Use of percutaneous microwave ablation in the treatment of retroperitoneal neoplasia in three dogs

William T. N. Culp From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Eric G. Johnson From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Carrie A. Palm From the Department of Medicine & Epidemiology (Palm), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Jenna H. Burton From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Robert B. Rebhun From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Carlos O. Rodriguez Jr From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Michael S. Kent From the Department of Surgical & Radiological Sciences (Culp, Johnson, Burton, Rebhun, Rodriguez, Kent), School of Veterinary Medicine, University of California-Davis, Davis, California;

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Craig B. Glaiberman Department of Clinical Studies, Sutter Medical Center, Sacramento, California (Glaiberman).

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Abstract

CASE DESCRIPTION

3 dogs with retroperitoneal masses (2 renal and 1 located near the diaphragm) were treated by percutaneous microwave ablation (MWA).

CLINICAL FINDINGS

Dogs between 11 and 13 years of age weighing between 13.7 and 43.8 kg had either a renal mass (n = 2) or a mass located in the caudodorsal aspect of the retroperitoneal space near the right side of the diaphragm (1). Cytology revealed that one of the renal masses and the mass located near the diaphragm were malignant neoplasias. Findings on cytologic evaluation of a sample of the other renal mass was nondiagnostic. Maximum mass diameters ranged between 1.4 and 2.5 cm.

TREATMENT AND OUTCOME

All dogs were treated by percutaneous MWA. Probes were directed into tumors by use of ultrasound and CT guidance, and microwave energy was applied to each mass. Findings on imaging of each mass following MWA was consistent with successful treatment. No intraprocedural or major postprocedural complications occurred, and all dogs were discharged from the hospital within 3 days of treatment. Two dogs died at 3 and 21 months after MWA with no known local recurrence; 1 dog was still alive 64 months after treatment.

CLINICAL RELEVANCE

Although the indications for MWA in the treatment of neoplasia in companion animals are limited, the outcomes of dogs in the present report provided preliminary evidence that percutaneous MWA can be safely used to effectively treat retroperitoneal neoplasia. This procedure was successfully performed with image guidance in all 3 dogs.

Abstract

CASE DESCRIPTION

3 dogs with retroperitoneal masses (2 renal and 1 located near the diaphragm) were treated by percutaneous microwave ablation (MWA).

CLINICAL FINDINGS

Dogs between 11 and 13 years of age weighing between 13.7 and 43.8 kg had either a renal mass (n = 2) or a mass located in the caudodorsal aspect of the retroperitoneal space near the right side of the diaphragm (1). Cytology revealed that one of the renal masses and the mass located near the diaphragm were malignant neoplasias. Findings on cytologic evaluation of a sample of the other renal mass was nondiagnostic. Maximum mass diameters ranged between 1.4 and 2.5 cm.

TREATMENT AND OUTCOME

All dogs were treated by percutaneous MWA. Probes were directed into tumors by use of ultrasound and CT guidance, and microwave energy was applied to each mass. Findings on imaging of each mass following MWA was consistent with successful treatment. No intraprocedural or major postprocedural complications occurred, and all dogs were discharged from the hospital within 3 days of treatment. Two dogs died at 3 and 21 months after MWA with no known local recurrence; 1 dog was still alive 64 months after treatment.

CLINICAL RELEVANCE

Although the indications for MWA in the treatment of neoplasia in companion animals are limited, the outcomes of dogs in the present report provided preliminary evidence that percutaneous MWA can be safely used to effectively treat retroperitoneal neoplasia. This procedure was successfully performed with image guidance in all 3 dogs.

Introduction

A 6-year-old 43.8-kg spayed female Labrador Retriever was evaluated for a mass affecting the left anal sac that was found during routine physical examination; no clinical signs had been reported. Results of clinical laboratory diagnostics and findings on thoracic radiography were unremarkable. A fine-needle aspirate of the mass revealed a diagnosis of probable squamous cell carcinoma, and the mass was subsequently removed by left anal sacculectomy by the referring veterinarian. Histologic evaluation revealed a squamous cell carcinoma with a marked desmoplastic fibrous response that was invasive into surrounding lymphatic vasculature and that was narrowly excised.

