Page 196 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition
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CHAPTER 11  Interventional Oncology  175


           sheaths are often placed intravascularly to allow for serial insertion   the ability of veterinarians to be able to offer advanced procedures
           of instrumentation into a blood vessel without concern for losing   such as stenting. Although stenting procedures are now consid-
                                                                 ered an important component of oncologic therapy, research is
           access. Sheaths can also be created with a “peel-away” design. This
  VetBooks.ir  type of sheath allows for instrumentation that may have a sizeable   still limited.
                                                                   Most of the available literature describing stenting in compan-
           external attachment piece to be introduced into a lumen or cavity.
             One of the main workhorses of most IO procedures is the guide   ion animals is focused on the description of the procedure and
           wire. The guide wire demonstrates much of its utility as a means   initial outcomes in a small cohort of animals; however, experience
           of getting into a location in the least invasive way possible and in   is growing, and our assessment of these procedures should expand
           a relatively safe fashion. The guide wire also acts as means of place-  with that growth. Stenting should be evaluated both from a pro-
           ment of specialized catheters, sheaths, stents, and balloons, as these   cedural and clinical outcome standpoint. In many human trials,
           devices very commonly are designed with an inner cannula that   the technical success of a stenting procedure is determined and
           allows them to slide over a guide wire into position. Guide wires   generally focuses on whether an obstruction could be bypassed
           are available in a variety of lengths, diameters, shapes, and stiff-  and whether a stent could be successfully placed. Clinical success
           ness; these variations are particularly important to consider when   is often defined as stent patency at a defined period of time after
           attempting to gain access through a severely obstructed lumen.  the procedure.
             Specialized catheters are generally used to gain or maintain   Intraluminal stent placement for the treatment of malignant
           access in a luminal structure (including blood vessel) or for estab-  urethral obstruction is likely the most commonly performed vet-
           lishing a coaxial system, by which a smaller catheter or other   erinary IO technique. Urethral stent placement is generally con-
           device can also be placed. Specialized catheters are often used to   sidered an attractive option for the relief of malignant obstruction,
           generate contrast studies when planning ballooning and stenting   as the procedure can be performed with minimal invasion and
           procedures. In companion animals, the catheters used for con-  often rapidly relieves obstruction. In dogs, female cats, and male
           trast studies and vascular procedures are generally 4 to 5 French in   cats that have had a perineal urethrostomy, urethral stents can be
           diameter, although larger catheters may be used to allow for easier   placed via the distal urethra without need for an incision.
           drainage of an obstructed organ. In addition, much smaller cath-  Several studies have described outcomes in dogs and cats
           eters (microcatheters) can be used to access smaller caliber vessels   undergoing urethral stenting. In four canine urethral stent studies,
           during intraarterial chemotherapy delivery or embolization; as an   95 dogs undergoing stent placement to treat malignant urethral
           example, the author commonly uses microcatheters that are 1.7   obstructions have been reported. 2–4  In the first study, both self-
           to 2.4 French in diameter for prostatic embolization procedures.  expanding and balloon-expandable stents were utilized to relieve
             Stents are devices placed into a luminal structure or passage to   obstruction, and stent placement was found to be rapid, safe, and
                                                                                              4
           counteract a disease-induced stricture. The use of stents is com-  effective at restoring luminal patency.  Stent placement was suc-
           monplace in medicine, and the disease processes treated by stent-  cessful in relieving urinary obstruction in 97% of cases in the four
           ing are constantly expanding. There are descriptions of stents used   published studies. 2–4,11  Reported major complications of urethral
           to treat malignant obstructions in the urethra, ureter, trachea,   stenting have included incontinence (26%–37%), reobstruction
           esophagus, colon, and blood vessels in companion animals. 2–10  In   (16%), and stent migration (11%) ; in addition, stranguria
                                                                                             2,3
           addition, stents have been used to treat malignant obstructions of   poststent placement is also a common finding.  Of 19 owners
                                                                                                      2,4
           the biliary tract and nasopharynx in the author’s clinic.  surveyed in one study, 16 were satisfied with the outcome and
             Stents can be simple catheters or other tube-like devices that are   felt that they would recommend urethral stent placement to other
           placed to simply allow for a lumen to be open, or specially designed   owners.   When treatment with nonsteroidal antiinflammatory
                                                                       3
           with varying metals in a variety of configurations and sizes. Stents   drugs (NSAIDs) before or chemotherapy after stent placement
           that contain interstices can be covered or uncovered, and can be   was pursued, median survival times were significantly increased
           delivered with a balloon (balloon-expandable stents) or be self-  versus when medical therapy was not pursued.  Urethral stent
                                                                                                      2
           expanding. If stents are covered, they can be impregnated with anti-  placement has been shown to be successful in relieving malignant
           neoplastic substances such as chemotherapy or radiation beads.  obstructions in cats as well. 12,13
             An embolic agent is a substance placed into a blood vessel to   Ureteral stenting to relieve a malignant obstruction is gener-
           cause temporary or permanent occlusion of blood flow. This altera-  ally required when a lower urinary tract carcinoma grows from
           tion of blood flow could be a partial or total occlusion of the blood   the bladder, urethra, and/or prostate to obstruct the ureteral
           supply depending on the eventual goal. Many different embolic   orifice in the bladder. This procedure can be performed with
           agents have been described, and advances in these products are con-  minimal invasion using ultrasound and fluoroscopic guidance
           tinuing on a regular basis. Currently, major embolic agents com-  (Fig. 11.1). As the ureteral orifice is generally obscured by the
           monly used during the treatment of neoplasia include particles and   tumor, it is usually recommended to percutaneously gain access
           liquid embolics such as cyanoacrylate or ethylene–vinyl alcohol   into the renal pelvis and place a guide wire across the obstruc-
           copolymer, although many other agents may be considered.   tion in an antegrade fashion. Dilation of a tract through the
                                                                 tumor then occurs from retrograde placement of a sheath and
                                                                 dilator. The ureteral stent is also passed retrograde through the
           Interventional Oncology Treatment                     stent and positioned with its pigtails in the renal pelvis and
           Categories                                            bladder. In one study evaluating 12 dogs undergoing ureteral
                                                                 stent placement via percutaneous access, stents were successfully
                                                                                5
           Stenting of Malignant Strictures                      placed in all dogs.  In dogs with azotemia before stent place-
                                                                 ment, improvements in blood urea nitrogen (BUN) and serum
                                                                                                        8
           The use of stents in the treatment of companion animals has   creatinine concentrations were noted in all dogs.  In addition,
           become commonplace over the past decade. Increased availability   all dogs with follow-up ultrasonography demonstrated improved
           of specialized equipment and imaging modalities has improved   hydronephrosis and hydroureter. 5
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