CN117677406A - Distributing particles - Google Patents

Abstract

There is provided a method of distributing particles, the method comprising: providing a plurality of microparticles at an insertion site in a medium; ultrasound is applied to the insertion site, which generates bubbles by cavitation at cavitation nuclei located at the insertion site and drives movement of the bubbles such that the bubbles drive movement of the microparticles to a desired spatial distribution in the tumor. The method may be a method of treating a tumor, and the microparticles may comprise a radioisotope for treating the tumor. Microparticles for use in the treatment of tumors by the methods are also disclosed.

Description

Distributing particles

Technical Field

The present invention relates to distributing microparticles, and in some aspects to treating solid tumors by distributing microparticles that include a radioisotope.

Background

Microparticles, such as microspheres, are used in many applications. For example, it may be used as a diagnostic tool in medical testing, or to alter the density of plastics to provide additional buoyancy. The radioactive particles, i.e. particles comprising at least one radioisotope, may be used for imaging or for manipulating substances inside a medium. They are widely used in medical imaging and diagnosis of various diseases. In many such applications, it can be difficult to locate the particles in their intended locations.

One particular application of the radioactive microparticles is in the treatment of tumors. Brachytherapy and its modern evolution selective in vivo radiation therapy (SIRT) involve implanting radionuclides, either as bulk solids (e.g., iodine crystals) or as colloidal suspensions (e.g., yttrium citrate suspensions), into a liquid medium. SIRT was developed to prolong and improve the quality of life of patients with unresectable hepatocellular carcinoma (HCC), for whom external radiation therapy (EBRT) is not appropriate due to the poor tolerance of the liver to radiation.

Current methods for SIRT use known grades of high purity isotopes to localize radiation using permanent biocompatible microspheres with calculated emission energy and treatment duration for a particular indication. Like brachytherapy, SIRT allows for precise, controlled radiation treatment of radiation-sensitive organs and tissues that would otherwise not be tolerant of large doses of unfocused diffuse radiation. Treatment is a minimally invasive treatment via femoral or radial access, enabling delivery through outpatient care and making it an attractive alternative to EBRT. Although the original therapeutic concept was developed in the 1950 s, the adoption and practice of SIRT as a palliative technique was not widely practiced until current SIRT products were approved in the 2000 s. SIRT remains a non-curative treatment, and several health authorities worldwide recommendations have been obtained.

The therapeutic effect of SIRT is hindered by the terminal distribution of the radioactive microspheres. Due to the limited penetration depth of radiation generated by beta emissions from commercial radioactive embolic agents, the therapeutic efficacy is directly related to the distribution of the microspheres within the tumor tissue. Treating larger areas requires a greater number of microspheres or a greater distribution of the same number of spheres, while reducing the radiation intensity.

The distribution of the microspheres is ultimately limited by the placement of the catheter in which they are delivered. In hypoxic solid tumors, large areas of the tumor may remain untreated, as the vasculature of the cancer is primarily present at the periphery of the solid lesion, preventing the delivery of radioactive microspheres to the center of the solid tumor. Thus, the irradiation energy and its subsequent irradiation depth are critical to the feasibility of the procedure. If the penetration depth of the radiation is extended to enable treatment of a larger proportion of tumours, it is expected that the prognosis of the patient will also be improved.

Disclosure of Invention

According to a first aspect of the present invention there is provided a method of distributing particles, the method comprising: providing a plurality of microparticles at an insertion site in a medium; and applying ultrasound to the insertion site, the ultrasound generating bubbles by cavitation at cavitation nuclei located at the insertion site and driving movement of the bubbles such that the bubbles drive movement of the microparticles to a desired spatial distribution in the medium.

Surprisingly, it has been found that micron-sized particles can be driven by bubbles, particularly microbubbles generated by ultrasound-induced cavitation. Ultrasonic induced cavitation of microbubbles has been used to entrain nanoscale particles in a media liquid, but unexpectedly, ultrasonic induced cavitation of microbubbles can also drive movement of much larger (and heavier) microscale particles. The entrainment mechanism used in the nanoscale range would not be possible in the very different size and mass ranges of the micron-sized particles. However, the present inventors have found that gas microbubbles generated by ultrasonic induced cavitation can impart kinetic energy directly to micron-sized particles, thereby driving the movement of the particles. Cavitation, as used herein, refers to the evolution (i.e., growth) and subsequent oscillation of bubbles of various sizes from cavitation nuclei. The bubbles may or may not subsequently collapse during the application of ultrasound.

The applied ultrasound performs two functions, namely generating bubbles suitable for driving particles by cavitation of the cavitation nuclei; and driving the movement of the bubble. Cavitation bubbles in turn drive the movement of the particles, thereby distributing the particles into a desired spatial distribution. In particular, microbubbles can disperse particles within a medium and/or translate particles within a medium.

In this way, the particles may be non-invasively diffused from their original position, avoiding the limited distribution of particles discussed above, such as where the limited capillary size prevents further diffusion. In particular, when the method is used to distribute microparticles in tumors, the microspheres can diffuse into the tumor even in cases where the vasculature is limited to the tumor center only.

In some embodiments, providing the plurality of microparticles at the insertion site may include providing the plurality of microparticles at a location where the cavitation nuclei are located. For example, cavitation nuclei may be endogenous to the insertion site. In other embodiments, the microparticles and cavitation nuclei may be provided to the insertion site together or separately as a composition. Cavitation nuclei that generate microbubbles may include endogenous nuclei already at the insertion site, as well as exogenous nuclei provided to the insertion site.

In general, exogenous cavitation nuclei may be preferred because high ultrasonic energy may be required to generate bubbles from endogenous cavitation nuclei. This may be due to a smaller number or inefficiency of endogenous cavitation nuclei or high activation energy when bubbles are generated from the endogenous cavitation nuclei. The high ultrasonic energy required for cavitation from endogenous nuclei may damage the surrounding medium.

The cavitation nuclei may comprise at least one of: microbubbles, nanobubbles, nanodroplets, and gas-stable nanoparticles, such as nanocups or nanocones, i.e., nanoscale gas-stable shells having voids that act as cavitation nuclei.

In some embodiments, each particle may include a radioisotope, such as a beta or gamma emitting radioisotope. The radioisotope may be one or more of the following: yttrium 90, iodine 125, copper 64, scandium 44, lutetium 176 or holmium 166. It has been found that the ultrasonic method of driving the particles provides an efficient and effective method for distributing the radioactive particles in or around the medium.

In some embodiments, the microparticles may comprise a therapeutic agent. As described above, the therapeutic agent may include one or more radioisotopes.

