Introduction

Simple renal cysts (SRCs) occur in both people and dogs, most commonly as incidental findings with unclear impact on renal function.^1–3 Clinical signs seen with SRCs include systemic hypertension, urinary tract obstruction, and abdominal pain.^1,2,4 Treatment of SRCs is recommended when clinical signs are present; percutaneous drainage with ethanol sclerotherapy is the standard of care for clinical SRCs in people, with literature suggesting low complication rates. Laparoscopic decortication and omentalization is advocated for treatment of recurrent SRCs in people but has not been reported in dogs.^5,6

Renal tumors are uncommon in dogs, and renal adenomas are exceedingly rare. A single case report describes a large renal adenoma found on necropsy in a 5 yr old dog, and a retrospective study of 82 dogs with renal tumors found no renal adenomas.^7,8 The objective of this case report is to describe an uncommon presentation and novel treatment of a cystic renal adenoma and to thus aid in the understanding and management of these tumors.

Case Report

A 12 yr old 13.5 kg (29.7 lb) male castrated Pembroke Welsh Corgi was referred to the NC State Veterinary Hospital for further evaluation of a suspected renal cyst at the cranial pole of the left kidney. The renal cyst was initially diagnosed ultrasonographically by the primary care veterinarian after the dog developed lethargy, abdominal pain, and trembling of 6 wk duration. A complete blood count and serum chemistry panel were unremarkable. A urinalysis revealed a urine specific gravity of 1.030 with an unremarkable sediment; urine aerobic culture was negative. Symmetric dimethylarginine levels were elevated at 20 μg/dL (reference range, 0–14 μg/dL) at that time. Radiographic findings included the presence of a mass effect in the region of the left kidney. Ultrasound findings showed a thin-walled, hypoechoic structure measuring approximately 8 cm in diameter, most consistent with a fluid-filled cyst associated with the cranial pole of the left kidney. The dog was transferred to a veterinary referral center where 250 mL of brown, turbulent fluid was drained percutaneously with ultrasound guidance. Cytology of the cystic fluid was consistent with prior hemorrhage with no evidence of inflammation, pathogenic organisms, or neoplasia. Aerobic culture of the cystic fluid was negative. The dog’s clinical signs temporarily abated after drainage but returned within a few weeks, at which time repeat ultrasound revealed the recurrence of the cystic lesion, along with a small amount of free perinephric fluid. Cytology of the perinephric fluid was similar to the previously aspirated cystic fluid. A volume of 150 mL of cystic fluid was aspirated, and cystic fluid creatinine (1.4 mg/dL) was equivalent to the dog’s serum creatinine.

One week following the second percutaneous aspiration, the dog presented to the NC State Veterinary Hospital. Physical examination revealed no abnormalities. Abdominal ultrasonography found a large, complex, 6 cm cystic structure associated with the cranial pole of the left kidney with focal changes to the adjacent renal parenchyma. Also, a small renal cyst (3.7 mm in diameter) was noted in the caudal pole of the left kidney. Packed cell volume, total solids, urine specific gravity, coagulation panel, and noninvasive blood pressure results were all within normal limits. Preprocedural blood urea nitrogen measured 16 mg/dL (reference range, 6–26 mg/dL) and creatinine measured 1.2 mg/dL (reference range, 0.7–1.5 mg/dL).

In order to maximize the preservation of renal function, ultrasound-guided percutaneous ethanol sclerotherapy of the renal cyst was recommended, as previously described.⁹ The dog was anesthetized and placed in right lateral recumbency. The left flank was clipped and aseptically prepared. Ultrasound was used to determine the position of the cyst, which measured approximately 7.4 cm in diameter in all directions. A 1.5-inch 22-gauge needle was inserted into the cystic cavity along the caudolateral aspect of the structure. Using an extension set with a 3-way valve and a 35 mL syringe, 300 mL of brown fluid was removed by aspiration. Based on volumetric calculations from ultrasound images, it was estimated that approximately 35 mL of fluid remained in the cyst after drainage, but further aspiration was not successful. Portions of the drained fluid were sampled and submitted for cytology and aerobic culture, and 150 mL of 95% ethanol was infused into the cyst. After 3 min, 125 mL of 95% ethanol and residual cystic fluid was slowly removed. Because of repeated clogging of the needle, the remaining 25 mL could not be removed. One hundred twenty milliliters of a 1:10 ratio 2% lidocaine:95% ethanol solution was infused into the cyst. After 3 min, 138 mL of the solution and residual ethanol and cystic fluid was slowly removed. The needle was then withdrawn. Approximately 40 mL was unable to be withdrawn from the cyst after both rounds of sclerotherapy with 95% ethanol. The cystic lesion was reassessed on ultrasound, and the postprocedural dimensions were measured at 7 cm in craniocaudal length and 3 cm in dorsoventral and lateromedial dimensions. The dog recovered uneventfully from general anesthesia and was given 0.45% NaCl IV at a rate of 3.3 mL/kg/hr while monitored for abnormal mentation and respiratory decompensation. Blood urea nitrogen (18 mg/dL; reference range, 6–26 mg/dL) and serum creatinine (0.9 mg/dL; reference range, 0.7–1.5 mg/dL) performed the following morning were both within normal limits. The dog was discharged from the hospital the day after ethanol sclerotherapy.