Because of the invasive nature of the mass, postoperative chemotherapy was recommended and was initiated at the University of California-Davis Veterinary Medical Teaching Hospital (UCD-VMTH). Carboplatin (270 mg/m2) was administered IV, and the patient underwent a recheck physical examination 3 weeks later. At the recheck examination, a recurrence of the squamous cell carcinoma was found in the region of the previous surgery, and mitoxantrone (5 mg/m2, IV) was administered. Owners elected to postpone further chemotherapy and move forward with surgical removal of the recurrent tumor 10 weeks after the original surgery. Histologic evaluation again revealed squamous cell carcinoma in the submitted surgical sample. The dog then returned 2 weeks later to undergo radiation therapy at the site of tumor resection. A total dose of 48 Gy was delivered in 16 daily 3-Gy fractions, with the only complication being severe moist desquamation of the skin overlying the treated region; the desquamation was mostly resolved at 2 weeks after radiation therapy.

During the next 58 months, the patient was reevaluated 32 times, which included 17 abdominal ultrasonographic examinations; other than treatment for pyoderma and generalized atopic dermatitis, no other major medical issues were encountered. When the dog was approximately 11.5 years old, abdominal ultrasonography was performed and revealed a 0.75-cm-diameter hypoechoic nodule in the left kidney. A fine-needle aspiration was performed for cytologic examination that revealed carcinoma. Treatment options that were offered to the owner included serial monitoring with abdominal ultrasonography, laparoscopic or open partial or complete nephrectomy, and percutaneous thermal ablation. The owner elected to monitor the nodule with serial ultrasonography at that time.

Approximately 6 months later, the dog was evaluated at the UCD-VMTH (at the age of approx 12 years). On physical examination, no cardiovascular or respiratory abnormalities were observed. The abdomen was soft and nonpainful on palpation. Multiple subcutaneous masses that had previously been diagnosed as lipomas were evident, as well as multiple areas of alopecia and cutaneous erythema. Findings on thoracic radiographs were unremarkable. On abdominal ultrasonography, the previously identified hypoechoic nodule in the cranial pole of the left kidney had grown to an approximately 1-cm-diameter mass (Figure 1).

Figure 1
Figure 1

Single parasagittal ultrasonographic (A) and transverse CT (B) images of the left renal mass (asterisks) in a 12-year-old spayed female Labrador Retriever.

Citation: Journal of the American Veterinary Medical Association 259, 10; 10.2460/javma.20.09.0505

The owner elected to move forward with treatment with percutaneous microwave ablation (MWA). Prior to anesthesia, a CBC and serum biochemical analysis were performed and the findings were unremarkable. The dog weighed 40.7 kg. Pre-anesthetic medications administered were hydromorphone (0.05 mg/kg, SC) and atropine (0.02 mg/kg, SC) for sedation. Anesthesia was induced with alfaxalone (1 mg/kg, IV) and midazolam (0.25 mg/kg, IV). After induction and intubation, anesthesia was maintained with isoflurane administered in oxygen. The anesthetized dog was transferred to the diagnostic imaging suite that housed a CT scanner (Lightspeed 16; GE Medical Systems) and ultrasound machine (iE 33; Philips Healthcare Solutions).

The dog was placed in dorsal recumbency and was subsequently placed in a slightly oblique position to maintain the left kidney at a higher level than the right kidney. Overlapping (ratio of 0.5:1) 2.0-mm-thick helical CT images were obtained prior to and after administration of a single dose of iodinated contrast media (iopamidol; Isovue-370, Bracco Diagnostics Inc; 370 mg/mL; 740 mg/kg). A pressure injector was used to administer the contrast media IV at a rate of 5 mL/s. Computed tomographic images were reconstructed in soft tissue and bone algorithms. The renal mass was imaged in the cranioventral cortex of the left kidney, was 1.1 × 1.2 × 1.4 cm in size, and was well circumscribed (Figure 1). The mass was minimally hyperattenuated on precontrast images and contrast-enhanced on postcontrast images.

