Primary renal neoplasia is relatively rare in humans and dogs (3,4). It accounts for 0.3%-1.7% of all canine primary neoplasias (9), and renal cell carcinoma (RCC) accounts for approximately 60% of all primary renal neoplasias (2,3). The clinical symptoms are nonspecific and include hematuria, lethargy, anorexia, polyuria, polydipsia, and weight loss (3). Nephrectomy is the most effective treatment for RCC. The average survival period of dogs that do not undergo nephrectomy is less than 1 month, and the average survival period of dogs that undergo nephrectomy is 16 months, regardless of tumor type (3,6). In addition, the survival time of dogs treated with surgery and chemotherapy is not significantly longer than that of dogs treated with surgery alone (3).

A 10-year-old female Pomeranian weighing 2.25 kg with a history of gradual weight loss for 9 weeks and an intraabdominal mass confirmed through X-ray and ultrasonography at a local hospital was referred to the Veterinary Medical Teaching Hospital of Jeju National University. Physical examination revealed abdominal distension and increased abdominal pressure. Low albumin (2.0 g/dL; reference range, 2.5-4.4 g/dL), glucose (57 mg/dL; reference range, 60-110 mg/dL), and potassium (3.6 mmol/L; reference range, 3.7-5.8 mmol/L) levels were detected, and there were no other abnormalities on blood analysis. In radiographic views, a mass occupying half of the abdominal cavity was observed (Fig. 1A), and a heterogeneously hyperechoic mass measuring 7.51 × 10.6 cm was found at the level of the left kidney on abdominal ultrasound (Fig. 1B), whereas the right kidney was normal. Contrast-enhanced computed tomography (CT; SOMATOM Emotion, Siemens, Erlangen, Germany; pitch 0.8, scan slice thickness 2.0 mm, rotation time 0.6 s, mAs 39, kVP 130, field of view of 15 cm) with 1 mL/kg of intravenous contrast agent (Omnipaque 100 mL, GE Healthcare, Chicago, IL, USA) in the supine position showed a unilateral left renal mass with a 10-cm diameter vessel contrast enhancement in the corticomedullary phase (Fig. 2A) and enlargement of the left ovary (Fig. 2B). Unilateral nephrectomy and ovariohysterectomy were also performed. The renal mass was adjacent to the aorta and vena cava and adhered to the small intestine, greater omentum, and left ovary. The renal mass was 12.5 × 10 × 7 cm in size (Fig. 3A), and the ovarian mass was 2 × 2 × 1 cm (Fig. 3B). After surgery, the patient weighed 1.88 kg, and the weight of the intraabdominal mass was 16.5% of the body weight.

Figure 1. Imaging of abdominal mass. (A) A mass occupying more than half of the abdominal cavity (yellow arrow) was identified on X-ray. (B) Heterogeneous hyperechoic mass at the level of left kidney was detected on abdominal ultrasound.

Figure 2. Contrast-enhanced computed tomography of abdominal mass. (A) Unilateral left renal mass with vessel contrast enhancement (yellow arrow) in the corticomedullary phase was identified. (B) In addition, enlargement of left ovary (yellow arrow) was detected.

Figure 3. Surgically removed mass. (A) Left kidney (12.5 × 10 × 7 cm) and (B) left ovary (2 × 2 × 1 cm; yellow arrow) were adherent to each other.

On histopathological examination, the renal mass was confirmed to be a renal tubular cell carcinoma. The neoplastic mass mainly consisted of cuboidal, columnar, or polygonal cells. In general, the eosinophilic cytoplasm and nucleus are pushed into the center or marginal regions of the cell. Large nucleoli in the cells were clearly observed, and the mitotic index was 37 at 10 fields of ×400 magnification. Necrosis was also observed in the mass (Fig. 4A). The left ovary was also diagnosed as carcinoma and showed a tubular pattern similar to that of the renal mass. It is highly likely that the RCC had metastasized to the ovary. The mitotic index was 1-5, and necrosis was observed inside the mass (Fig. 4B).