The dog remained free of clinical signs for 1 wk, after which trembling and panting were observed by the owner. The following week, the dog was presented to the NC State Veterinary Hospital for re-evaluation. Physical examination revealed mild to moderate mid-abdominal discomfort on palpation. Focused renal ultrasound revealed a left renal cystic structure similar in size to presclerotherapy measurements; the smaller cyst in the caudal pole of the left kidney appeared unchanged with a measured diameter of 3.4 mm.

Given the recurrence of the left renal cyst after percutaneous ethanol sclerotherapy, laparoscopic deroofing and omentalization was recommended. The dog was placed in dorsal recumbency and the surgical table was tilted 10 degrees in right lateral oblique to assist in visualization of the left kidney. A 2 cm incision was made through skin, subcutaneous tissue, and linea alba immediately caudal to the umbilicus for placement of a single-incision laparoscopic surgery port^a. A 5 mm 30-degree endoscope^b and 5 mm blunt probe were introduced to explore the abdomen. The cranial pole of the left kidney appeared enlarged and irregular with omental adhesions (Figure 1). The omental adhesions were removed using babcock forceps^c and a vessel-sealing device^d. Metzenbaum scissors^e were used to make a 1 cm initial incision through the wall of the renal cyst, and a 5 mm irrigation-suction cannula^f was introduced to withdraw cystic fluid. The vessel-sealing device was then used to remove the cranial wall of the cyst as close to normal renal parenchyma as possible. Approximately 75% of the cyst wall was removed and submitted for histopathology. The remaining 25% of the medial cyst wall was adhered to the left limb of the pancreas and was left intact. Babcock forceps were then used to pull omentum into the remaining cystic cavity, where an endoscopic hemostatic clip applicator^g was used to secure the omentum within the remaining cystic cavity. The port site was closed in routine fashion and the dog recovered from anesthesia uneventfully. The dog was discharged from the hospital the following day after a renal panel confirmed normal blood urea nitrogen (13 mg/dL; reference range, 6–26 mg/dL) and serum creatinine (0.7 mg/dL; reference range, 0.7–1.5 mg/dL) levels.

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Histopathology of the cyst wall revealed a renal adenoma with cyst formation. Intraluminal fibrin and hemorrhage were accompanied by lymphoplasmacytic inflammation and mesothelial cell hypertrophy. Marked, focal lymphoplasmacytic interstitial nephritis was noted in adjacent renal parenchyma that merged with an area of fibrous connective tissue representing the outer wall of the cyst. Arising from the fibrovascular stroma, a well-demarcated densely cellular neoplasm contained neoplastic epithelial cells arranged in tubular structures forming papillary-like projections into the cystic lumen. Within this neoplasm, mild anisocytosis and anisokaryosis were noted, with 1 mitotic figure observed in 10 high-magnification fields.

The dog presented for re-evaluation 1 mo after laparoscopic deroofing and omentalization. The owner reported that the dog was doing well. Focused urinary ultrasound did not identify the previously described cystic structure but instead detected a 2.5 cm by 3 cm ovoid nodular structure extending into the cranial retroperitoneal space from the cranial pole of the left kidney. This structure was partially fluid filled and thought to represent omentum, but residual or recurrent adenomatous tissue could not be excluded. Mild degenerative renal changes were appreciated bilaterally, with associated small, cortical renal cysts; in particular, two 3 mm cysts were observed in the left renal cortex distinct from the cystic structure at the cranial pole. Although metastasis cannot be excluded, these lesions were thought to be consistent with degenerative changes during both this and subsequent ultrasounds. A second focused urinary ultrasound performed 3 mo postoperatively revealed similar changes, with the lesion at the cranial pole of the left kidney measuring approximately 2.8 cm by 1.7 cm. A urinalysis performed by the referring veterinarian 9 mo postoperatively was unremarkable, with a urine specific gravity of 1.037. A third focal ultrasound performed 11 mo postoperatively confirmed continued presence of a fluid-filled cystic lesion disrupting the cranial pole of the left kidney with a maximal diameter of 3.75 cm. The lesion was irregularly shaped with a thin outer wall and hyperechoic extensions of tissue within the lesion, consistent with persistent omentalization or recurrent renal neoplasia. A chemistry panel and complete blood count performed at this time were unremarkable, and physical examination was similarly unremarkable. The dog was doing well with no clinical signs. At re-evaluation 18 mo postoperatively, the dog continued to display no clinical signs consistent with renal cyst recurrence. Complete blood count, chemistry panel, urinalysis, and thoracic radiographs were unremarkable. Abdominal ultrasound revealed that the fluid-filled cystic lesion disrupting the cranial pole of the left kidney was nearly identical in appearance and size to the previous ultrasound. Additionally, several small cysts were identified in the right kidney and caudal cortex of the left kidney; these were described as static compared with previous ultrasounds and consistent with degenerative changes. Several rounded hypoechoic nodules were noted in the hepatic parenchyma; cytology obtained via fine-needle aspirate revealed mild vacuolar change consistent with hyperplastic or regenerative nodules.