In some embodiments, the medium may be tissue, such as human tissue. The tissue may be located in the patient or may have been removed from the patient. The tissue may be a tumor or a part of a tumor.

In particular embodiments, the method may be a method of treating a tumor by distributing microparticles comprising a radioisotope. The tumor may be a tumor of the liver, brain, pancreas, kidney, lung, throat, neck or intestine, and in particular may be glioma, glioblastoma or meningioma.

According to a second aspect of the present invention there is provided a method for treating a solid tumour, the method comprising: providing a plurality of microparticles at an insertion site in a patient tissue, wherein the plurality of microparticles comprises at least one radioisotope; ultrasound is applied to the injection insertion site, the ultrasound generating bubbles from cavitation nuclei located at the insertion site, and driving movement of the bubbles such that the bubbles drive movement of the microparticles into a spatial distribution for providing radiation for treatment of the tumor. The insertion site may be within the tumor, or outside the tumor, e.g., adjacent to the tumor. The spatial distribution may be a distribution across a tumor.

Any embodiment of the first aspect of the invention may be combined with the second aspect, in particular embodiments relating to the nature of ultrasound, cavitation nuclei and/or microparticles.

According to a third aspect of the present invention there is provided a plurality of microparticles, the microparticles comprising a radioisotope, for use in the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a solid tumour.

According to a fourth aspect of the present invention there is provided a plurality of cavitation nuclei for use in the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a solid tumour.

According to a fifth aspect of the present invention there is provided a composition comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, for use in the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a solid tumour.

According to a sixth aspect of the present invention there is provided a product comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, as a combined preparation for simultaneous, separate or sequential treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a solid tumour. Thus, cavitation nuclei and microparticles can be supplied and inserted separately into the body, but are used together to treat tumors by the application of ultrasound.

According to a seventh aspect of the present invention there is provided the use of a plurality of microparticles, including a radioisotope, in the manufacture of a medicament for the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a tumour.

According to an eighth aspect of the present invention there is provided the use of a plurality of cavitation nuclei in the manufacture of a medicament for the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a tumour.

According to a ninth aspect of the present invention there is provided the use of a composition comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, in the manufacture of a medicament for the treatment of a tumour by a method according to any embodiment of the second aspect of the present invention or any embodiment of the first aspect, wherein the method is a method of treating a solid tumour.

Drawings

In order to allow a better understanding, embodiments of the invention will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which:

FIG. 1 (a) shows a method of distributing microparticles according to the present invention;

FIG. 1 (b) shows a specific example of a first step of the method of FIG. 1 (a);

FIG. 1 (c) shows another example of a first step of the method of FIG. 1 (a);

FIG. 2 schematically illustrates the method of FIG. 1 (a) applied to a microparticle;

FIG. 3 illustrates the formation of bubbles using a nanocup;

FIG. 4 is a schematic diagram of an electronic setup for ultrasound generation from a HIFU transducer;

Fig. 5 and 6 show μct images to show the effect of focal pressure on particle distribution;

FIG. 7 shows a μCT image to show the effect of well diameter on particle distribution; and

fig. 8 shows a μct image to show the effect of duty cycle on particle distribution.

Detailed Description

Fig. 1 (a) shows a method of distributing particles using ultrasound according to the present invention. As discussed above, it may be difficult to move the particles to their intended locations in the medium. The particles may accumulate at the insertion site rather than moving to a desired distribution, particularly where capillaries are used to introduce the particles into the medium. The method of fig. 1 (a) improves the distribution of particles by applying ultrasonic energy.

The method of fig. 1 (a) is used to distribute particles in a medium. The medium may be a material such as plastic, wherein the particles are used to alter a material property such as density or buoyancy, or for diagnostic purposes, e.g. to act as externally detectable radiotracers.

The medium may be tissue, such as human tissue. The tissue may be in vivo or ex vivo. The microparticles may be used in tissue for diagnostic purposes (e.g., radiotracers) or non-diagnostic and non-therapeutic purposes (such as altering the characteristics of tissue).

The microparticles may alternatively or additionally be used in tissue to treat a disease. In particular, the method may be used to distribute particles into a spatial distribution for the treatment of tumors. The tumor may be a solid tumor. In some embodiments, the method may be a method of treating a tumor by distributing microparticles, each microparticle comprising a therapeutic agent, such as a radioisotope. In other words, the method may be used to provide selective in vivo radiation therapy (SIRT).

SIRT has been used to treat a variety of tumor types, but is most commonly used to treat liver cancer, such as hepatocellular carcinoma (HCC), cholangiocarcinoma, and colorectal cancer liver metastasis (mCRC). Other examples of SIRT treatment include pancreatic neuroendocrine tumors (pNET), lung tumors, and CNS tumors, such as gliomas. It is desirable to be able to translate SIRT technology to other areas of the body where external beam radiation is still currently used as part of first line therapy as well as to improve tumors that SIRT can already treat. For example, glioblastoma is the second most frequently reported brain tumor following meningioma, and is also the most common malignancy. Glioblastomas account for 15.4% of all primary brain tumors, and 45.6% of primary malignant brain tumors in the united states. Glioblastoma multiforme (GBM) patients have an average survival of 1 year, with only 5% of individuals living for 5 years or more, and no preventive strategy or standardized second line treatment available; these distressing figures reflect the limited treatments available, with most procedures leading to recurrence and disease progression within 10 to 30 weeks. The method of fig. 1 (a) can be used to treat glioblastoma as well as glioma and meningioma by providing a non-invasive ultrasound procedure to distribute the radioactive microparticles into a distribution for treating the tumor. In general, the method can be used to treat solid tumors of the liver, brain, pancreas, kidney, lung, throat, neck, or intestine.

Although the following description most often refers to the use of microparticles for the treatment of tumors, it should be understood that the methods discussed may be equally applicable to non-therapeutic uses, such as in non-biological materials.

Injection of microparticles

The method of fig. 1 (a) begins at step 101, wherein a plurality of microparticles are provided at an insertion site in a medium. The insertion site is also provided with or has included a cavitation nucleus, as discussed in more detail below with respect to fig. 1 (b) and 1 (c). Providing the microparticles may include injecting or otherwise inserting a plurality of microparticles at a common location. Where the particles are introduced via a capillary or cavity (naturally occurring or manufactured), the insertion site may be separated from the point where the particles were initially introduced into the capillary. For example, the insertion site may be located at the end of the capillary tube. Where the method is used to treat a tumor, providing may include providing a plurality of microparticles at an insertion site in a patient's tissue. The tissue may be the tumor itself, or tumor residue (after incomplete resection) or tissue adjacent to or surrounding the tumor or residue.

