Introduction

Primary renal tumors in cats are rare, with the majority of tumors reported as lymphoma and carcinomas.¹ Most cats with renal carcinomas present for nonspecific clinical signs including anorexia and weight loss.¹ The metastatic rate for completely staged feline renal carcinomas has been reported to be 64%, with the most common metastatic site being the lungs.¹ Other reported primary renal tumors in cats include nephroblastoma, leiomyosarcoma, adenoma, and hemangiosarcoma.^1,2

Although perirenal pseudocysts are well documented in cats, malignant tumors in this location are rare.³ Perirenal neoplasia has been documented in other species, including a metastatic myxosarcoma in a horse and a liposarcoma in a pig.^4,5

Hemoabdomen is rare in cats, with approximately half of the cases having an underlying neoplastic condition, most commonly hemangiosarcoma.⁶ Hemoabdomen has not been reported secondary to a renal myxosarcoma.

The current report describes the clinicopathologic findings of the first documented case of a primary perirenal/renal myxosarcoma with associated hemoabdomen in a cat.

Case Report

A 12 yr old spayed female domestic shorthair was presented for a firm, large mass palpated in the cranial abdominal cavity by the referring veterinarian. The cat had an acute history of lethargy, behavioral isolation, anorexia, and a pendulous abdomen. An initial physical examination was not obtained as a result of the patient’s fractious nature. Once the cat was sedated a physical examination was performed and blood was obtained for a complete blood count (CBC), biochemistry profile, and coagulation panel. The physical examination did not reveal any clinically significant abnormalities other than the large cranial abdominal mass and a body condition score of 2–3/9. Biochemistry panel results revealed hyperglycemia 187 mg/dL (85–115), an elevation in creatinine kinase 603 U/L (0–300), panhypoproteinemia (total protein 4.4 g/dL [6.5–8.7], albumin 2.1 g/dL [2.7–4.1], globulin 2.3 g/dL [3.3–4.9], and hypercholesterolemia 181 mg/dL [95–175]), and azotemia (blood urea nitrogen [BUN] 35 mg/dL [18–30], and creatinine 2.34 mg/dL [0.80–2.20]) with hypocalcemia 8.7 mg/dL (9.1–10.8). The CBC showed a normocytic, normochromic regenerative anemia (hematocrit 24.8% [30.0–48.0], packed cell volume [PCV] 25% [29–48], and reticulocyte count 126.8 × 10³/µL), along with a thrombocytopenia 43 × 10³/µL (190–368) with few large forms. The total white blood cell count was elevated and was characterized by a leukocytosis 13.1 × 10³/µL (6.0–10.0) with a mild left shift (band neutrophils 0.4 × 10³/µL (0.0–0.3) and lymphopenia 0.5 × 10³/µL (1.5–7.0). An elevated prothrombin time 14.7 s (9.0–11.5) and partial thromboplastin time 20.0 s (11.0–14.0) on the coagulation panel indicated prolonged coagulation. Abdominal ultrasound was performed using a 5–8 MHz curved array transducer^a. A moderate amount of echogenic peritoneal fluid was present. A large hypoechoic, heterogenous, and irregularly shaped mass measuring at least 10.4 × 8.8 cm in size was attached to the caudal pole of the right kidney infiltrating its capsular margin and hilar region. The mass was encompassing the entire cranial abdomen. Color Doppler flow was used to confirm renal origin of the mass and vessels could be traced that crossed the cortex of the kidney into the mass (Figure 1). The left kidney was displaced caudally by the mass and was unremarkable. The remainder of the abdominal examination was unremarkable. Based on ultrasound, differential diagnoses consisted of renal adenocarcinoma, nephroblastoma, and hemangiosarcoma. Ultrasound-guided fine-needle aspirates of the renal mass were obtained. Results of cytologic evaluation of the renal mass demonstrated low cellularity that hindered a definitive cytologic diagnosis. However, the presence of spindle cells could have indicated that the sample was of normal smooth muscle or a mesenchymal tumor. The peritoneal effusion was also cytologically examined and interpreted as acute hemorrhagic effusion.

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An abdominal exploratory laparotomy was performed. Upon opening the abdominal cavity, a moderate amount of hemorrhage and a large, 12.0 × 8.5 × 6.5 cm, pale tan to dark red, soft, gelatinous, well-vascularized mass grossly infiltrating the right kidney were found. A right nephrectomy was completed with all surrounding major anatomical structures spared. Impression smears of the mass were submitted to the clinical pathology lab but were nondiagnostic. The mass was submitted for routine histopathologic examination (Figures 2A, B). The specimen was fixed in 10% neutral buffered formalin solution and routinely processed, and 4.5 µm sections stained with hematoxylin and eosin (H&E) were examined. Histologically, the mass was a partially encapsulated, well-demarcated neoplasm compressing and invading the renal parenchyma. The loosely to densely packed neoplastic spindle to stellate cells were arranged in thin bundles and streams separated by an abundant lightly basophilic, myxoid matrix in a fine fibrovascular stroma (Figure 2C). The extracellular myxoid matrix showed positive staining with Alcian Blue stain at pH 2.5 (Figure 2C, insert). The neoplastic cells had variable distinct cell borders and scant lightly eosinophilic cytoplasm. The nuclei were oval to elongated with stippled chromatin and often one small nucleolus. Anisocytosis and anisokaryosis were moderate. The mitotic index was 14 per 10 high-power fields. There were multifocal hemorrhagic and rare necrotic foci throughout the tumor.

