnessing the immune system for the treatment of brain pathologies is a promising strategy. Here, we contend that MR image-guided focused ultrasound (FUS) represents a noninvasive approach that will allow for favorable therapeutic immunomodulation in the setting of the central nervous system. One obstacle to effective immunotherapeutic drug delivery to the brain is the blood brain barrier (BBB), which refers to the specialized structure of brain capillaries that prevents transport of most therapeutics from the blood into brain tissue. When applied in the presence of circulating microbubbles, FUS can safely and transiently open the BBB to facilitate the delivery of immunotherapeutic agents into the brain parenchyma. Furthermore, it has been demonstrated that physical perturbations of the tissue microenvironment via FUS can modulate immune response in both normal and diseased tissue. In this review article, we provide an overview of FUS energy regimens and corresponding tissue bioeffects, followed by a review of the literature pertaining to FUS for therapeutic antibody delivery in normal brain and preclinical models of brain disease. We provide an overview of studies that demonstrate FUS-mediated immune modulation in both the brain and peripheral settings. Finally, we provide remarks on challenges facing FUS immunotherapy and opportunities for future expansion in this area.

The response of MRI contrast parameters in in vitro tissues and tissue mimicking phantoms to fractionation by histotripsy

Histotripsy is a non-invasive, focused ultrasound lesioning technique that can ablate precise volumes of soft tissue using a novel mechanical fractionation mechanism. Previous research suggests that magnetic resonance imaging (MRI) may be a sensitive image-based feedback mechanism for histotripsy. However, there are insufficient data to form some unified understanding of the response of the MR contrast mechanisms in tissues to histotripsy. In this paper, we investigate the response of the MR contrast parameters R1, R2, and the apparent diffusion coefficient (ADC) to various treatment levels of histotripsy in in vitro porcine liver, kidney, muscle, and blood clot as well in formulations of bovine red blood cells suspended in agar gel. We also make a histological analysis of histotripsy lesions in porcine liver. We find that R2 and the ADC are both sensitive to ablation in all materials tested here, and the degree of response varies with tissue type. Correspondingly, under histologic analysis, the porcine liver exhibited various levels of mechanical disruption and necrotic debris that are characteristic of histotripsy. While the area of intact red blood cells and nuclei found within these lesions both decreased with increasing amounts of treatment, the area of red blood cells decreased much more rapidly than the area of intact nuclei. Additionally, the decrease in area of intact red blood cells saturated at the same treatment levels at which the response of the R2 saturated while the area of intact nuclei appeared to vary linearly with the response of the ADC.

In vivo photoacoustics and high frequency ultrasound imaging of mechanical high intensity focused ultrasound (HIFU) ablation

The thermal effect of high intensity focused ultrasound (HIFU) has been clinically exploited over a decade, while the mechanical HIFU is still largely confined to laboratory investigations. This is in part due to the lack of adequate imaging techniques to better understand the in-vivo pathological and immunological effects caused by the mechanical treatment. In this work, we explore the use of high frequency ultrasound (US) and photoacoustics (PA) as a potential tool to evaluate the effect of mechanical ablation in-vivo, e.g. boiling histotripsy. Two mice bearing a neuroblastoma tumor in the right leg were ablated using an MRI-HIFU system conceived for small animals and monitored using MRI thermometry. High frequency US and PA imaging were performed before and after the HIFU treatment. Afterwards, the tumor was resected for further assessment and evaluation of the ablated region using histopathology. High frequency US imaging revealed the presence of liquefied regions in the treated area together with fragmentized tissue which appeared with different reflecting proprieties compared to the surrounding tissue. Photoacoustic imaging on the other hand revealed the presence of deoxygenated blood within the tumor after the ablation due to the destruction of blood vessel network while color Doppler imaging confirmed the blood vessel network destruction within the tumor. The treated area and the presence of red blood cells detected by photoacoustics were further confirmed by the histopathology. This feasibility study demonstrates the potential of high frequency US and PA approach for assessing in-vivo the effect of mechanical HIFU tumor ablation.

Thermal and mechanical high-intensity focused ultrasound: perspectives on tumor ablation, immune effects and combination strategies

Tumor ablation technologies, such as radiofrequency-, cryo- or high-intensity focused ultrasound (HIFU) ablation will destroy tumor tissue in a minimally invasive manner. Ablation generates large volumes of tumor debris in situ, releasing multiple bio-molecules like tumor antigens and damage-associated molecular patterns. To initiate an adaptive antitumor immune response, antigen-presenting cells need to take up tumor antigens and, following activation, present them to immune effector cells. The impact of the type of tumor ablation on the precise nature, availability and suitability of the tumor debris for immune response induction, however, is poorly understood. In this review, we focus on immune effects after HIFU-mediated ablation and compare these to findings using other ablation technologies. HIFU can be used both for thermal and mechanical destruction of tissue, inducing coagulative necrosis or subcellular fragmentation, respectively. Preclinical and clinical results of HIFU tumor ablation show increased infiltration and activation of CD4⁺ and CD8⁺ T cells. As previously observed for other types of tumor ablation technologies, however, this ablation-induced enhanced infiltration alone appears insufficient to generate consistent protective antitumor immunity. Therapies combining ablation with immune stimulation are therefore expected to be key to boost HIFU-induced immune effects and to achieve systemic, long-lasting, antitumor immunity.