Abstract
Background
The first clinical transcatheter embolization procedure was reported in 1972, utilizing autologous clot as the embolic agent. Since then, embolization procedures have become progressively more sophisticated and the number, variety, and complexity of embolic agents has exponentially increased [11].
Embolization is a therapeutic method of modern medicine, with the aim of a partial or complete occlusion of vessels. The mainly angiographically controlled intervention for arterial or venous occlusion is meanwhile an integral part in multimodal treatment concepts. The minimally invasive endovascular embolization procedure works gentler than many other therapeutic strategies, covering broad tasks – from and range of indications.
Different types of embolisates are currently on the market and clinical used depending on the purpose of temporary or permanent occlusion. Vascular embolisates are particles or fluids, which can be released into the bloodstream through a catheter to occlude the target vessel mechanically and/or biologically. This definition excludes vessel-blocking agents or devices such as balloons and coils, which are positioned at the target site [10]. These are subdiveded in liqiud (Histoacryl® tissue glue made of n- Butyl-2-Cyanoacrylat (Braun®), Lipiodol® Ultra- Fluid injecion solution (Guerbet GmbH) or Onyx® Liquid Embolic System (ev® The Endovascular Company)), particulate embolisates such as classic particles (polyvinlyl alcohol – particles (Ivalon® Surgical), Gelfoam® Plus Hemostasis Kit (Baxter) or EmboCept® S 45 mmg/7,5 ml Degradable Starch Microspheres (PharmaCept)), deformable particles (Bead Block®, 100–1200 μm (Terumo), Embozene® Microspheres (Celo®Nova BioSciences, Inc.)) or loadable particlaes (e.g. DC Beads®, 100–700 μm (Terumo) or Embozene Tandem™ 100 μm, (Celo®Nova BioSciences,Inc.)) as well as metallic embolisates (e.g. Axium® Detachable Coil System (ev3® The Endovascular Company)).
Meanwhile, there is an increasing number of existing therapeutic uses of embolization. Transcatheter embolization is used for different types of e.g. endoscopically or surgical unmanageable arterial bleeding [9] (preoperative) portal vein embolization [21] embolization of malformations [6] or endoleaks [13]. Furthermore one oft the major application of embolisates is tumor therapy, more precisely the transarterial chemoembolization of liver dominate tumors [1] or hepatic metastases of CRC [4, 7], NET [2], Melanoma [16], Mamma Carcinoma [5].
Conventional transarterial chemoembolization (cTACE) using a mixture of a chemotherapeutic agent (e.g. doxorubicin or cisplatin) and lipiodol [3] is a recommended option of care for the treatment of intermediate stage hepatocellular carcinoma (HCC). The introduction of TACE with drug eluting beads (DEB-TACE) [23] was primarily developed to enhance the delivery of the chemotherapeutic agent while minimizing systemic toxicity and to provide a standardized embolising effect. Drug eluting beads are embolic microspheres loaded with a chemotherapeutic agent (mostly doxorubicin) with the ability of slow drug release, which should ensure high local and low systemic drug concentrations. Indeed, systemic levels of doxorubicin were significantly lower in patients receiving DEB-TACE compared to patients receiving cTACE with lipiodol [17].
Objective
The aim of this study was to investigate various embolization particles, focusing particles on their physical properties. For this purpose different embolisates had been investigated in vitro and the morphological variability and elasticity of the particles, mainly during and after their passage through a catheter was of particular interest.
Methods
Behavior oft the particles in a catheter
Various embolization particles were observed in a microscope to investigate the morphological and elastic properties. The outer shape of the particles before catheter passage and their change during and after the passage of the catheter was of interest. The catheter used in the clinic for embolization therapies was imitated by preparing a simulation catheter made of glass and plastic with a comparable diameter. To facilitate comparison, microscopic images were provided with a scale. Embolisates that constitutes the microscope, as clear spheres, were stained blue partly in order to achieve an improved presentation.
Preparation of the simulation catheter
The aim was to provide a transparent glass and plastic catheter corresponding to the diameter of the dimensions of a catheter used clinically for embolization therapies. Thus, the in vitro catheter demands on soft skills of embolization as a catheter used in vivo. The advantage of the transparent catheter simulation in this field is the possibility of microscopic observation of the embolisates directly in the catheter.
Producing the simulation catheter following materials were used: Glass tube: length 30 cm (Schott Ruhrglas®) Plastic tube: cover for transportation of a Dorado® PTA Dilatation Catheter (Bard Medical®) Bunsen burner Wire: Synchro2® STANDARD 0.014 in×200 cm, REF: 2641, LOT: B30905 (Stryker® Neurovascular) Histoacryl® tissue glue of n- Butyl-2-Cyanoacrylat (Braun®) Connection Discofix® C three-way valve for medical applications (Braun®) Inflation apparatus: Everest 20 ml REF: AC3205P, LOT: 50572691 (Meditronic, Inc.)