Discussion

This report describes the first laparoscopic deroofing and omentalization of a cystic renal adenoma in a dog. Ethanol sclerotherapy of renal cysts in dogs has been reported in a small case series and one case report.^4,9 Improvement in clinical signs, including resolution of systemic hypertension, was observed in the four dogs that received percutaneous drainage and sclerotherapy of the cyst with no evidence of recurrence on ultrasonography 3–4 wk after the procedure.⁴ One dog experienced bleeding during ethanol injection and the procedure was aborted.⁴ Long-term follow-up for the four dogs in the case series was not reported; the dog in the case report had no sonographic evidence of the cyst 6 mo after sclerotherapy.^4,9 The majority of people treated with ethanol sclerotherapy for renal cysts report improved flank pain following a single ethanol sclerotherapy procedure.^10–12 In people, recurrence of clinical signs from the cyst is the most common complication of ethanol sclerotherapy, with recurrence rates of 32–100% reported after a single treatment.^5,13 Whereas some studies suggest that multiple sclerotherapy treatments decrease the risk of recurrence in people, others advocate laparoscopic decortication as the treatment of choice, particularly for recurrent renal cysts.^5,6,12,13 Although no complications were observed during ethanol sclerotherapy in this case, the owner elected laparoscopic deroofing and omentalization rather than a second ethanol sclerotherapy treatment owing to the rapid recurrence of cyst size and clinical signs following ethanol sclerotherapy. Surgical management of renal cysts is not well described in veterinary literature, although partial resection and omentalization is described for the management of prostatic cysts in dogs.¹⁴

An unexpected finding following laparoscopic deroofing in this case was the diagnosis of a renal adenoma. Despite multiple cytological evaluations of cystic fluid, no evidence of neoplasia was found prior to laparoscopic deroofing. Had a neoplastic lesion been identified preoperatively, excision of the adenoma via nephrectomy may have been considered. Given the dog’s elevated symmetric dimethylarginine and the owner’s wishes to preserve renal function, deroofing and omentalization may still have been elected over more aggressive resection in the absence of malignancy. In this case, omentalization of the cranial aspect of the left kidney was performed to enhance drainage of residual cystic fluid. Although omentum is thought to enhance lymphatic drainage, evidence suggests that omentum may also promote tumor growth via immunological and metabolic mechanisms.¹⁵ It is therefore possible that omentalization of the cystic adenoma may increase the risk of adenoma regrowth. In cases in which a tumor is suspected but deroofing is elected over nephrectomy to preserve renal function, deroofing without concurrent omentalization may be prudent to minimize the risk of tumor progression, although it is likely that omentum will adhere to the debrided cyst even without intentional omentalization. Further study is needed to evaluate the effect of omentum on renal tumor progression. Given the absence of ultrasonographically apparent progression of the renal adenoma during the 18 mo following deroofing and omentalization, palliative laparoscopic treatment of cystic renal adenomas may provide durable clinical improvement while minimizing loss of renal function.

Malignant transformation of renal cysts is reported in people, and development of a renal carcinoma was reported following ethanol sclerotherapy.^16,17 Thus, it is possible that the lesion reported here may have initially been an SRC that transformed to a renal adenoma following initial aspirations or sclerotherapy, although the relatively short timeline of this case makes this very unlikely. Because of the potential for recurrence of the adenoma or malignant transformation to a carcinoma, continued ultrasonographic monitoring of this case is warranted.

This report demonstrates that laparoscopic deroofing and omentalization is a safe and feasible minimally invasive alternative for management of cystic renal adenomas causing clinical signs, particularly for lesions that fail to respond to ethanol sclerotherapy. Owing to the challenge of differentiating adenomatous tissue from nonadenomatous renal parenchyma intraoperatively, laparoscopic decortication may be associated with a higher risk of incomplete resection of renal adenomas compared with open decortication. There is a paucity of literature describing incompletely resected renal adenomas in people and dogs, making any prediction of outcome with incomplete resection difficult. The absence of progression observed in this case over 18 mo suggests that renal adenomas may progress slowly enough to achieve sustained clinical improvement with incomplete resection in some cases. Because of the benign nature of renal adenomas, the most likely consequence of incomplete resection is local recurrence, in which case repeated decortication or nephrectomy may be considered.

Conclusion

Veterinarians should be aware of the potential for renal adenomas to present as large cystic lesions. This case report demonstrates that laparoscopic deroofing and omentalization can provide safe and durable palliation of clinical signs associated with a cystic adenoma. Given the potential for renal tumors to appear similar to SRCs, biopsy prior to omentalization may be warranted. Further study is indicated to evaluate whether omentalization affects progression of incompletely excised cystic renal tumors.