After obtaining CT images, the abdomen was clipped, prepared with aseptic technique, and draped. The ultrasound transducer was then positioned over the region of the left kidney, and the mass in the cranial pole of the left kidney was visualized. Three probe lengths were available for the ablation system (EmPrint Ablation System; Medtronic) that was used, and a 15-cm-length probe was selected. On initial introduction of the probe through the skin, the probe started to bend and it was removed. To avoid breakage of the probe, an introducer was placed percutaneously by use of ultrasound guidance into the abdomen in the area of the mass. A stab incision was made in the skin in line with the left kidney, approximately halfway between the vertebral bodies and the ventral abdominal midline. An 18-gauge over-the-needle catheter (Becton, Dickinson and Company) was inserted into the abdomen, and the needle was removed (Figure 2). A J-tipped guidewire (Infiniti Medical) was inserted through the catheter into the abdomen, and the catheter was removed. An introducer sheath-dilator combination (Infiniti Medical) was inserted into the abdomen over the guidewire, and the dilator and guidewire were removed, leaving the sheath in position. The microwave probe was then placed through the sheath, into the abdomen, and into the mass by use of ultrasound guidance. Ultrasound guidance was used to guide the probe into position to avoid thermal injury to adjacent structures.

Figure 2
Figure 2

Intraprocedural images of the introduction of a sheath and microwave ablation (MWA) probe in the dog of Figure 1. A—An 18-gauge over-the-needle catheter (caret symbol) has been inserted percutaneously into the abdomen, and a J-tipped guidewire (white arrow) has been inserted through the catheter into the abdomen. B—An introducer sheath (asterisk) has been placed in the abdomen over the guidewire, the guidewire has been removed, and the MWA antenna (plus sign) has been inserted into the sheath to facilitate introduction into the abdomen.

Citation: Journal of the American Veterinary Medical Association 259, 10; 10.2460/javma.20.09.0505

A CT scan was then performed to confirm the appropriate probe location within the renal mass (Figure 3) that would result in ablation of the entire lesion; the CT images were obtained in the region of the kidney exclusively. Generally, with a nodule size of approximately 1.4 cm in diameter, a total ablation zone diameter of 3.4 cm is desired; however, because of overall kidney size and surrounding healthy renal parenchyma, an ablation zone with a diameter of approximately 2.5 cm was selected. To achieve this ablation zone, the generator (Becton, Dickinson and Company) power was set at 45 W with an ablation time of 3 minutes. Ablation was monitored by use of ultra-sonography, and echogenic change was observed in the mass secondary to microbubbles generated during treatment (consistent with tissue ablation). No complications were encountered during MWA treatment.

Figure 3
Figure 3

A CT-angiogram multiplanar volume reformation image of the left kidney (LK) of the dog in Figure 1 undergoing MWA of a renal mass. The MWA probe can be seen in place within the tumor.

Citation: Journal of the American Veterinary Medical Association 259, 10; 10.2460/javma.20.09.0505

The probe and sheath were then removed from the abdomen, and the dog recovered uneventfully from anesthesia. Anesthesia time was 135 minutes, and procedural treatment time (from start of the CT scan until probe removal) was 92 minutes. The dog was discharged from the hospital on the same day as treatment, and carprofen (2.2 mg/kg, PO, q 12 h) was administered. Because of the dog's longstanding osteoarthritis, carprofen treatment was to be continued indefinitely. The dog was evaluated 8 times during the next 2 years. Approximately 2 years after MWA, abdominal ultrasonography in conjunction with power Doppler ultrasonography revealed an avascular mass that was slightly smaller in size (0.9 mm maximum diameter), compared with pretreatment measurements. The dog was alive as of 64 months after MWA.

A 13-year-old 19.2-kg castrated male Soft-Coated Wheaten Terrier crossbred dog was admitted to the UCD-VMTH for further evaluation and possible treatment of a mass on the right kidney that had been detected approximately 1 month earlier by the referring veterinarian. The mass was identified incidentally on abdominal ultrasonographic examination during a routine geriatric evaluation.

At the time of evaluation at the UCD-VMTH, the dog was bright, alert, and responsive. Vital parameters (heart rate, respiratory rate, and rectal temperature) were within reference limits, and hydration was adequate. The dog had multiple subcutaneous masses, opacities of both intraocular lenses (suspected to be cataracts), moderate dental disease, and mild alopecia on the dorsal aspect of the tail. The dog had a low-grade, intermittent (grade 1/6) left-sided heart murmur. No arrhythmias were detected, and the dog's femoral pulses were strong, synchronous, and symmetric. The dog had a slightly tense abdomen but did not show overt signs of pain on abdominal palpation. Findings on the remainder of the physical examination were unremarkable.