Figure 4. Histopathologic findings. (A) The left kidney mass mainly consisted of cuboidal, columnar, or polygonal cells with malignancy, which was consistent with renal cell carcinoma (H&E, ×400). (B) Malignant tubular pattern similar to the left kidney mass was detected in the left ovary (H&E, ×400).

As RCC is resistant to chemotherapy (3), we decided to follow-up the patient without chemotherapy. At day 7 after surgery, serum chemistry showed restored albumin (3.0 g/dL; reference range, 2.5-4.4 g/dL), glucose (94 mg/dL; reference range, 60-110 mg/dL), and potassium (4.2 mmol/L; reference range, 3.7-5.8 mmol/L) levels. At day 42 and 150 after surgery, the dog weighed 2.3 kg, and there was no evidence of recurrence or further metastasis of the tumor on X-ray and abdominal ultrasound. Unfortunately, the patient was lost to follow-up after the last visit (150 days after surgery).

In this case, the mass was sufficiently large to occupy half of the abdominal cavity and was 16.5% of the body weight. In previous studies, RCC did not attain sufficiently massive size to occupy half of the abdominal cavity (10,12,14), however, nephroblastoma could be massive (1,13). Lucke et al. have reported that variant sizes of RCC (20 × 16 × 9, 6 × 2 × 2, 7 × 5 × 7, 12 × 6, and 8 × 7 × 5 cm) were observed in medium- and large-breed dogs including Collie, Alsatian, Boxer, Afghan, and Labrador (12). Furthermore, in human, RCC with a diameter of >20 cm is extremely rare and can be diagnosed as a huge RCC (4). In the present case, it is noteworthy that the renal mass was remarkably larger (12.5 × 10 × 7 cm in a dog weighing 2.5 kg) than typical canine RCC.

Ovarian metastasis of RCC was observed in this case. While ovarian primary neoplasia usually shows a cystic or papillary proliferative pattern on histopathology, in this case, a tubular pattern similar to RCC was detected. In addition, the left kidney and left ovary were adherent to each other. Therefore, it was tempting to speculate that RCC had metastasized to the ovary in this case. According to previous studies on human RCC, ovarian metastasis was confirmed in only 0.5% of all cases (7,15). To the best of our knowledge, this is the first case report of ovarian metastasis of RCC in dogs.

Tanaka et al. (17) recently revealed that contrast-enhanced CT could be helpful in characterizing primary renal tumors. The previous study demonstrated that CT imaging patterns of various primary renal tumors are different. In RCC, angiographic enhancement in the corticomedullary phase could be observed, however, in renal lymphoma, angiographic enhancement was not observed in any phase. In this case, because angiography enhancement was confirmed in the corticomedullary phase, CT image analysis results were consistent with those of the previous study (17). In addition, CT could be useful for screening local or distant metastases before surgical planning.

In the present case, hypoglycaemia (57 mg/dL) was detected. Although urinalysis (for detecting urine glucose) and determining serum concentration of insulin like growth factor were not performed, the patient could be tentatively diagnosed with non-islet cell tumor-induced paraneoplastic syndrome because hypoglycemia was normalized only after surgery without additional treatment. In human and equine medicine, a single rare case of paraneoplastic hypoglycemia due to RCC has been reported (5,16). Although hypoglycemia due to non-islet cell tumors has been reported in dogs (8,11), paraneoplastic hypoglycemia due to RCC in dogs has not yet been reported.

To the best of our knowledge, this is the first case report of a huge canine RCC with ovarian metastasis. Furthermore, this patient was tentatively diagnosed with paraneoplastic hypoglycemia, which normalized with surgical removal of the tumor.

This study was supported by the National Research Foundation (NRF) of Korea grant funded by the Korea government (MSIT) (NRF-2021R1F1A1063399).

We thank veterinary pathologists of PATH (Seoul, Korea) for providing diagnostic supports.

The authors have no conflicting interests.