such as ethanol, hot saline, and acetic acid. Additionally, high‐intensity focused ultrasound is showing promise in successful treatment of certain tumors, including prostatic neoplasia (Rebillard et al. 2008). These procedures can be performed with minimally invasive image‐guided techniques. Multiple tumor types have been treated with ablation, and the research into the use of tumor ablation is vast and growing infinitely. Because many human tumors are diagnosed at a stage when resection is not possible, tumor ablation offers an alternative option for treatment, and this therapy may also benefit our veterinary patients.
RFA has been the most studied form of ablative therapy (Pentecost 2006). RFA can be performed with either monopolar or bipolar radiofrequency systems (Pentecost 2006). An electrical pole (or poles in bipolar systems) located on a probe is placed within the tumor tissue (Rose et al. 2006; Mahnken et al. 2009). Radiofrequency waves are converted to heat, and this thermal damage causes subsequent tissue destruction (D’Ippolito and Goldberg 2002; Kunkle and Uzzo 2008). Control of the area that is exposed to RFA is essential to prevent damage to surrounding normal tissue (D’Ippolito and Goldberg 2002). In human medicine, general recommendations for focal ablation with RFA are to aim for homogenous necrosis of the entire tumor, as well as a surrounding region of normal tissue at least 1‐cm thick (Rose et al. 2006).
Most clinical experience with RFA has been with treatment of hepatic malignancies, particularly hepatocellular carcinoma. Treatment of metastatic colonic disease has also been reported (Lencioni et al. 2009; Padma et al. 2009). RFA is considered by some as the treatment of choice for early‐stage, nonresectable hepatocellular carcinoma (Lencioni et al. 2009). Across several studies, RFA has been shown to be an effective treatment modality for liver tumor destruction and results in an acceptable level of morbidity (Lencioni and Crocetti 2007). Other tumor types that are being treated with RFA include renal, pulmonary, and breast tumors (Noguchi et al. 2006; Rose et al. 2006; Abbas et al. 2009; Carraway et al. 2009; Raman et al. 2009; Vogl et al. 2009). RFA has been used to treat canine primary hyperparathyroidism (Pollard et al. 2001) and feline hyperthyroidism (Mallery et al. 2003), but the clinical use of RFA to treat malignant neoplasia in companion animals has yet to be investigated.
Cryoablation uses alternating freeze–thaw cycles that cause intracellular ice crystal formation, cellular dehydration, and microcirculatory failure, which result in ischemia and cytotoxicity (Vestal 2005; Raman et al. 2009; Vogl et al. 2009). Cryoablation is being successfully used to treat small renal masses, liver neoplasia, and prostatic neoplasia, as well as bone and soft tissue metastases in humans (Vestal 2005; Kunkle and Uzzo 2008; Callstrom and Kurup 2009; Padma et al. 2009; Raman et al. 2009). Improved outcome with decreased morbidity has been reported with the use of cryotherapy to treat prostatic neoplasia (Vestal 2005).
Microwave and laser ablation are newer ablation strategies, but the use of these systems is growing quickly as the clinical utility is being discovered. Microwave ablation works by heating the water molecules in tissues, with resultant coagulation necrosis and cell death (Simon et al. 2005; Abbas et al. 2009). Microwave ablation has been evaluated in the treatment of liver, lung, kidney, adrenal gland, and bone neoplasia in humans (Simon et al. 2005; Lencioni and Crocetti 2008; Moser et al. 2008; Abbas et al. 2009). Laser ablation is performed with a neodymium‐YAG laser. This device elevates the temperature of tumor tissue and also results in coagulative necrosis (Vogl et al. 2009). In the author’s clinic, a focal renal tumor has been treated successfully with microwave ablation. In that case, no long‐term complications were encountered, and follow‐up ultrasound evaluations demonstrated no tumor recurrence (Culp et al. 2017). Additionally, six other clinical cases have been reported, including five dogs with liver neoplasia and one dog with a metastatic pulmonary lesion (Mazzacari et al. 2017; Yang et al. 2017). No complications were reported in those cases (Mazzacari et al. 2017; Yang et al. 2017). The metastatic pulmonary nodule was secondary to a scapular and humeral osteosarcoma; additionally, this dog was diagnosed with hypertrophic osteopathy. After microwave ablation, the patient demonstrated resolution of the clinical signs of hypertrophic osteopathy (Mazzacari et al. 2017). Clinical reports of laser ablation are lacking, but proposed applications include liver, lung, and bone tumors (Pacella et al. 2001; Lencioni and Crocetti 2008; Moser et al. 2008; Vogl et al. 2009).