Fig. 2 (a) and 2 (b) show schematic examples of the method of fig. 1 (a). Fig. 2 (a) shows a medium 201 in which particles are to be distributed. A plurality of particles 203 are inserted into the cavity 202 where they accumulate at an insertion site 204. Subsequently, the particles 203 are distributed in the medium 201 by applying ultrasound from the ultrasound transducer 205, as will be discussed in more detail below in relation to step 102 of the method of fig. 1 (a).

In a particular example, the microparticles may be microspheres. Microspheres are micron-sized particles, which may be solid or hollow, and which are approximately spherical.

The particles may comprise or consist of ceramic. Such ceramics may include a radioisotope, such as yttrium 90, and one or more additional elements, such as silicon, aluminum, gallium, strontium manganese, or titanium. Since the starting materials are usually in the form of salts, such as oxides or carbonates, the ceramic also usually comprises oxygen. In one approach, the ceramic may be yttrium aluminum silicate glass. Ceramic materials may be particularly useful for providing inert, relatively incompressible particulates. One particular example for SIRT is manufactured by Biocompatibles UK Ltd (part of Boston science Co.) Microspheres are a combination of three high purity metal oxides, yttrium, aluminum and silicon, which mix and melt together at extreme temperatures to produce a solid (YAS) glass. The glass was crushed, powdered and spheroidized over an open flame to form YAS microspheres (see, e.g., US4789501, which is incorporated herein by reference). Further examples of ceramic SIRT microspheres are described in WO16082045 and WO 05087274.

The methods disclosed herein may be generally applied to microparticles, which are particles having a size (which may be a diameter) in the range of 1 μm to 1000 μm. The particles may have an average size of 200 μm or less, or preferably 150 μm or less, or more preferably 120 μm or less. The microparticles may have a minimum size of 1 μm or more, or preferably 5 μm or more, or more preferably 10 μm or more, or 20 μm or more, or 50 μm or more, or 100 μm or more. For example, the number of the cells to be processed,the microspheres have a size range of 15 to 35 μm, with an average size in the range of 20 to 30 μm, which is well suited for the present method and placement in human tissue. The particle size may be determined by Scanning Electron Microscopy (SEM), optical microscopy, and/or laser diffraction particle size analysis.

As discussed above, radioactive microparticles are particularly useful applications of microparticles. To this end, the microparticles may comprise at least one radioisotope. The radioisotope may be a beta or gamma emitting radioisotope such as yttrium 90, iodine 125, copper 64, scandium 44, lutetium 176 or holmium 166. In particular, for the treatment of tumors, isotope selection is screened for energy emission (electron volts, eV) and efficacy is weighed against systemic toxicity of radioisotope decay products. Iodine 125 and yttrium 90 are particularly useful because iodine 125 has a poorer bioavailability than iodine 127 and the inert decay products of yttrium 90 and zirconium 90. There is an increasing trend towards radiotherapy using isotopes of atypical emission beta (e.g. Cu-64, sc-44, lu-176, ho-166) because of their consistent imaging potential, enabling real-time monitoring of surgery via Single Photon Emission Computed Tomography (SPECT) and Magnetic Resonance Imaging (MRI).

To produce radioactive microspheres from YAS glass, YAS microspheres including Y89 are neutron bombarded in a nuclear reactor to produce yttrium 90 as the sole radioactive component. Other isotopes formed as a result of neutron bombardment of silicon or aluminum are considered to be stable or not significant in composition. The enriched Y90 isotope underwent beta decay to Zr90 with a half-life of 64.1h and decay energy of 0.93MeV average energy. Electron emission from an unstable Y90 atom is decelerated by electrostatic repulsive force of neighboring atoms; this deceleration and kinetic energy loss is emitted to surrounding cells (bremsstrahlung or cerenkov) with "braking" gamma radiation and can be detected externally for imaging. The energy emitted into adjacent cells can lead to DNA double strand breaks, where downstream signaling of the DNA damage can lead to cell necrosis.

The microparticles may be selected to have a level of radioactivity suitable to provide a clinically acceptable dose of radiation absorption for the intended treatment.The activity of each microsphere at the time of calibration was 2500Bq. Generally, the microparticles may have an activity of at least 10Bq, preferably at least 40 Bq. In some embodiments, the activity is at least 500, preferably at least 100 and most preferably at least 2000Bq per ball. The maximum activity of each ball is determined by factors such as the isotope selected, the number of balls to be delivered, and the like. In some embodiments, the maximum value may be 3000Bq, preferably 5000Bq, to provide optimal treatment while minimizing damage to surrounding healthy tissue. The total activity of a dose of microspheres may range from 3GBq to 20 GBq.

The goal of tumor uptake dose depends on the sensitivity of the tumor tissue and surrounding healthy tissue to radiation. Absorbed doses of at least 50Gy to tumour tissue are desirable, but preferably at least 150Gy. For example, the dose for a liver tumor may be between 50 and 500Gy, typically at least 150Gy and preferably at least 200Gy. LEGACY studies have recently shown that doses of at least 400Gy provide high levels of response in liver tumors such as HCC (Ann Oncol.2020;31 (journal 4): S692-S693). In the treatment of CNS tumors, such as gliomas, doses between 35Gy and 115Gy have been reported (Passiak et al EJNMIMI Res.2020; 10:96.) and in lung tumors about 250Gy has been used (US 10,232,063). Herein, the dose refers to the dose absorbed by tumor tissue. The dose may also be measured as the dose to perfused tissue, i.e., including tumor tissue and some surrounding tissue. The dose to perfused tissue will typically be lower than the dose to tumor tissue.

The microparticles may have a density of 10g/ml or less, or preferably 5g/ml or less, or more preferably 4g/ml or less. The microparticles may have a density of 1g/ml or greater, or preferably 2g/ml or greater, or more preferably 3g/ml or greater. For example, the number of the cells to be processed, The microparticles have a particle size of 3.6gml ^-1 Is a relative density of (c). These relatively high densities result from the use of substantially incompressible materials, which are ideal for SIRT and other applications of the microspheres. The combination of high density and micron-sized particles produces relatively heavy particles, at least compared to nanoparticle systems. Here, the density is the density of individual particles. The density may be determined by the raw material (e.g., glass) from which the particles were formed prior to spheroidization.