The glass and plastic tube were heated rotating directly over the flame of a Bunsen burner. After a short time the heated materials melt as far a way that they can be manually deformed for pulling apart the ends and thus to reduce the diameter of the tube. Then the tube was removed from the heat for hardening. Afterwards the wire of a in-vivo used catheter (Synchro2® STANDARD 0.014 in×200 cm, (Stryker® Neurovascular)) is used to set the final diameter of the simulation catheter. Therefore a 0.014 inch wire was placed in the glass/plastic tube. At the point of greatest resistance of the glass/plastic catheter was broken manually. Thus, the tip of the catheter had a similar diameter as the real catheter wire. The other end was occluded using Histoacryl® tissue glue made of n-Butyl-2-Cyanoacrylat (Braun®). Thus the particles could be injected using a Discofix® C 3 way walve (Braun®).
Preparation oft the scale
To provide an idea of the proportions of the various particulate embolisates and to quantify a possible deformation a special scale was necessary
Materials used: wire: ev 3™ NITREX™.014”×80 cm, (ev3 Endovascular, Inc.) Microscopy images: Microscope: USB Digital- microscpe 1,3 MP, Art. No.: 88-54000 (Bresser) Software: Future WinJoe Power Point: Microsoft Windows 8, Version 6.2 (Build 9200) © Microsoft Corporation Microsoft PowerPoint 2013 Paint: Microsoft Windows 8, Version 6.2 (Build 9200) © Microsoft Corporation
Tested particulate embolisates
(see Table 1)
Coloration of the EmboCept®- particles with royal blue ink
As the EmboCept®- particles appear not only transparent in the microscope and also have a quite small diameter, we tinted the particles with blue ink (Pelikan® 4001 TP6) for a better evaluation oft the morphology.
The EmboCept®- particles have been removed from the original package using a syringe (2 ml: Omnifix® Solo (Braun®)) and a needle (Sterican® needle, Art.-No./Ref.:4657519, Pharmacy code: 2050798 (Braun®)). Then a few drops of the royal blue ink were added (step 1) and mixed by shaking. Afterwards we injected the solution on the filter paper (Intrapur®Lipid 1,2 μm (Braun®)) (step 2) und 2×10 ml 0,9% NaCl Miniplasco (Braun®) filled in a 20 ml syringe (Omnifix® (Braun®)) to wash out the abandoned ink, which was not connected with the particles. (step 3). As a result only the 50 μm EmboCept®- particles stayed on the 1,2 μm filter.
Analysis
A scale was designed for grading the morphologic changes of particle shape after their catheter passage (grade: 1 no changes; 2 small changes in <50%; 3 small changes in >50%; 4 big changes in <50%; 5 big changes in >50%; 6 complete deformation/destruction) as well as for the appearance of a deformation during catheter passage (0 = not necessary, 1 = necessary). Furthermore the morphologic changes of particles after catheter passage were classified in changes in the basic shape oft the particle compared to their shape before the passage (e.g. before passage round, afterwards oval), changes in size with subdivision, if the new diameter was smaller or bigger than before the catheter passage (0 = no; 1 = yes).
All tested particles were evaluated microscopically by four blinded reviewers (2 interventional radiologists with 25 and 8 years experience, 1 resident for radiology with 3 years experience and 1 resident in plastic surgery as a non-specialist in radiology using the defined graduation).
Data acquisition and data analysis was performed using Microsoft Excel 2011 Version 12.0.
Results
Behavior of the different particles during the in vitro study
EmboCept®
The EmboCept® – particles appear to be homogeneous spherical particles with slightly varying diameter. The variation stands at around 50–100 μm. During catheter passage there is no relevant deformation of the EmboCept®- particles due to the catheter diameter of 0.014 inch (= ca.355 μm). The average value reviewed was 1.75 without any changes in the basic round shape and also without any change in its size.
Behaviour contour SE® microspheres
Inspecting the native Contour SE™ Microspheres, they are – pursuant the producer’s data – small irregular flakes of polyvinyl alcohol with an already inhomogeneous morphology before passing the catheter. The passage of the catheter was unproblematic, because the size of the embolisate with a diameter of 300–500 μm lies in the range of the diameter of the simulation catheter. Due to the preliminary morphology of the Contour SE® Microspheres is not finally investigable whether and what changes to the particles result from contact with the limitations of the glass catheter. Regardless this fact the average value reviewed was 3,8 without any changes in the basic shapeand size.
