Interventional radiology (IR) and interventinal oncology (IO) involve the use of contemporary imaging modalities such as fluoroscopy, endoscopy, ultrasound, CT, and MRI to gain access to different structures in order to deliver materials for therapeutic purposes. IO is a subspecialty of radiology in human medicine and techniques have been widely utilized in the last 30 years to produce minimally invasive diagnostic and therapeutic outcomes. Applications of IO in veterinary medicine are also being performed for the same purpose. This is a rapidly evoloving area of veterinary medicine and combines the expertise of many specialties.
Minimally invasive therapeutics offer the advantages of smaller incisions, decreased pain, shortened anesthesia times and shorter length-of-stay compared to traditional open surgical approaches. Currently in veterinary medicine, laproscopy, thoracoscopy, minimally invasive orthopedic procedures, endourology, interventional radiology and interventional oncology are meeting this demand.
The primary disadvantages of IO include the required technical expertise, the specialized equipment necessary (fluoroscopy with or without digital subtraction capabilities), and the large initial capital investment necessary to provide a suitable inventory of catheters, guidewires, balloons, stents and coils.
Specific IO Interventions
Malignant Urethral Obstruction
Transitional cell carcinoma, prostatic carcinoma, and other intrapelvic neoplasia may result in urethral obstruction. Traditional therapy has been centered on diverting urine via surgical placement of a cystostomy tube while pursuing traditional tumor-directed therapies. Cystostomy tube placement requires surgery and requires significant owner maintenance for the duration of the pet’s life. In addition, complications including tube dislodgement and recurrent, frequently multi-drug resistant urinary tract infection are not uncommon. Using IO techniques, an intraluminal self-expanding metallic urethral stent can be placed (non-surgically) via the vulva or penis to open the urethral lumen. Note that stents for this purpose are very different than those used for other applications. Using fluoroscopy, the length and width of the obstruction can be very precisely measured and a stent of an appropriate length and width to span the obstruction chosen. The stent is deployed from a delivery system introduced via the urethral orifice. Patients are able to urinate immediately after stent placement. The greatest complication of the procedure is incontinence. The overall incidence of incontinence after stent placement is <25% with varying degrees of incontinence reported in cases. In the authors experience, no patients have died in the short term due to recurrent urethral obstruction. However, in patients with urethral or bladder sarcoma, the local disease has progressed rapidly and tumor growth through the interstices of the stent has been observed. This highlights the necessity to obtain a diagnosis prior to stent placement. With the symptom of the neoplastic condition palliated, chemotherapy, radiation therapy or other adjunctive treatments may be utilized to address the underlying neoplasia. Most dogs with neoplasia of the lower urinary tract do not develop obstruction. However, stranguria and poillakiuria accompanied by an enlarged bladder is consistent with partial or complete obstruction of the lower urinary tract. In cases of partial obstruction, some oncologists will prefer to treat with piroxicam and traditional chemotherapeutics to shrink the tumor and palliate the signs of obstruction while urine is diverted via a urinary catheter. Alternatively, and in cases of complete obstruction, after medical stabilization, a palliative measure (urethral stent or cystostomy tube) for relief of the obstruction is indicated.
A retrograde positive contrast urethrocystogram in a 12 year old, male castrated Labradore retriever. There is contrast extravasation into the prostatic tumor and a proximal urethral obstruction secondary to a prostatic carcinoma. A marker catheter is in the colon within a red rubber catheter. Measurements for width and length will be performed to select an appropriately sized self-expanding metallic stent.
Ureteral Stenting for Malignant Obstruction
Ureteral stents are double pigtail, multiple fenestration, tubes made from plastics, silicone, or silastic. Stents have been placed successfully as a long-term treatment option in veterinary patients. Such stents are most often placed in an antegrade manner because the tumor often obstructs visualization of the ureteral papilla for a retrograde placement. The antegrade technique requires percutaneous pyelocentesis with a renal access needle or an over-the needle IV catheter (18-G in dogs; 22-G in cats). This can be performed using ultrasound or fluoroscopy. The guidewire is passed down the ureter guided by an ureteropyelogram and into the urinary bladder and out the penis or vulva. The stent is then placed in a retrograde fashion over the wire to keep the hole in the kidney as small as possible. These procedures can also be accomplished intraoperatively. Successful placement of ureteral stent in cats and dogs is high.
An Inifinit Medical ureteral stent used to relieve malignant ureteral obstructions.
A left lateral radiograph of a patient post ureteral stent placement for unilateral ureteral orifice obstruction from a trigonal transitional cell carcinoma.
Partial or Complete Bening or Malignant Tracheal Obstruction
Tracheal stenting offers a non-invasive method of immediately restoring luminal diameter and airflow across both the intra- and extra-thoracic trachea or the mainstem bronchi during a short anesthetic period. This can be performed for intraluminal or extraluminal airway obstructions. Indications include patients with diffuse disease, nonresectable intra- or extraluminal disease, patients intolerable to invasive surgery, or for pet owners electing non-surgical options to alleviate airway obstructions. Recommendations on stent type and size vary based on the discretion of the clinician performing the procedure. In most cases, the use of a woven nitinol reconstrainable foreshortening stent that spans the entire region of obstruction and at least 1 cm of airway in front of and behind the obstruction is recommended. To prevent migration within the trachea, a stent is often chosen that is 10-20% greater than the maximal tracheal diameter. Airway diameter measurements obtained from radiographs, fluoroscopy, bronchoscopy and CT have been used to select the appropriate stent size.