In the case of microparticles for the treatment of tumors, the insertion site may be within the tumor. Alternatively, the insertion site may be adjacent to the tumor, for example, at a distance sufficiently close to the tumor that the ultrasound energy discussed in step 102 below is able to drive the particles into the tumor. The insertion site may be on the tumor surface, or within 5cm, or within 2cm, or within 1cm of the tumor surface. In particular, the insertion site may be located within a cavity or capillary vessel formed by resecting a portion of a tumor. The method of fig. 1 (a) may include resecting a portion of a tumor to form a cavity. Alternatively, the insertion site may be located at the edge of a tumor formed by resecting a portion of the tumor. The method of fig. 1 (a) may include resecting a portion of a tumor to form a tumor margin or tumor residue.

The delivery of the microparticles may be performed with sterile 0.9% saline, so that the balloon is flushed from a v-glass vial located within the auxiliary acrylic stent, through the microcatheter and into the arterial vasculature. Placement of the tip and the inner diameter of the microcatheter are determined by operator, availability and regional preference. Delivery may be through a catheter to a point in the vasculature that serves the general volume of tissue, rather than the tumor itself. This process relies on tumor-selective local blood supply so that in reality most particles eventually enter the tumor, yet some percentage still enters the surrounding tissue. Alternatively, the microparticles may be delivered directly to the vessel feeding the tumor. This may be referred to as super-selective delivery because almost all particles enter the tumor and only a limited amount enters the surrounding tissue.

Radiation therapy was completed within 2 weeks, with patient follow-up performed after 6 weeks; including further imaging of parenchymal and necrotic tissue, further histology may be required to assess surgical performance.

Cavitation nuclei

Returning to step 101 of the method, particles 203 are provided at an insertion site 204 where cavitation nuclei are located. Cavitation nuclei can be considered any material that produces expanding bubbles that undergo inertial (collapse) or non-inertial cavitation upon exposure to ultrasound. The maximum diameter of the expansion bubble may be in the range of 1 μm to 500 μm. Thus, the bubbles generated may be referred to as microbubbles. The size of the bubbles may oscillate according to the frequency of the applied ultrasound field. The density of the expanded bubbles is typically much lower than the density of the microspheres. The density (or average density) of the expanded foam may be in the range of 0.5kg/m ³ To 2kg/m ³ In the range of (2), or in the range of 0.9kg/m ³ To 1.1kg/m ³ Within a range of (2)。

In some embodiments, an exogenous cavitation nucleus is provided at the insertion site 204. In some embodiments, step 101 includes providing a composition of both microparticles 203 and cavitation nuclei to the insertion site 204. Fig. 1 (b) and 1 (c) show two alternative embodiments using a combined composition, wherein cavitation nuclei and microparticles 203 are provided in separate steps.

In a first alternative shown in fig. 1 (b), step 101 includes a first step 1001 of providing microparticles 203 at an insertion site 204, for example, using the insertion method discussed above. After insertion of the microparticles 204, the method proceeds to step 1002, where cavitation nuclei are provided to the insertion site 204. The process of providing cavitation nuclei may be substantially similar to the process of inserting microparticles discussed above.

In a second alternative shown in fig. 1 (c), these steps are reversed. Thus, in fig. 1 (c), step 101 of the method includes a first step 1101 of providing cavitation nuclei at the insertion site 204. Next, at step 1102, the microparticles 203 are provided at the insertion site 204.

Alternatively, the insertion site 204 where the plurality of particles are provided in step 101 may be a location where the cavitation nuclei are already located. In particular, the cavitation nuclei may be endogenous to the insertion site. In such embodiments, there may be no step of providing exogenous cavitation nuclei. Alternatively, exogenous cavitation nuclei may be provided to sites already having endogenous cavitation nuclei to enhance cavitation effects. Generally, exogenous cavitation nuclei are preferred because they often require less ultrasonic energy to generate bubbles. Thus reducing the risk of damage to surrounding tissue by ultrasound.

Suitable cavitation nuclei include microbubbles, nanobubbles, gas-stable nanoparticles (e.g., nanocups), and nanodroplets. Each of these will be discussed in more detail below. Such cavitation nuclei have been used in combination with ultrasound for nanoscale applications such as drug delivery. These nanoscale particles are very different in scale from the dense, incompressible microspheres used in the microparticles of the present invention, particularly SIRT. The inventors surprisingly found that these cavitation nuclei and the bubbles they generate can be applied to larger and larger mass microparticle systems and thus help overcome the limited distribution of microparticles in SIRT and other microparticle applications.

Microbubbles and nanobubbles

Cavitation nuclei may be or include Microbubbles (MB) and/or nanobubbles. Microbubbles and nanobubbles are very small air pockets that typically contain a perfluorocarbon gas core coated with a phospholipid. Upon exposure to ultrasound, these balloons expand, thereby increasing the diameter of the bubbles. Where the cavitation nuclei include microbubbles and/or nanobubbles, generating bubbles by cavitation at the cavitation nuclei includes growing/developing microbubbles/nanobubbles into larger bubbles suitable for driving the particles.

Microbubbles have been shown to occur naturally in vivo, in porcine kidney and in porcine liver, and observations of microbubbles are linearly proportional to the concentration of human Red Blood Cells (RBCs) in vitro.

The use of high intensity focused ultrasound to evaporate a liquid (via thermal or mechanical stress) in the focal region of an ultrasound transducer may generate a sufficient number to create a cloud of microbubbles in the body.

When exposed to an external ultrasound field, the aerated MB will expand and contract, the amplitude of which depends on the energy and frequency of the field. At low amplitudes the oscillation of the bubbles is largely linear, but as the amplitude increases the behavior of the bubbles becomes more and more nonlinear, whereby the radial expansion and contraction may vary significantly with the maximum volume of the MB depending on the wave pressure. When the resonant frequency of the bubble exceeds the ultrasonic frequency, the bubble collapses, losing periodic oscillations, which is known as inertial, unstable, or transient cavitation. Such unstable cavitation events can produce new smaller MBs (nuclei) with different critical excitation pressures and oscillation frequencies. Under ultrasonic exposure, the bubbles will increase the free energy within the system, thereby increasing the temperature and volume of the bubbles, and any dissolved gas from the surrounding liquid combines with the bubbles to further increase their volumetric expansion. In theory, the expansion of the bubble increases the relative concentration of any entrapped gas in the solution at the bubble (defined as the rectifying diffusion liquid interface) to further facilitate the inflow of gas and subsequent expansion of MB.