Behaviour of embosphere® microspheres during the in vitro study
The homogeneous spherical Embosphere® Microspheres – particles can be prepared well in spite of its transparent appearance and are macroscopically evident. Comparing the diameter of the particles (700–900 μm) and the simulation catheter (approximately 355 μm) it is notable that a passage of Embosphere® Microspheres – particles is only possible with deformation. The necessary change of shape for passing the catheter is almost reversible due to their elasticity. The average value reviewed was 2 without any changes in the basic shape but changes in size meaning, that the particles show a bigger diameter than before after the catheter passage (see Fig. 1).
Behaviour of embozene® microspheres during the vitro study
Embozene® Microspheres, 400 μm
The homogeneous spherical Embosphere® Microspheres – particles can be displayed well in spite of its transparent appearance and are macroscopically evident. Comparing the diameter of these particles (700–900μm) and the simulation catheter (approximately 355 μm) reveals that a passage of Embosphere® Microspheres – particles only by their deformation is possible. Therefor the passage of the catheter necessary change of shape is reversible and elastic. The average value reviewed was 2 without any changes in the basic shape but changes in their size, by showing a maximum diameter smaller than before the catheter passage.
Embozene® Microspheres, 500 μm
The manufacturer’s instructions for the red Embozene® microspheres with a size of 500 μm state a design specification of 530 μm±50 μm and a recommended catheter for intervention of 0.016 inches (about 400 μm). Similarly to the blue colored Embozene® Microspheres they are elastically deformable due to their diameter for smooth catheter passage. The average value reviewed was 1 without any changes in the basic shape after the catheter passage. Nevertheless the Embozene of 500 μm showed mostly changes in their size with a maximum diameter that was smaller than before. (please see Fig. 2)
Embozene® Microspheres, 1300 μm
The average value reviewed was 2. There were no relevant changes in the basic shape after the catheter passage. The particles only lost partly their sharp, smooth margins. Nevertheless the Embozene of 1300 μm showed mostly changes in their size with a maximum diameter that was smaller thanbefore.
Behaviour of DC beads®–particles
Due to its diameter of 300–500 μm, the catheter passage was unproblematic and the particles remained nearly constant in shape. Yet we found changes in their size after the passage of the catheter by observing diameters smaller than before in the majority of the particles. The average value reviewed was 2.0 (see Fig. 3).
Behaviour of embozene tandem® – particle
Since the particles have a small diameter of about 100 μm, there is no relevant impairment of the particle shape and size caused by the walls of the simulation catheter, so that the morphological character of Embozene Tandem® – particles after catheter passage remains unchanged. The average value reviewed was 1 (see Fig. 4).
Discussion
Embolization procedures have become progressively more sophisticated and the number, variety, and complexity of embolic agents has exponentially increased.
Once it had been decided that the vessel can be treated and consecutively sacrificed, determining the best embolic agent to use, the operator should know the length and the size of the vessel, which should be embolized as well as if the tissue is supplied by the vessel to remain viable after theembolization [11].
The huge number of agents at an operator’s disposal may lead to confusion in determining which agent to use for any given clinical scenario. The most appropriate embolic agent would differ for different clinical scenarios:
The aim of each procedure is different, and thus the embolic agent used is different, too. It is vital to recognize the important characteristic of all embolic agents: regardless of the makeup of the agent, the smaller the agent, the greater the likelihood of ischemia of the organ being supplied by the embolized vessel.
For many years since the 1970 s galantine sponge and polyvinyl alcohol particles were conventional and widely used particulate agents. Nevertheless the occlusion level of these conventional agents is not predictable because of the irregular shape and their variability in size. Generally they tend to aggregate in vessels more proximately than they were thought to, but their tiny little fragments can also migrate into capillary beds. Consequently it is very difficult to perform a controlled target embolization using these agents.
Osuga et al. [14] evaluated porous gelatin particles (GP). Corresponding to our study they investigated the size, the distribution as well as the incidence of fragments of GP before and after microcatheter passage. 1 mm- and 2 mm- gelatin particles were injected into 3 microcatheters, all with a different inner-diameters by three interventional radiologists. The particles were also stained with methylene blue ink, and measured by a digital microscope. The change of mean particle diameter after catheter passage ranged within 10%. Osuga et al found out that the particles after passage through the catheter were uniform in size, distribution and contained fragments. They did not change significantly in shape after microcatheter passage.
To avoid the disadvantages of these conventional embolic agents, spherical embolic agents or microspheres have been developed since the 1990s.
Advantages of these microspheres are, that the particles are uniform in shape and size and also easy to inject through a microcatheter. They can – corresponding to the particle size – travel distally to vessels, so the occlusion level can be predicted according to the particle size chosen. Thus, new bland microspheres and DEB may bring a significant advancement to embolization for primary liver tumors as well as hepatic metastases from various cancers [15].
