et al. 2002; Valji 2006; Jamshidi et al. 2008; Lewis 2008; Hanawa 2009). Ureteral stents are composed of polyethylene, polyurethane, hydrogel, silicone, or thermoplastic polymer and are formed into a tube; metal ureteral stents have also been used (Auge and Preminger 2002; Liatsikos et al. 2009; Venkatesan et al. 2010).
Several different methods are used to fabricate stents. The majority of stents are made from laser cutting (Stoeckel et al. 2002). Other fabrication techniques include photochemical etching, waterjet cutting, braiding, knitting, and coiling (Stoeckel et al. 2002). The interventional radiologist should be familiar with the method used to make each stent as this affects the deployment and eventual configuration of the stent. For instance, braided designs will shorten after expansion, and it is critical to understand this if the appropriate size stent is to be selected (Stoeckel et al. 2002).
Vascular obstructions may require stent placement to restore the vascular lumen. Budd–Chiari syndrome is manifested by hepatic venous outflow obstruction secondary to a myriad of diseases, including malignant neoplasia (Beckett and Olliff 2008). The primary nonvascular regions where stent placement can be beneficial include the biliary tree, esophagus, colon, urethra, ureter, trachea/bronchus, and lacrimal duct (Pron et al. 1999). Malignant obstructions are the primary indications for placement of stents in these locations (Pron et al. 1999).
Recent advances in stent technology have included development of drug‐eluting stents, removable stents, radioactive stents, and absorbable stents. Drug‐eluting stents are used commonly in human medicine, and drugs such as paclitaxel and cisplatin have been embedded into the coating on the stent (Ong and Serruys 2005; Lewis 2008; Chao et al. 2013; Kim et al. 2014). Drug‐eluting stents are most commonly used for cardiovascular disease in humans (Lewis 2008), although clinical cases of hepatobiliary malignancy have been treated with drug‐eluting stents (Suk et al. 2007). Additionally, paclitaxel‐eluting stents have been evaluated in the urinary tracts of pigs and dogs (Shin et al. 2005; Liatsikos et al. 2007). Removable and absorbable stents are being used in human IR (Lootz et al. 2001; Tammela and Talja 2003; Grabow et al. 2005; Lewis 2008; McLoughlin and Byrne 2008; Kotsar et al. 2010); however, the application for removable and absorbable stents in veterinary IO is likely to be limited. Further research is needed to evaluate the use of radioactive stents, but early research and clinical results are hopeful (Liu et al. 2007, 2009). These stents provide an intraluminal source of brachytherapy with the goal of local tumor control (Balter 1998; Liu et al. 2007, 2009).
Approaches
Vascular
The majority of veterinary IO vascular procedures are performed through major vessels including the carotid artery, femoral artery, femoral vein, and jugular vein. For all approaches, the hair over the surgical site is clipped, and the site is prepared with an aseptic technique and draped. The approach to the carotid artery and jugular vein is made with the animal in dorsal recumbency; the neck is outstretched (often with a towel under) and the head is maintained parallel to the tabletop. For a femoral artery or vein approach, the patient is also placed in dorsal recumbency and the femoral pulse is palpated. The hind limb will often need to be pulled gently away from the body to allow for appropriate exposure of the inguinal region.
In human medicine, ultrasound guidance is often used to gain vascular access (Ahmad et al. 2008; Arthurs et al. 2008); in veterinary patients, this is less common but should be explored further. A 1–2 cm skin incision is made directly over the vessel to be accessed; generally, the incision is parallel to the direction of the vessel. The subcutaneous tissue is bluntly and sharply dissected until the desired vessel is easily palpable or visible. The tissue surrounding the vessel is then gently dissected, and the vessel is manipulated to allow circumferential freeing of the vessel. A length of at least 1 cm of vessel should be freed from the surrounding tissue. When approaching the femoral or carotid artery, the vessels may be ligated (in dogs) at the conclusion of the procedure (Perkins and Edmark 1971; Moss 1974; Clendenin and Conrad 1979a, 1979b). A stab incision into the skin can be used when approaching the jugular vein or femoral vein, and dissection is generally not necessary. Given the lower pressures found in the venous system, ligation is not necessary, as gentle pressure applied at the conclusion of the procedure will often maintain hemostasis.
The original technique for gaining vascular access was described by Seldinger (1953), and this technique remains the primary means for obtaining vascular access for IO procedures (Higgs et al. 2005). An over‐the‐needle IV catheter (generally 18 or 22 gauge) is used to puncture the vessel, and the catheter is then advanced into the vessel. When puncturing the vessel, the needle should be advanced into the vessel at a 45° angle (Valji 2006; Stavropoulos et al. 2006). When a sufficient flash of blood has been noted (in humans, this is considered a 4‐ to 6‐inch spurt in a normotensive person), the needle is removed, and a guidewire is introduced into the IV catheter and subsequently into the vessel. The IV catheter is then removed by backing it out of the vessel over the guidewire. A vascular access sheath‐dilator combination is placed over the guidewire and gentle pressure is applied to manipulate the vascular access sheath‐dilator combination into the vessel. A slight twisting motion may be necessary to introduce the vascular access sheath‐dilator combination into the vessel. The dilator is removed from the vascular access sheath over the guidewire.
With the guidewire and vascular access sheath in place, specific vessels can be selected. A catheter is often placed over the guidewire and through the vascular access sheath to perform a myriad of diagnostic and treatment techniques. Agents such as contrast and embolic materials can be injected through the catheter when the area of interest has been identified. Additionally, other catheters and guidewires can be used through the specialized catheter using the coaxial technique.
Natural Orifices
A surgical approach to address malignant obstructions in the trachea, esophagus, colon, and urethra is generally not necessary when pursuing IO treatment options. Furthermore, ureteral stents can be placed cystoscopically, and biliary stents can be placed with endoscopic guidance. For the placement of most tracheal, esophageal, colonic, and urethral stents, the patient is positioned in lateral recumbency on the fluoroscopy table. The fluoroscopy monitor and necessary equipment should be within reach of the interventional radiologist. Standard sets should include an appropriately sized stent, sterile saline, sterile bowls, guidewire, and marker catheter. For esophageal, colonic, and urethral stents, a straight or slightly angled‐tip catheter and iodinated contrast material should be easily accessible. An access sheath is recommended for the placement of urethral stents and should be placed in the distal urethra (Weisse et al. 2006).
Nonvascular Interventional Oncology Techniques
Stents
Tracheobronchial Neoplasia
Tracheobronchial stenosis secondary to neoplasia can result in severe and often life‐threatening clinical signs in human and veterinary patients. When possible, resection of the stenotic region should be attempted (Shin et al. 2006; Withrow 2007). Neoplastic disease may be extensive, however, precluding the successful use of surgery. Intraluminal tracheobronchial stenting has developed as a treatment option in cases where surgery is not recommended or not elected by human patients (Lee 2000; Husain et al. 2007; Kim et al. 2009).
Intraluminal tracheal stenting has been well described in veterinary medicine for the treatment of tracheal collapse (Moritz et al. 2004; Sura and Krahwinkel 2008). Currently, only one case of intraluminal tracheal stenting to treat tracheal neoplasia has been documented in the veterinary literature (Culp et al. 2007). In that cat, a 2.6 cm long tracheal carcinoma was diagnosed, and the owners elected to perform intraluminal tracheal stenting to palliate the clinical signs. The clinical signs were relieved for a period of six weeks until metastatic disease was noted in the pulmonary parenchyma and the owners elected euthanasia. No complications associated with the intraluminal tracheal stenting procedure or postprocedure