Synthetic microbubbles consist mainly of gas nuclei (which are typically perfluorocarbons, stabilized by a liquid or protein shell) to prevent dissolution of the gas from the larger bubbles (> 1 μm) into the surrounding solution; wherein stable bubbles smaller than 1 μm are called "nanobubbles". Because core-shell chemistry affects the stability and cavitation threshold of microbubble populations, substrates of interest (cytotoxins, metal particles, proteins, viruses) are typically bound to surfaces.

Gas stable nanoparticle (nanocup)

In some embodiments, the cavitation nuclei may be or include gas-stable nanoparticles in the form of cups or cones. The nanocup is a cavitation nucleus based on nanoscale gas-stable polymers.

The nanocup may be considered an improvement over the individual microbubbles. One of the biggest disadvantages of microbubbles is that they cannot cross membranes because of their relative size (> 1 μm) compared to the endocellular (100 to 800 nm), whereas eukaryotic cells are typically in the range of 5 to 10 μm. Cavitation events with synthetic microbubbles typically cannot last more than one to two minutes due to rapid depletion under ultrasound. However, the use of the nanocup increases the duration of continuous cavitation by a factor of four.

Nano-sized (< 1 μm) hollow polymer spheres can be prepared via seed thermally initiated emulsion polymerization. When the organic monomers react together, a polymer shell or lens is created on the surface of the suspension droplet. Due to the osmotic pressure on the unsupported polymeric membrane, the surface shell collapses inward, creating a polymeric disk or "nanocup". Changing the monomer composition changes the viscosity within the particle during polymerization and the size of the cavity that is subsequently created within the nanocup.

Fig. 3 schematically illustrates a process of forming bubbles from a nanocup 301. In a first step, the small cavities of the concave polymeric nanocups 301 have nanobubbles trapped on their surface 302, which surface 302 is a nucleation site for cavitation events. In a second step, the gas nanobubbles 302 expand radially upon exposure to ultrasound and outside the cavity in which they are located. In a third step, the nanobubbles 302 reach a critical size, where the contact angle between the bubbles and the nanocup 301 approaches its maximum value, and the nanobubbles 302 detach themselves from the nanocup 301 (at this point, the free nanobubbles 302 may be of micrometer size and may be considered microbubbles). The nanobubbles/microbubbles 302 continue to expand. Eventually, a complete bubble 303 is formed. The maximum radius of the bubbles 303 is independent of the cavity or acoustic pressure of the nanocup, but is directly related to the frequency of the focused ultrasound waves. The ultrasonic amplitude determines the number of nucleated bubbles and thus the frequency of non-spherical collapse. For further details on bubble formation from the nanocup, please refer to Kwan, JJ et al, "ultra-high dynamics from micron-sized inertial cavitation of nanoparticles," applied Physics review "6, 1-8 (2016), which is incorporated herein by reference.

Nanometer liquid drop

Unlike microbubbles with a stable gas core, nanodroplets have a liquid (e.g., perfluoropentane, perfluorohexane) perfluorocarbon liquid core that is stabilized by a similar lipid or phospholipid molecule on the surface. Nanodroplets have advantages over microbubbles and nanocups, notably increased stability compared to their gaseous counterparts, particularly in the in vivo circulation. Upon exposure to ultrasound, the highly volatile liquid evaporates into a gas, thereby generating bubbles. However, the energy required for this phase change is much higher than for a gaseous core cavitation agent, but is still an option for sensitive therapeutic delivery applications.

Applying ultrasound

Returning to the method of fig. 1 (a), the method proceeds to step 102. At step 102, ultrasound is applied to the insertion site 204 to generate bubbles by cavitation at cavitation nuclei located at the insertion site. The applied ultrasound field drives the movement of the generated bubbles and their clouds so that the bubbles drive the movement of the particles to the desired spatial distribution in the medium. The applied ultrasound may cause cavitation of an exogenous cavitation core or a natural cavitation core within the insertion site.

The applied ultrasound performs two functions. Which first generates bubbles by cavitation at the cavitation nuclei, as discussed above with respect to step 101. It also drives the movement of these generated bubbles via acoustic radiation forces acting on the bubbles. Cavitation bubbles in turn impart kinetic energy to the particles, thereby moving the particles to a desired spatial distribution. The microparticles may be dispersed and/or translated from their location at the insertion site. The direction and intensity of the ultrasound may be selected to provide a particular movement of the particles.

An example of this process is shown in fig. 2 (a) and 2 (b). In fig. 2 (a), an ultrasonic generator 205 generates an ultrasonic beam focused on the insertion site 204. This utilizes cavitation nuclei at the insertion site 204 (bubbles and cavitation nuclei are not shown for clarity). As shown in fig. 2 (b), the bubbles drive the movement of the particles 203, in this case causing them to move into the medium 201 and to spread out, thereby creating a desired spatial distribution of the particles in the medium 201. The term "desired spatial distribution" is used herein to refer to a general distribution pattern of particles (e.g., dispersed relative to an initial insertion site). The desired spatial distribution depends on the intended use of the particles.

It has been found that the combination of ultrasound applied to the site with cavitation nuclei and particles is able to distribute the particles more distally and more efficiently than conventional methods of inserting particles. The maximum displacement of the microparticles from the insertion site resulting from this method can be in the range of 1mm to 4cm, allowing the microparticles to penetrate deeper into the tumor than conventional SIRT methods.

Notably, the process of driving movement of microparticles with bubbles is different from the mechanism used in nanoscale ultrasound drug delivery. In the latter case, moving the bubbles causes fluid flow, which in turn entrains the nanoparticles. This process of entraining particles is not applicable to the larger and heavier particles contemplated in the present invention.

In this method for the use of radioactive particles (such as those described above) In the case of treating tumors, the microbubbles may drive the particles into a spatial distribution to provide radiation to treat the tumor. This may include driving the microparticles into the tumor (with the insertion site in the tissue surrounding the tumor) and/or through the tumor. The generated spatial distribution canTo penetrate deeper into the tumor than was previously possible and to be more dispersed within the tumor than would otherwise be possible, thereby greatly increasing the amount of tumor that can be reached and treated. Thus, a much more effective treatment is possible.

Ultrasound is broadly defined as any frequency above the human audible range (20 kHz). The medical application of traditional ultrasound as a diagnostic imaging modality (ultrasound examination) is well established; a large part of the area of the human body is visualized via different tissue densities. Medical ultrasound examinations use very low energy inputs and frequencies, thereby creating a safe and non-destructive method suitable for imaging sensitive tissues and organs.