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Positive and negative immunoreactivity of the neoplastic cells to vimentin and cytokeratin AE1/AE3, respectively, confirmed a mesenchymal cell origin of the neoplasm and invasion of the neoplasm into the renal parenchyma (Figure 2D). The negative immunohistochemistry results to desmin, alpha smooth muscle actin, and CD117 (c-kit) made a muscular origin and gastrointestinal stromal tumor unlikely. Round cell tumors, including lymphosarcoma, were ruled out by histomorphology and with negative immunoreactivity to CD20 (B-cell marker), CD3 (T-cell marker), and CD18 (histiocytic cell marker). Therefore, based on the histomorphology with the presence of abundant myxoid matrix and immunohistochemical results, a diagnosis of a well-differentiated myxosarcoma was made.

Two days of postoperative care was given, in which the patient received intravenous fluid therapy and a constant rate infusion of fentanyl (3 µg/kg/hr). Based on a PCV of 15% and total solids of 3.2 g/dL, a 1 U transfusion of 23 mL type A feline red blood cells was given. Approximately 12 hr after the transfusion, the patient’s PCV had increased to 20% and the total solids to 5.0 g/dL. A 1 day postoperative renal biochemistry profile showed a mildly elevated BUN 31 mg/dL (18–30), normal creatinine 1.93 mg/dL (0.80–2.20), mild hypophosphatemia 3.9 mg/dL (4.0–6.6), hypoalbuminemia 2.0 g/dL (2.7–4.1), and mildly elevated bicarbonate 21.3 mmol/L (11.0–21.0). The patient started eating and was discharged with buprenorphine^b 0.02 mg/kg transmucosally q 8 hr as needed for pain.

Seven days after surgery, the patient returned for lethargy and inappetence. A CBC, biochemistry panel, and urinalysis were performed. The CBC revealed a normocytic, normochromic, nonregenerative anemia (hematocrit 23.1% [30.0–48.0], PCV 23% [29–48], reticulocyte count of 58.7 × 10³/µL), thrombocytopenia 103 × 10³/µL (190–368) with a few clumps present, and a leukocytosis 13.7 × 10³/µL (6.0–10.0) characterized by a mature neutrophilia (segmented neutrophils 12.6 × 10³/µL [2.5–12.5]) and lymphopenia 1.0 × 10³/µL (1.5–7.0). The biochemistry panel showed a panhypoproteinemia (total protein 5.6 g/dL [6.5–8.7], albumin 2.4 g/dL [2.7–4.1], globulin 3.2 g/dL [3.3–4.9]), and hypercholesterolemia 238 mg/dL [95–175]. A free catch urinalysis showed a specific gravity of 1.024. The urine dipstick showed only a trace amount of protein. The patient was given 100 mL of subcutaneous fluid therapy to help correct dehydration and was discharged with buprenorphine^b 0.01 mg/kg transmucosally q 8 hr.

Five months after nephrectomy, a repeat renal panel revealed the following similar findings: BUN 34 mg/dL (18–30) and creatinine 2.19 mg/dL (0.80–2.20) with hypophosphatemia 3.9 mg/dL (4.0–6.6) and hypoalbuminemia 2.6 g/dL (2.7–4.1). The patent’s azotemia was considered stable, and owners elected no further treatments be implemented at that time. Fourteen months postoperatively the patient had no evidence of recurrence or metastatic disease based on repeat staging tests including a CBC, biochemistry panel, abdominal ultrasound, and thoracic radiographs.

Discussion

To the authors’ knowledge, this is the first reported case of a perirenal/renal myxosarcoma in a cat. This is also the first case of a myxosarcoma associated with hemoperiotoneum in a cat. Myxosarcomas are considered soft tissue sarcomas that are of fibroblastic origin and have an exuberant amount of mucopolysacchardes in a myxoid matrix.⁷ Histological changes are used to distinguish a myxosarcoma from its benign counterpart, myxoma. Although subtle, an increase in the number of mitotic figures, cellularity, and nuclear pleomorphism is warranted.⁸

Myxosarcomas are commonly found in the subcutaneous tissue of the thorax or limbs of middle-aged to older domestic animals.⁷ Myxosarcomas have also been documented in the heart, cerebrum, and eye.⁸ Although visceral myxosarcomas are rare, intestinal myxosarcomas have been previously reported in the dog and the horse, all of whom had metastatic disease.⁹ High metastatic rates have also been reported for splenic stromal sarcomas in dogs.¹⁰ Little information is available regarding the biological behavior of visceral soft tissue sarcomas in cats.

Although the overall occurrence of spontaneous hemoperitoneum is rare, in a retrospective case study of 65 cats, 46% of spontaneous hemoperitoneums were as a result of neoplasia and 54% were as result of non-neoplastic diseases.⁶ Hemangiosarcoma was the most common neoplastic cause, with median survival times of neoplastic causes being 11 days.⁶ In this study, 8 of the 65 cats who were diagnosed with spontaneous hemoperitoneum recovered and were discharged.⁶ To the authors’ knowledge, this is the first documented case of a spontaneous hemoperitoneum of myxosarcoma origin.

Conclusion

It appears nephrectomy in cats with perirenal/renal myxosarcoma can aid in achieving a complete clinical remission and a prolonged survival. The cat in this report remains free of metastatic disease at the time of writing, 14 mo after nephrectomy.