After admission to the UCD-VMTH, clinical laboratory testing included a CBC, serum biochemical analysis, and urinalysis. Results of the CBC were within reference limits. The serum biochemical analysis revealed azotemia with high serum BUN (46 mg/dL; reference range, 11 to 33 mg/dL) and creatinine (1.7 mg/dL; reference range, 0.8 to 1.5 mg/dL) concentrations. Urine specific gravity was 1.021 without any abnormal findings on urinalysis. Findings on thoracic radiography were within reference limits.

Computed tomography was performed as described for the first dog of this report, and a renal mass was observed on the caudal pole of the right kidney; the renal mass measured approximately 2.2 × 2.5 × 2.5 cm. On CT evaluation the mass was considered most consistent with malignant neoplasia. Degenerative renal changes were observed as well as several renal cortical cysts. Multiple splenic nodules, with benign imaging characteristics, were observed. Additionally, a mass associated with the gastric cardia was evident. The gastric cardia mass was in a location that was inaccessible for a fine-needle aspiration. Treatment of the identified renal mass and gastric mass was discussed with the owner, who elected not to perform an open abdominal exploration with subsequent biopsy and instead chose to treat the renal mass percutaneously with MWA and to monitor the gastric mass.

The dog returned 1 week later for MWA of the renal mass. The dog received morphine (0.5 mg/kg, IM) and atropine (0.02 mg/kg, SC) for sedation. Anesthesia was induced with alfaxalone (1 mg/kg, IV) and midazolam (0.2 mg/kg, IV). After anesthetic induction and intubation, anesthesia was maintained with isoflurane administered in oxygen.

The anesthetized dog was transferred to the diagnostic imaging room for MWA. A 16-gauge biopsy needle (BioPince; Argon Medical Devices) was introduced into the renal tumor 2 times (by use of ultrasound guidance), and biopsy specimens were obtained. The MWA probe was introduced into the abdomen as described for the first dog of this report through an introducer sheath. Computed tomographic images in the region of the kidney were obtained to confirm placement of the probe, and it was noticed that the descending duodenum was abutting the medioventral aspect of the renal mass. An introducer sheath with associated dilator (AccuStick introducer system; Boston Scientific) was placed percutaneously through the body wall over a J-tipped guidewire (Infiniti Medical) at the right flank, bisecting the region between kidney and small intestine. Once the radiopaque sheath tip was in the region of the duodenum, 60 mL of sterile saline (0.9% NaCl) solution was infused, followed by a second infusion of 60 mL of sterile saline solution; finally, 30 mL of air was infused into the space by the kidney to induce anatomic separation between the duodenum and kidney. A sufficient increase in distance between the kidney and duodenum was obtained, and an additional 8 CT scans were obtained to evaluate positioning of the probe within the renal mass; this number of scans was necessary because there was concern about the proximity of the probe to the duodenum, and confirmation of sufficient distance was necessary prior to performing MWA. Microwave ablation was performed 3 times under ultrasound guidance, and echogenic change was observed in the mass secondary to microbubbles generated during treatments (Figure 4). The treatments consisted of the following: 45 W for 3 minutes, 75 W for 3 minutes, and 60 W for 3 minutes. Ultrasonography was used to perform a final urinary tract evaluation, and no abnormalities were observed with the exception of a single structure suspected to be a small blood clot within the lumen of the bladder; it was suspected that renal hematuria induced during ablation caused blood to travel from the renal pelvis antegrade into the bladder.

Figure 4
Figure 4

Intraprocedural image during ultrasound (US)-guided MWA probe (P) placement. The clinician is using the ultrasound transducer (T) to guide the MWA antenna into the renal mass. After placement, CT was performed to assess location prior to treatment.