In medical applications, ultrasound waves are most often generated using Piezoelectric (PZT) ceramics; in the event of a displacement in the shape of the element, acoustic waves are generated as a result of a series of compressions and decompressions originating from the transducer surface. In an unfocused flat transducer, the wave will expand into a series of concentric concave wavefronts (spaced a single wavelength) so that the wave speed fluctuates as it travels through the substance; resulting in a high pressure band and a low pressure band. The intensity of sound is relative to the acoustic impedance, viscosity and its elastic behavior (Young's modulus; elastic or viscoelastic) of the material. During wave propagation, the particles may move in the direction of the wave (longitudinal or compression) or perpendicular to the wave (transverse or shear).

As the ultrasound waves pass through tissue or medium, energy is lost by scattering or thermal deposition; wherein the effective range of ultrasound is constrained by the energy input. The amplitude of the wave slowly dissipates inversely proportional to the propagation distance.

High Intensity Focused Ultrasound (HIFU)

In some embodiments, the applied ultrasound is focused ultrasound, with the focal point disposed at or near the insertion site. Preferably, the applied ultrasound is High Intensity Focused Ultrasound (HIFU).

HIFU is achieved by placing a low-speed confocal lens at the boundary of the PZT element that generates ultrasound, narrowing the wavefront to a single controlled focus (determined by the curvature of the lens) at a fixed distance from the applanation element. The lens acts by increasing the impedance for the wave propagation rate towards the center of the element, allowing the wave fronts (compression and decompression) at the transducer edges to reach the middle wave fronts at the same time; thus, the amplitude of the signal generated at the focal spot is increased without increasing the voltage applied to the transducer. After passing through the focal point (far field), the wavefront diverges and the signal strength is lost as the distance increases.

As a result of focusing the ultrasonic waves to a small focal region, a significant pressure gradient is created compared to the pressure gradient outside the focal region. The pressure gradient of the sound wave acts on an in-focus obstacle, known as the Acoustic Radiation Force (ARF), which can be approximated as:

Equation 1: acoustic radiation force on object

F: in terms of (kgs) ^-2 cm ^-2 ) Expressed force

A: absorption coefficient of material (Npcm) ^-1 )

I: average intensity of sound wave over time at ultrasonic focus (Wcm ^-2 )

C: sound velocity in medium (cms) ^-1 )

Absorbing energy from HIFU waves and converting it into kinetic energy can create localized temperature peaks within the tissue, which can lead to tissue destruction. Tissue coagulation was induced in vivo at 43 ℃, which is a combination of protein denaturation and permanent cell damage. Thermal ablation is associated with thermal doses exceeding 43 ℃ over a given period of time; wherein the thermal dose required for ablation varies with tissue and type. The overall heat resistance of the brain is particularly low, with each region having its own distinct damage threshold. Thermal stress is hindered by convection to surrounding locations; via blood circulation, cell permeability and perfusion between tissues increases, resulting in surgery with lower selectivity compared to mechanical stress. While receiving lower intensity ultrasound than the focal region, the pre-focal tissue does experience pre-focal energy deposition and heating. Pre-focal heating is an important issue in relation to in vivo treatment using HIFU, where undesirable collateral damage may cause thermal ablation in front of the focal spot, resulting in skin burns due to continued exposure of pre-focal tissue to converging HIFU wavefronts.

In order to provide a balance between efficient driving of the bubbles and limiting damage to surrounding tissue, the ultrasound applied in the present method may apply a peak negative focus pressure in the range of 1 to 20MPa at the insertion site.

Apparatus and method for controlling the operation of a device

Fig. 4 shows an example of an ultrasound setup 205 that may be used to perform step 102 of the method of fig. 1 (a). The transducer 401 may be configured to generate ultrasound with parameters selected by the waveform generator 404 to provide a desired particle distribution. The parameters may be selected to promote cavitation of bubbles from the cavitation nuclei and desired movement of microbubbles so as to provide a desired particle distribution. In a particular example, transducer 401 may generate ultrasound having a fundamental frequency in the range of 0.1 to 5 MHz. The generated ultrasound may have a pulse repetition frequency in the range of 0.1 to 10 Hz. The generated ultrasound may have a duty cycle (continuous wave) in the range of 1% to 100%. These parameters have been found to be particularly effective in producing the desired movement of particles, particularly in tumor-like media.

The ultrasound generator 205 of fig. 4 includes a HIFU transducer 401 with its impedance matching network 402, which is driven by the voltage output of an amplifier 403. In the example shown, the output voltage of the amplifier 403 is monitored by an oscilloscope 406 using a 1mΩ high impedance cable 405 to check if there is an anomaly in the transducer drive signal. The input voltage and other ultrasonic parameters are controlled by an arbitrary waveform generator 404. Although shown separately in fig. 4, linear array 407 is inserted concentrically within HIFU transducer 401 (represented by the dashed line within transducer 408). The linear array 407 is used for real-time passive acoustic mapping and post-exposure imaging operated by the imaging controller box 408. An imaging controller box 408 is connected to the waveform generator 404 to ensure that the transmit signal and the receive signal are synchronized.

When used to move particles in the body, ultrasound imaging may be used to identify the region to which HIFU pulses should be applied. Since the physically co-aligned HIFU transducer 401 and linear array 407 have coincident focal points, identifying the imaging focal point allows identification of the HIFU target focal point. Alternatively, the ultrasound probe may be inserted directly into the post-operative tissue cavity.

Results

Experiments were performed to investigate the subjectThe microspheres are distributed in the hydrogel medium to mimic tumors. Wells are formed in the medium to act as cavities for transporting the particles. Thus, the insertion site provided with the particles is in the well. Ultrasound was applied using the apparatus shown in fig. 4. The transducer and the coaxially aligned linear array are aligned, first the depth of focus of the transducer is aligned with a centralized 1mm stainless steel spike rod, which is inserted into the well to be tested. A pulse receiver is initially used to send short, low energy pulses to the transducer so that the soft hydrogel is not unnecessarily exposed to HIFU. The transducer was adjusted in 0.1mm increments until the maximum amplitude of the received signal was observed on the oscilloscope. Thereafter, a B-mode image is captured (usingThe system acts as an image controller 408). The x, y coordinates of the b-mode image are used to align subsequent wells in the same medium for visual confirmation prior to ultrasound exposure. After initial alignment, the pulse receiver is replaced with a waveform generator. In this case, the pulse repetition frequency or interval (PRF) is defined by +. >The instrument determines and the remaining parameters are determined via a waveform generator.