The author prefers to induce the patient in the fluoroscopy suite. Anesthesia should include a preanesthetic (if appropriate for the individual) and preoxygenation. Propofol induction is generally appropriate in most patients. Alternative induction protocols tailored to patient needs and concurrent problems are always appropriate. The author always performs a thorough laryngeal examination to identify laryngeal collapse or another laryngeal pathology/dysfunction prior to placement of the tracheal stent. A large endotracheal tube (>4mm internal diameter) with a radiopaque marking all the way to the tip should be utilized. Initially, the endotracheal tube may need to be placed relatively distally down the trachea to bypass the area of collapse. This may not be possible in dogs with diffuse tracheal collapse. Monitoring should include ECG, SpO2, blood pressure, and ETCO2 if possible.
Right lateral radiograph of a 9-year-old, male castrated, beagle mix that presented for dyspnea and an upper airway obstruction. The patient was intubated and diagnosed with a non resectable tracheal carcinoma. A palliative tracheal stent was placed to provide a patent airway. The patient was euthanized 4 months later secondary to clinical signs associated with metastatic disease.
Transarterial Bland Embolization (TAE)/Transarterial Chemoembolization (TACE)/Intraarterial Chemotherapy (IA) of Liver Tumors
The techniques and responses of liver tumors to transarterial embolization/chemoembolization has been described and performed for about a decade in small animal patients. Most of the techniques have been adapted from human medicine. The vascular anatomy of the liver makes it an ideal candidate for such procedures. Most the blood supplying the liver comes from the portal vein, but tumors of the liver receive the majority, if not all, of their blood supply from the hepatic arterial system. This allows for targeting of the major tumor blood supply while preserving blood flow to the normal liver.
To perform liver embolization/chemoembolization, the arterial blood supply is accessed from the femoral artery. Utilizing fluoroscopy and angiographic studies with digital subtraction angiography, the hepatic artery, a branch of the celiac artery, is selected, and an angiographic map of the hepatic tumors blood supply is generated. Once the hepatic arterial system supplying the tumor is identified, an embolic agent (with or without chemotherapy) can be delivered to the tumor. It is important to identify the gastroduodenal artery (a branch of the hepatic artery) and avoid embolization of this artery and its terminal branches. It should be noted that the first line of therapy for liver tumors is surgical resection. Transarterial therapeutic modalities are reserved for patients with nonresectable liver tumors as is defined by ultrasound, CT, and/or abdominal exploratory.
TAE and IA chemotherapy have been performed in small animal patients with nasal tumors, liver tumors, and urogenital tumors as well as some musculoskeletal tumors. A thorough understanding of the vascular anatomy is essential to avoid non target tissue or organ embolization.
Prostatic artery embolization and chemoembolization have been performed in the treatment of prostatic carcinoma in dogs. This is a successful technique that reduces tumor size overtime. If the patient has obstructive disease from the tumor, it is common that the urethra is first stented with a SEMS to provide a patent urethral lumen and then embolization performed after the patient has been stabilized and the stent allowed to be incorporated in to the urothelium. Bladder tumors have also been treated with transarterial chemotherapy. The cystic artery supplying the tumor should not be embolized but can be accessed through the carotid artery or by using a special reverse curve catheter placed through the femoral artery. Once the cystic artery is selected, chemotherapy can be directly delivered to the tumor. TAC of urinary bladder tumors uses similar doses of systemic chemotherapy but the systemic effect is greatly reduced, leading to less side effects and, in some cases, a more profound effect on local tumor control.
Ablative techniques for IO are either chemical or thermal in nature. Chemical ablation involves intratumor injection of liquid agents (like ethanol or chemotherapy). Thermal ablation uses heat energy or freezing agents applied directly to a tumor to incite cell death. The most common modalities for thermal ablative techniques include radiofrequency ablation (RFA), cryoablation, microwave ablation, and laser ablation. Cryoablation uses varying freeze-thaw cycles and results in the development of intracellular ice crystals and cellular dehydration, both of which lead to cellular damage and death. RFA, cryoablation, and microwave ablation use probes that are introduced into tumor tissue percutaneously or through a natural orifice. Guidance of the probes into the appropriate location is performed with image guidance, which may include fluoroscopy, CT, or MRI, although ultrasound is generally the most utilized imaging modality in veterinary medicine. These procedures can also be performed with direct access to the tumor via laparotomy, thoracotomy, or with endoscopic-guidance. Liver tumors, renal neoplasia, bone tumors, and nasal tumors are examples of disease in which ablative techniques have been used in veterinary medicine. Microwave ablation and laser ablation have tremendous potential in IO but are newer modalities in small animal medicine.
Dr Steve J. Mehler DVM, DACVS
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