Citation: Journal of the American Veterinary Medical Association 259, 10; 10.2460/javma.20.09.0505

The probe was removed from the abdomen, and the dog recovered from anesthesia. Anesthesia time was 240 minutes, and procedural treatment time was 110 minutes. The dog recovered uneventfully from anesthesia and was monitored overnight and received a maintenance amount of fluids IV. Focal urinary tract ultrasonography was performed the morning following MWA. A small amount of subcapsular hemorrhage was present on the right kidney in the area of the ablation. Serum biochemical analysis performed the next day showed slightly progressive azotemia with high serum BUN (46 mg/dL) and creatinine (2.5 mg/ dL) concentrations. The dog demonstrated no abnormal clinical signs and was discharged from the hospital 2 days later with a serum BUN concentration of 39 mg/dL and serum creatinine concentration of 2.5 mg/dL. Histologic examination results of the biopsy specimens were inconclusive for neoplasia. Cells in the submitted sample had bland morphological features with minimal anisocytosis and anisokaryosis with no mitotic figures; however, the tissue was fragmented and compressed, preventing a diagnosis of malignancy.

The dog was evaluated 3 months later for lethargy of 1 month's duration. On serum biochemical analysis, the serum creatinine (1.6 mg/dL) concentration had improved. Abdominal ultrasonography revealed that the renal mass was avascular and slightly decreased in size from pretreatment measurements; 2 planes were measured and revealed the mass to be 2 × 2.3 cm. However, multiple enlarged lymph nodes were observed throughout the mesentery and in the inguinal region. The most likely differential diagnosis was metastatic neoplasia. The owner elected not to perform aspiration of the lymph nodes. Additionally, mild bilateral pleural effusion was evident on thoracic radiographs. The previously detected gastric mass, which had a benign appearance during the previous ultrasonographic examination, now appeared to be extending through the gastric wall and obscuring the normal mural architecture of the gastric wall, suggesting a more aggressive process. During the next week, the dog's lethargy progressed and severe vomiting and diarrhea developed. The owners elected euthanasia and did not authorize a necropsy.

An 11-year-old 13.7-kg spayed female Australian Cattle Dog–Australian Shepherd crossbred dog was evaluated by the Neurology Service at the UCDVMTH for cognitive dysfunction, signs of back pain, and focal seizures manifested by right-sided facial muscle tremors and slight head movements. Brain and vertebral column MRI scans were performed. Multifocal intervertebral disk disease was diagnosed, and no intracranial abnormalities were identified. A retroperitoneal mass located near the dorsal aspect of the right side of the diaphragm was observed. Ultra-sound-guided aspiration of the mass was performed; cytologic examination of the aspirate specimen resulted in a diagnosis of malignant neoplasia with a suspicion of a neuroendocrine tumor. On immuno-chemical staining, smear specimens of the mass were strongly vimentin and chromogranin positive and cytokeratin negative. The dog was discharged from the hospital with a plan to return for treatment of the retroperitoneal mass.

The dog was reevaluated 2 months later and was bright, alert, and responsive. Findings on a CBC, serum biochemical analysis, and urinalysis were all within reference limits. No evidence of pulmonary metastatic disease was evident on thoracic radio-graphs. A diffuse bronchointerstitial pulmonary pattern was observed, likely reflecting age-related changes, although a component of chronic inflamma-tory airway disease was also possible. A suspect sliding hiatal hernia was also noticed. A grade 3 of 6 left apical systolic murmur was auscultated. On echocardiography, thickening of the mitral valve with regurgitation and moderate left ventricular wall thickening was found.

The dog received butorphanol (0.3 mg/kg, IM) and atropine (0.02 mg/kg, SC) for sedation. Anesthesia was induced with etomidate (0.35 mg/kg, IV) and midazolam (0.3 mg/kg, IV). After induction and intubation, anesthesia was maintained with isoflurane administered in oxygen to effect.

The anesthetized dog was transferred to the diagnostic imaging suite for CT and MWA. Computed tomography was performed as described for the first dog of this report. A 1.0 × 1.2 × 2.3-cm mass was observed in the retroperitoneal space near the right dorsal aspect of the diaphragm (Figure 5). Additionally, a mammary mass was found that had not been previously palpated. A 16-gauge biopsy needle (BioPince; Argon Medical Devices) was introduced into the retroperitoneal tumor (with ultrasound guidance), and a biopsy specimen was obtained. The MWA probe was introduced into the abdomen as described; the probe was subsequently guided into the mass by use of ultrasonography. The position of the probe within the mass was confirmed on CT evaluation of the retroperitoneal region from the kidney to the diaphragm. The generator (Becton, Dickinson and Company) power was set at 45 W with an ablation time of 3 minutes with the goal of generating an ablation zone that would extend beyond the mass. Ablation was performed 2 times, with each treatment being monitored with ultrasound guidance, and echogenic change was observed in the mass secondary to microbubbles generated during treatments. After ablation, a CT scan was again performed (Figure 5) to confirm the area of treatment and to evaluate for any complications; none were observed. Anesthesia time was 210 minutes, and procedural treatment time was 120 minutes.