Hydrogel as human body mimicking tissue

Any tissue, whether in vivo, ex vivo or in vitro, is unique to an individual organism, and is not only between organisms but also the same organismThe tissue of the object has congenital differences. Thus, it is inherently challenging to produce tissue motifs that mimic tissue properties in several key characteristics. Hydrogels have been used for decades as in vitro substitutes for mammalian tissue for ultrasound imaging; its sound velocity, sound attenuation and sound impedance are similar to those of several cancer indications. Minor changes can be made to the composition of the hydrogel to replicate tissue stiffness, porosity, acoustic properties, and cellular structure and environment. As a result, hydrogels can mimic many of the properties found in vivo. Since hydrogels consisted primarily of water, although modeled and tested as solids; its solids content is similar to that of many animal tissues, satisfying the relative sound velocity (about 1540ms ^-1 ) Attenuation (about 0.5 dBcm) ^-1 MHz ^-1 ) And backscattering coefficient (at 10 ^-5 To 10 ^-2 In the range between 2 and 7 MHz).

The honeycomb structure is a complex mixture of bilayers, microfilaments and tubules, aqueous and organic liquid phases, osmotic pressure and enzyme catalysis of endogenous substances and is considered to be almost impossible to reproduce in vitro. Hydrogels are therefore a compromise allowing to mimic the material properties similar to the expected study of their cellular counterparts; however, the design of hydrogels is not perfect and replicating one in vivo property is often at the expense of another material property due to the complexity of mimicking the cell structure, e.g., replicating the stiffness of a material to mimic the tissue would typically compromise the permeability of the material. In an attempt to improve the various differences in hydrogels, several additives were evaluated, including mixing different polymers (with or without modification) with inorganic additives or inks.

Although having acoustic properties similar to those of tissue, hydrogels have a microstructure significantly different from that of tissue; there is an isolated fluid cell that redistributes over time under stress within the polymer network. Although particularly convenient for modeling the material properties of tissue, the resulting gel-model is manufactured and subsequently consistent with natural variability due to its heterogeneous material properties. Despite the drawbacks of current hydrogel phantoms, due to ex vivo tissue ⁷⁵ Is effective in settingGlue tissue mimics remain the most suitable substitute for simulated tissue.

For the measurements discussed herein, agar-based hydrogels were used as tissue mimics. Compared with PVA freeze-thaw hydrogels, agar gels are clear, are well-known, have less variation, and are relatively fast to produce. Agar (3kDa Mw,Sigma Aldrich) was added to deionized water at 0.5 or 1% w/v (10 μm,type 1) degassing for at least 2 hours under vacuum, and then heating the suspension to a temperature under microwave irradiation>85 ℃ and then poured into a phantom mold, and allowed to cool at 4 ℃ for at least 12 hours. The parameter changes and experimental results discussed later are based on 0.5% agarose gel.

Parameters (parameters)

Nanometer cup (from)) And Sonovue->Selected as the cavitation agent for most experiments.1-5X 10 as supplied according to the manufacturer's instructions ⁸ Individual particle mL ^-1 The concentration was reconstituted and used. The nanocup was used at 1:9 dilution (1.0X10 ⁹ Particle mL ^-1 ) Thereby diluting the provided suspension with de-aerated, filtered de-ionized water to match +.>Concentration. Both cavitating agents are used in equal volumes. 20mg of YAS glass microspheres (three size ranges were used [ ]<15. 15-35, 35+μm) of Biocompatible UK Ltd) to each 250 μl of diluted nanocup injection; reducing the chance of cavitation agents precipitating and phase separating from the cold radioactive embolic agent.

The pulse repetition frequency was set to 3.3Hz; this is to allow equivalent duty cycles to be achieved for the various frequencies studied, which is limited by the 50,000 cycle burst of the waveform generator. Each test well was subjected to a total of 300 ten thousand cycles, thereby adjusting the ultrasound exposure time and the number of cycles per ultrasound burst as needed.

The fundamental frequencies studied were 0.5, 1.1, 1.5 and 3.3MHz. Each frequency was then also tested at multiple focal pressures; 0.5MHz (1.4, 3.0, 3.8 MPa), 1.1MHz (2.6 MPa), 3.3MHz (7.7, 11.7, 10.9 MPa). As with pressure, the Duty Cycle (DC) varies with each fundamental frequency; 0.5MHz (2.5%, 5%, 16.5% DC), 1.1MHz (9.7% DC) and 3.3MHz (1%, 2.5%, 5% DC).

The glass is colorless and transparent, wherein the microspheres alone are difficult to visualize under a bright field microscope. However, due to its relatively high density (3.4 gcm ^-3 ) The ceramic can thus be visualized via x-ray tomography. Under X-ray exposure, the higher density object appears brighter in the image according to the gray scale and corresponding Hounsfield value.

After applying ultrasound to study the distribution of microspheres, X-ray micro-computed tomography (μct) images were taken. The muct image of the well is generated from 512 consecutive image slices (DICOM, 512 x 512 pixels), which are superimposed on the z-plane, to create a 512 x 512 voxel image in which the x-ray acquisition settings are stored within the file image stack metadata. Once reconstructed, the distribution of microspheres can be measured manually in a DICOM file viewer, but this is currently limited by operator interpretation and deviation.

Fig. 5 shows a μct image to illustrate the effect of increasing the focal pressure of the applied ultrasound. In fig. 5 (a), the peak negative focal pressure is 1.4MPa. In FIG. 5 (b), the focal pressure is 3.0MPa. In fig. 5 (c), the peak negative focal pressure is 3.8MPa. In all cases, the fundamental frequency was 0.5MHz,3.3Hz PRF. In fig. (a) and (b), DC is 5%, and in fig. 5 (c), DC is 16.5%. Ultrasound is applied to all images from left to right.

Fig. 6 shows that the peak negative focus pressures are 7.7MPa (6 (a)), 11.7MPa (6 (b)) and 10.9MPa (6 (c)) for the fundamental frequency of 3.3 MHz. 5% DC, 3.3HzPRF and 4mm channels were used. Agar cracking at 11.7MPa (5 (b) 0) and planar fission at 10.9MPa (5 (c)) were observed. Pre-focal extravasation was observed at 11.7MPa (5 (b)) and 10.7MPa peak negative focal pressure (5 (c)). Ultrasound is applied to all images from left to right.

These figures show that increasing peak negative pressure in the HIFU focal region increases the penetration depth of the microspheres. The observed peak negative pressure reduction (6 (c)) is due to the nonlinearity of the ultrasound field. The lower frequency (0.5 MHz) appears to have a more pronounced effect on a larger proportion of deposited microspheres, which is believed to be due primarily to the size of the focal region to which the spheres are exposed. Higher fundamental frequencies will produce more discrete, smaller extravasation microsphere channels than some of the omnidirectional bursts seen at lower frequencies; this is related to the size of the transducer focus which decreases with increasing frequency.