PEI with ultrasound guidance is a well‐described technique for treating hepatocellular carcinoma but has also been used to treat other malignant neoplasia in humans (Lencioni and Crocetti 2008; Moser et al. 2008; Mahnken et al. 2009; Padma et al. 2009). Alcohol stimulates coagulation necrosis as it induces cellular dehydration and causes thrombosis‐induced ischemia (Lencioni and Crocetti 2008; Moser et al. 2008; Mahnken et al. 2009). PEI has the advantage of low morbidity but may not cause complete tumor necrosis as the ethanol spreads inhomogeneously (Mahnken et al. 2009). Another disadvantage of PEI is that a margin of normal tissue surrounding the tumor is not treated, causing small satellite lesions to be missed (Mahnken et al. 2009). PEI has recently been described as a treatment method for dogs with primary hyperparathyroidism; however, in comparison to surgery and heat ablation, PEI had less successful outcomes and more side effects (Rasor et al. 2007).
Vascular Interventional Oncology Techniques
Stents
Vascular stents can be placed as a palliative means of recanalizing vascular obstructions that occur secondary to neoplasia (Mónaco et al. 2003; Novellas et al. 2009). After placement of the stent, clinical signs often resolve (Mónaco et al. 2003; Novellas et al. 2009). Malignancy is the cause of superior vena cava syndrome (obstruction of the superior vena cava preventing venous return) in 90% of human cases, and the placement of vascular stents has become the treatment of choice (Mónaco et al. 2003).
Budd–Chiari syndrome occurs when venous flow at the level of the hepatic veins and/or vena cava is obstructed (Cura et al. 2009; Xue et al. 2009). The success rate of stent placement in humans as manifested by alleviation of pleural effusion, ascites, and hepatomegaly has been reported to be up to 97% (Qiao et al. 2005; Xue et al. 2009). Three dogs undergoing stent placement to treat Budd–Chiari syndrome have been reported (Schlisksup et al. 2009). All dogs had confirmed or suspected neoplasia causing Budd–Chiari syndrome. A stent or stents were placed into the left hepatic vein and/or the caudal vena cava to relieve the obstruction. All dogs experienced relief of clinical signs, and survival time ranged from 7 to 20 months (Schlisksup et al. 2009).
Intraarterial Chemotherapy
Conventional chemotherapy involves the administration of a drug into a peripheral vein, which results in systemic dosing of the drug. When given intravenously, the drug also undergoes dilution prior to reaching the tumor (Vogl et al. 2008). The administration of intraarterial chemotherapy by interventional radiologists is common practice in human medicine. The major reason for administration of chemotherapy directly into a tumor’s arterial supply is that a higher concentration of drug can be accumulated locally (at the tumor site), with fewer systemic side effects (von Scheel and Golde 1984; Mortimer et al. 1988; Sileni et al. 1992; Kovács et al. 1999; Furutani et al. 2002). Intraarterial chemotherapy has not gained universal acceptance, likely due to variable success rates; however, head and neck malignancies have shown an overall response rate of 82–95% when treated with intraarterial chemotherapy (Mortimer et al. 1988; Sileni et al. 1992; Korogi et al. 1995).
Intraarterial chemotherapy administration has been evaluated in canine patients with lower urinary tract neoplasia (Culp et al. 2015b). In that study, a comparison between dogs receiving intraarterial chemotherapy and dogs receiving conventional intravenous chemotherapy was performed, and subsequent changes in tumor size were determined. Dogs undergoing intraarterial chemotherapy demonstrated a significantly greater decrease in several measurements including length, length percentage, width percentage, longest unidimensional measurement, and longest unidimensional measurement percentage. Additionally, adverse events such as anemia, lethargy, and anorexia were significantly less likely to occur in the intraarterial chemotherapy group vs. the intravenous chemotherapy group. Superselection of the arterial blood supply to the lower urinary tract was shown to be feasible, with minimal complications (Culp et al. 2015b).
In humans, intraarterial chemotherapy is often administered