Figure 5
Figure 5

Single transverse postcontrast CT images from an 11-year-old spayed female Australian Cattle Dog–Australian Shepherd crossbred dog with a retroperitoneal mass (asterisks) near the diaphragm. The mass can be seen prior to (panel A) and after (panel B) MWA. Notice that the image obtained after MWA (panel B) lacks central contrast enhancement indicating a positive therapeutic response.

Citation: Journal of the American Veterinary Medical Association 259, 10; 10.2460/javma.20.09.0505

The dog recovered from anesthesia without any complications and was discharged from the hospital 2 days after treatment. Findings on cytologic examination of the biopsy specimen of the retroperitoneal mass were nondiagnostic. Fine-needle aspiration of the mammary mass had been performed prior to hospital discharge; cytologic examination of the aspirate confirmed the presence of a mammary carcinoma. The mammary mass was removed approximately 2 months after ablation, and histologic evaluation revealed a mammary adenocarcinoma.

The dog was regularly evaluated after treatment with physical examinations and abdominal ultrasonography. There was progressive deterioration of the neurologic status; however, clinical signs related to the retroperitoneal mass were not found. Abdominal ultrasonography performed 5 and 15 months after MWA showed no evidence of the retroperitoneal mass. The dog was euthanized 21 months after treatment for worsening neurologic status; a necropsy was not performed.

Discussion

Results of this case series demonstrated that MWA could be successfully used to treat retroperitoneal neoplasia. Although technically challenging, this percutaneous, minimally invasive procedure was successfully performed with image guidance. Additionally, dogs experienced minimal complications secondary to MWA, hospitalization was short following MWA, and the primary treated tumors did not progress in any dog during the follow-up periods.

The treatment of neoplasia in humans through chemical or thermal ablation is an established technique. Chemical ablation is less commonly pursued in the treatment of neoplasia, with most reports describing the use of absolute ethanol ablation.1,2 Thermal ablation uses either heating or cooling to cause tumor cell necrosis. The major thermal ablation techniques that cause tumor necrosis by heat are radiofrequency ablation, laser ablation, high-intensity focused ultrasound, and MWA.1,2,3 Microwave ablation is performed by the insertion of a probe that can transmit microwave energy directly to tumor cells.1 The transmission of microwaves agitates water molecules within tumor cells, causing friction and heat and subsequent cell death.1

The advantages of MWA over other thermal ablation techniques have been described.1 Radiofrequency ablation generates heat by creating an alternating electrical current between the probes that are inserted into the tumor and the grounding pad on which the patient is placed. The grounding pads used during radiofrequency ablation require a relatively large surface area of hairless skin to allow for sufficient grounding pad contact to prevent burns secondary to incomplete grounding. Microwave ablation uses electromagnetic radiation that produces no electrical current, and therefore, grounding pads are not necessary.1,3 Additionally, the electrical current associated with radio-frequency ablation results in water vapor generation, desiccation, and charring, resulting in increased tissue impedance that decreases the temperature that can be achieved in a tumor. Microwave ablation is not affected by these factors and can therefore generate higher intratumoral temperatures and pressures that improve tumor penetration.1,3,4 Microwave ablation is also able to generate larger ablation zones, compared with radiofrequency ablation, which results in decreased MWA applicator use (ie, less probes and less treatment times) and a greater ability of MWA to obtain a tumor ablative treatment margin.3,4 Lastly, MWA is generally faster than radiofrequency ablation and is not affected by heat sinks.4

In humans, MWA has been applied in many different locations, with techniques described for intra-abdominal percutaneous treatment of numerous tumors including those of the pancreas,5 kidneys,6,7,8 and liver.9 Microwave ablation is particularly suited for the treatment of renal tumors, and the reported success rate in humans is high.10,11,12 The effectiveness of MWA is dependent on many factors. Generally, MWA is reserved for small masses (< 4 cm in diameter) that can be targeted with image guidance. Image guidance can be performed with direct visualization or palpation (ie, through open or endoscopic surgery) or percutaneously, if an appropriate image can be achieved and the mass is accessible. In humans, ablation techniques are often chosen for patients who are not suitable candidates for open surgery or have declined open surgery.8,12,13