Figure 7 shows the effect of well diameter on microsphere distribution. The well diameters were 4mm (7 (a)), 2mm (7 (b)) and 1mm (7 (c)). The ultrasonic parameters are as follows: 0.5MHz 3.3Hz PRF,5% DC. 35+μm is used And (3) microspheres. An ultrasound field is applied to all three images from left to right.

Although not a major parameter, well diameter does appear to have an effect on the distribution of microsphere projections, especially at low fundamental frequencies, where the focal region comprises a majority of the volume of the well.

Fig. 8 shows an image similar to that of fig. 7, but with 2.5% DC, rather than 5%, showing the effect of DC on microsphere distribution. Using 15-35 μmMicroparticles. No microsphere extravasation was seen below 1% DC when compared to the control sample not exposed to ultrasound.

Taken together, these results demonstrate that the application of ultrasound to the combination of microspheres and cavitation nuclei does distribute the microparticles into tumor-like media, thereby demonstrating that the present invention can improve treatment of tumors by distributing the microparticles more effectively than is possible in traditional SIRT methods.

Claims (40)

1. A method of distributing microparticles, the method comprising:

providing a plurality of microparticles at an insertion site in a medium; and

ultrasound is applied to the insertion site, which generates bubbles by cavitation at cavitation nuclei located at the insertion site and drives movement of the bubbles such that the bubbles drive movement of the microparticles to a desired spatial distribution in the medium.

2. The method of claim 1, wherein the microparticles are microspheres.

3. A method according to any preceding claim, wherein the particles of the plurality of particles have an average size of 200 μm or less, or preferably 150 μm or less or more preferably 120 μm or less.

4. A method according to any preceding claim, wherein the particles of the plurality of particles have an average size of 1 μm or more, or preferably 5 μm or more, or more preferably 10 μm or more, or 20 μm or more, or 50 μm or more, or 100 μm or more.

5. A method according to any preceding claim, wherein the particles of the plurality of particles have a density of 10g/ml or less, or preferably 5g/ml or less, or more preferably 4g/ml or less.

6. A method according to any preceding claim, wherein the particles of the plurality of particles have a density of 1g/ml or more, or preferably 2g/ml or more, or more preferably 3g/ml or more.

7. A method according to any preceding claim, wherein the particles comprise ceramic.

8. A method according to any preceding claim, wherein the microparticles comprise at least one radioisotope.

9. The method of claim 8, wherein the radioisotope is a beta or gamma emitting radioisotope.

10. The method of claim 9, wherein the radioisotope is yttrium 90, iodine 125, copper 64, scandium 44, lutetium 176, or holmium 166.

11. The method of claim 10, wherein the particulates comprise yttrium aluminum silicate glass.

12. The method of any one of claims 8 to 11, wherein the microparticles emit radiation having an activity of 10Bq or greater.

13. The method of any one of claims 8 to 12, wherein the microparticles emit radiation having an activity of 5000Bq or less.

14. The method of any one of claims 1 to 13, wherein the cavitation nuclei are exogenous to the medium, and wherein the method further comprises providing a plurality of cavitation nuclei at the insertion site.

15. The method of claim 14, wherein a composition comprising the plurality of microparticles and the plurality of cavitation nuclei is provided at the insertion site.

16. The method of claim 14, wherein the plurality of microparticles and the plurality of cavitation nuclei are provided at the insertion site in separate steps.

17. A method according to any preceding claim, wherein the cavitation nuclei are endogenous to the medium.

18. The method of any preceding claim, wherein the cavitation nuclei comprise at least one of: microbubbles, nanobubbles, nanodroplets, and gas-stable nanoparticles.

19. The method of any preceding claim, wherein the ultrasound has a fundamental frequency in the range of 0.1 to 5 MHz.

20. The method of any preceding claim, wherein the ultrasound has a pulse repetition frequency in the range of 0.1 to 10 Hz.

21. The method of any preceding claim, wherein the ultrasound has a duty cycle in the range of 1% to 100%.

22. The method of any preceding claim, wherein the ultrasound applies a peak pressure in the range of 1 to 20MPa at the insertion site.

23. The method of any preceding claim, wherein the medium is tissue of a patient.

24. The method of claim 23, wherein the method is a method of treating a tumor in a patient, the plurality of microparticles comprises at least one radioisotope, and the spatial distribution is used to provide radiation to treat the tumor, each microparticle comprising a radioisotope.

25. The method of claim 24, wherein the insertion site is within the tumor.

26. The method of claim 24, wherein the insertion site is adjacent to the tumor.

27. The method of any one of claims 24 to 26, wherein the spatial distribution is a distribution across the tumor.

28. The method of claim 26, wherein the insertion site is within a cavity formed by resecting a portion of the tumor, or wherein the insertion site is at a tumor margin formed by resecting a portion of the tumor.

29. The method of any one of claims 24 to 28, wherein the tumor is a solid tumor.

30. The method of claim 29, wherein the solid tumor is a tumor of the liver, brain, pancreas, kidney, lung, throat, neck, or intestine.

31. The method of claim 30, wherein the tumor is a glioma, glioblastoma, or meningioma.

32. A plurality of microparticles comprising a radioisotope for use in the treatment of a tumor by the method according to any one of claims 23 to 31.

33. A plurality of cavitation nuclei for treating a tumor by the method of any one of claims 23 to 31.

34. A composition comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, for use in treating a tumor by the method of any one of claims 23 to 31.

35. A product comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, the product being for simultaneous or sequential use as a combined preparation in the treatment of a tumour by a method according to any of claims 23 to 31.

36. The microparticle of claim 32, the composition of claim 34, or the product of claim 35, wherein the radioisotope is a beta or gamma emitting radioisotope.

37. The microparticle of claim 32, the composition of claim 34, or the product of claim 35, wherein the radioisotope is yttrium 90, iodine 125, copper 64, scandium 44, lutetium 176, or holmium 166.

38. Use of a plurality of microparticles, including a radioisotope, in the manufacture of a medicament for treating a tumour by a method according to any one of claims 23 to 31.

39. Use of a plurality of cavitation nuclei in the manufacture of a medicament for treating a tumor by the method of any one of claims 23 to 31.

40. Use of a composition comprising a plurality of cavitation nuclei and a plurality of microparticles, the microparticles comprising a radioisotope, in the manufacture of a medicament for treating a tumor by the method of any one of claims 23 to 31.