Particulate embolisates are currently the most commonly used agents for permanent small-vessel occlusion. The size of the particles used range between 30 μm and 1300 μm. (in our study 100 μm – 1300 μm). Particles are radiolucent, but mixed with contrast they can be gently injected with a small syringe (1 to 10 mL) using fluoroscopic guidance. Use of small particles, especially in solid organs such as the liver, may lead to focal necrosis and predispose to abscess formation [12].
Stampfl et al. [20] investigated in their study specific inflammatory and foreign body reactions after 4 and 12 weeks according to the Banff 97 classification after porcine liver embolization with different spherical embolic agents of various sizes: (40–120 μm (Embozene, Embosphere), and 100–300 μm, 500–700 μm, and 700–900 μm (Embozene, Embosphere, Bead Block, and Contour SE). Allover the foreign body reaction was pronounced after embolization with smaller particles, especially in small Embosphere particles. They found out that the numbers of giant cells with Embosphere 100–300 μm were statistically higher compared with other materials, which were of corresponding size (P < 0.0001). The stated out that giant cell and inflammatory reactions after embolization depend on the embolic material but also that the overall inflammatory reaction was low. Conculasionally Stamfl et al. found out that giant cells as a indicator for foreign body reaction were more frequently associated with a small particle size.
The same group investigated the early and long-term effects of microparticle embolization in mini-pig models regarding liver [19] and kidney [18]. In their liver focused study they evaluate trisacryl-gelatin microspheres (range 40–120 μm) for acute and chronic tissue embolization. Their results show that after embolization oft he liver, no parenchymal necrosis did occur; there were only signs of vessel wall disintegration. Also the bile ducts stayed intact. A distinct foreign body reaction with little leukocytic infiltration and giant cells was found, but without signs of major inflammation. Regarding the results after the embolization of the kidney using 40–120-microm trisacryl-gelatin microspheres (Embospheres) segmental renal infarction occurred after chronic partial embolization despite recanalization during follow-up. Only mild specific intra-arterial foreign body reactions were found.
These studies underline the importance about the knowledge and prediction of the stability in morphology of the embolization agents as examined and shown in our study to predict the area embolized and avoid undesirable side effects such as wrong embolization because of a possible changed diameter/size after catheter passage.
Turjman et al. [22] described the requirements for the ideal particulate embolic material: easiness of passing through a micro-catheter, effectiveness in producing vascular occlusion, homogeneous embolization, easy availability, biocompatibility, relatively benign inflammatory response, and no carcinogenesis. The easiness of passing through a micro-catheter is mainly related to the shape of the particles; nearly spheroid or oval particles with a smooth surface and no spine are ideal for this purpose. This is correlating with our results, where the particles showed deformation during their passage through the catheter where necessary. This is correlating with our results, where the particles showed deformation during their passage through the catheter where necessary. Shape also has an effect on the response of the surrounding tissues [8].
Extruded PVA particles (Boston Scientific, Natick, MA) are a newer agent, compressible and regular in shape. In theory, these particles should be less likely to cause an occlusion of the catheter.
Trisacryl gelatin microspheres (Embospheres; Biosphere Medical, Boston, MA) are made from trisacryl gelatin; these spheres are round, homogeneous, and compressible. In theory, they provide improved delivery and vessel occlusion due to decreased clumping [12].
Bead Block (Biocompatibles UK Ltd., Franham, UK) are compressible and hydrophilic PVA hydrogel particles with a theoretical decreased risk of clumping. They are available suspended in sterile water in prepackaged syringes. Although FDA-approved as an embolic agent, Bead Block is now frequently used as a drug-eluting bead for transcatheter chemoembolization procedures and known to be a safe and effective treatment option for liver dominant tumors with favorable pharmacokinetic profile [12].
In conclusion a stable shape and especially shape of the particles used for embolization and consecutively stable quality after their passage through the catheter is necessary to predict the results, success and consequences of the embolization procedure and further avoid possible complications.
In conclusion all tested embolization particles are, regarding their morphological properties in sense of homogenous shape before catheter passage, necessary deformation during catheter passage and renewed shape with varying changes in morphology after catheter passage (without complete deformation), a safe treatment option and due to their stable shape and size the result and success of the therapy predictable by minimizing complications. Tested in vitro no loss of functionality regarding physical properties should be expected.
Limitation
It has to be annotated that the study was performed using an in-vitro-design. Furthermore results were gained in one run of the experiment as well as under subjective graduation of the reviewers.
Clinical relevance
Embolization with particles is a safe treatment option as a minimally invasive endovascular procedure and works gentler than many other therapeutic strategies.
Embolization with particles covers broad tasks – from and range of indications.
During catheter passage all particles were elastically deformable, where necessary.
After catheter passage no loss of functionality due to a loss of basic shape was seen.