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3 Interventional Oncology
William T.N. Culp
Interventional radiology (IR) is a specialty that uses different imaging modalities to direct minimally invasive diagnostic and therapeutic procedures. IR has become a well‐established and integral specialty in human medicine and is rapidly growing in veterinary medicine. The influx of IR techniques in veterinary medicine allows veterinary clinicians the ability to offer patients advanced treatment options that were previously unavailable. Interventional oncology (IO) is a subspecialty of IR that is focused on the treatment of oncologic diseases.
When performing IO procedures, it is essential for the veterinary clinician to have a firm grasp of different imaging modalities and basic surgical procedures, as surgically approaching blood vessels is often necessary. IO procedures such as vascular stenting, intraarterial chemotherapy, and transarterial embolization/chemoembolization are performed intravascularly, and specialized sheaths, guidewires, and catheters are needed for these interventions. Nonvascular diseases such as malignant obstructions and effusions can also be treated with IO techniques and involve the placement of stents and long‐term catheters.
Many of the current applications of IO in veterinary patients are palliative; in these cases, the primary goal is to improve the quality of life while causing minimal morbidity. IO can also provide treatment options in cases that were previously considered untreatable. Reports on the use of IO in veterinary patients are limited, but investigation of IO applications in human medicine offers insight into the vast benefits that this expanding specialty can offer for our veterinary patients. A systematic discussion of the imaging, instrumentation, and techniques involved in IO will be discussed below.
Imaging
Complete knowledge of the vascular anatomy is mandatory for performing vascular interventions. Additionally, the interventional radiologist should have a thorough understanding of the imaging modalities and contrast agents that are used to perform IO procedures. While imaging modalities such as fluoroscopy, computed tomography (CT), and magnetic resonance imaging (MRI) are commonly employed by veterinary clinicians, the use of these modalities for IO treatments is largely unreported, aside from isolated case reports and small case series.
Modalities
Stenting procedures can be performed solely with digital radiography, although fluoroscopy is superior, as it allows for real‐time evaluation of the anatomy. Fluoroscopy is mandatory when performing IO procedures that require vascular interventions. A fluoroscopy unit (C‐arm) with specifications including digital subtraction, road‐mapping ability, collimation, and low patient radiation dosing is ideal. Ceiling mounting should be pursued when possible, and the C‐arm should have the ability to acquire complex oblique views. Newer units allow the interventional radiologist to perform image acquisition and most other C‐arm operations at the bedside, eliminating the need for an assistant to perform these tasks in a control room. Recently, the angiographic anatomy of the abdominal arterial blood supply was reported to provide a guide to individuals that are performing interventional procedures related to the abdominal organs (Culp et al. 2015a). Additionally, that study determined common locations for vascular branching.
While angiography performed with fluoroscopic guidance allows for excellent evaluation of the direction and velocity of blood flow, the images obtained are in two‐dimensional planes and only display the lumen of the vessel (Green and Parker 2003). Computed tomographic angiography (CTA) and magnetic resonance angiography (MRA) are rapidly developing imaging modalities that have certain advantages over fluoroscopy, including noninvasive angiographic image acquisition, less patient postprocedure discomfort, and volumetric and cross‐sectional image analysis (Green and Parker 2003; Hellinger and Rubin 2006; Thornton and Grist 2006). The volumetric and cross‐sectional image analysis that is obtained with CTA and MRA allows vessels to be evaluated in multiple directions with a single scan, whereas several images and injections of contrast are necessary to gain the same information using fluoroscopy (Hellinger and Rubin 2006). Advances are being made that allow CTA and MRA to be performed simultaneously with interventional techniques, which may result in more efficient and accurate IO procedures in the future (Ladd et al. 2000; Hellinger and Rubin 2006; Thornton and Grist 2006; Kos et al. 2008).
Ultrasound has many applications in IR. Many of the disease processes that may require IO treatments can be diagnosed by ultrasonography. In dogs, hepatocellular carcinoma, a tumor commonly treated by embolization or chemoembolization and radiofrequency ablation (RFA) in humans (Okusaka et al. 2009; Hiraoka et al. 2010), is easily identified by abdominal ultrasound (Liptak et al. 2004b). In one study of dogs with hepatocellular carcinoma, diagnosis of a hepatic mass was made with ultrasound in 93.5% of cases (Liptak et al. 2004b; Dernell et al. 2004). Other tumors that may require an IO treatment, such as urethral, colonic, and thyroid neoplasia, can also be evaluated by ultrasound (Hume et al. 2006; Weisse et al. 2006; Barber 2007). In a recent case series of dogs with masses obstructing hepatic venous outflow, masses were often identified by ultrasonography (Schlisksup et al. 2009). In addition to the diagnostic utility of ultrasound, this modality is regularly employed in humans to aid in obtaining vascular access (to initiate the Seldinger technique) during the performance of IR procedures (Longo et al. 1994; Dodd et al. 1996; Ahmad et al. 2008; Arthurs et al. 2008). Ultrasound can also be used to perform procedures, including the placement of drainage catheters, percutaneous biopsies, percutaneous ethanol injection (PEI) of hepatic neoplasia, and RFA (Longo et al. 1994; Dodd et al. 1996; Solbiati 1998).
Contrast Agents
Contrast agents are required components of most intravascular (IV) procedures and many stenting procedures. The predominant contrast agents used for angiography include iodinated agents (fluoroscopy, CT), gadolinium‐based agents (MRI), and carbon dioxide (CT) (Ehrmann et al. 1994; Moresco et al. 2000; Spinosa et al. 2000, 2001; Brown et al. 2003; Namasivayam et al. 2006;