There are many studies7,10,11,12,14,15 comparing the outcomes of surgery versus MWA and other thermal ablation techniques (eg, cryoablation and radiofrequency ablation). For small renal tumors (similar to tumors for which MWA would be considered), surgical techniques are generally focused on partial nephrectomy or nephron-sparing surgery.8 When treating small renal tumors, the choice of which treatment to pursue is often clinician dependent. Major factors to consider include patient-related factors (eg, age and comorbidities), surgical complications (eg, bleeding, urine leakage, and urinoma development), tumor control, and overall survival time. All dogs of the present report had small tumors that could be accessed percutaneously with image guidance, and several had comorbidities that made more invasive open surgeries less ideal. For the dogs of the present report, the primary goal in treating with MWA was to obtain local tumor control with minimal morbidity and minimal hospitalization times.

In contrast to the abundant data in humans, descriptions of thermal ablation in veterinary patients are rare. In particular, descriptions of MWA in companion animals are mostly limited to MWA applied during surgery (both open and minimally invasive)16,17 or in a research setting.18,19 Ten clinical cases have been reported, including 7 dogs with liver neoplasia16,20 and 3 dogs with a metastatic pulmonary lesion.17,21 Recently, a percutaneous technique for treating 2 dogs with pulmonary metastatic lesions was described.21 Although some minor complications were encountered, both of those dogs were effectively treated and were discharged from the hospital.21 To the authors' knowledge, the present report is the first on percutaneous MWA of retroperitoneal masses in dogs.

Technical aspects and limitations of MWA should be considered before these procedures are performed in companion animals. First, it is important to understand the specific ablation system used during MWA, as ablation zones vary with each individual system. The amount of tissue undergoing ablation and thermal damage is dependent on the power (in W) and the time that the energy is applied. Generally speaking, when either the power or time is increased, the ablation zone will increase, so both factors need to always be prescribed carefully. Thermal profiles have been established on the basis of research done in ex vivo samples, which can help to guide clinical decisions regarding each ablation zone. The goal of MWA is to treat the entire tumor as well as a margin of normal tissue surrounding the tumor whenever possible; this is not always possible in some situations because of tumor location within the treated organ or because of close proximity of critical nontarget tissues. Additionally, MWA requires a strong understanding of image guidance and the ability to direct a microwave probe into a tumor without causing trauma to adjacent organs while also avoiding non-target structures within the organ, such as the renal pelvis and blood vessels in the kidney. Finally, it is important to remember that MWA is not appropriate for all cases. Some masses may be too big and therefore out of the range of the ablation zone of a particular probe or in a location where the entire mass cannot be effectively treated.

Several limitations of the present report should be considered. Only 3 dogs were included in this cohort, and unfortunately, although imaging results were highly suggestive of the presence of neoplasia, a complete histopathologic diagnosis was only obtained for 1 of the 3 dogs. The equipment used during MWA is expensive and requires extensive training and expertise. Additionally, during MWA procedures, it is likely that a large number of CT scans may be performed, thus increasing radiation exposure. Attempts should be made to limit this exposure to both the patient and operators. Lastly, because of the delayed timing of advanced diagnostics, biologic behavior of many companion animal tumors, and stoic nature of veterinary patients, many tumors in locations (eg, lung, liver, and kidneys) that may be considered for percutaneous MWA in humans are too large in veterinary patients for this modality at the time of diagnosis.

In conclusion, minimally invasive percutaneous MWA was successfully used to treat retroperitoneal neoplasia in 3 dogs. The success described for the dogs of the present report should be used to cautiously consider application of this modality in patients with tumors that are of appropriate size to permit complete ablation and that have a location that allows for percutaneous image-guided access with low risk of damage to adjacent nontarget organs. Future investigation is needed to evaluate MWA in companion animals, as well as other thermal ablation techniques.

Acknowledgments

No external funding was used in this report. The authors declare that there were no conflicts of interest.

The authors thank Rodney Burrell for technical assistance during the performance of the MWA procedures.

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