Primary renal tumors are rare in companion animals. In cats, the most common primary renal tumor is lymphoma,1,2 and in dogs, metastatic neoplasia most commonly occurs in the kidneys.2 A retrospective study of feline primary renal tumors (excluding lymphoma) identified only 19 cases in a period of six years
from four veterinary colleges and one private specialty clinic.1 In both cats and dogs, most primary renal tumors are epithelial in origin, and benign tumors are rare.1-3 Feline renal transitional cell carcinoma is nearly as common as feline renal cell carcinoma.3 This is in contrast to dogs, in which renal cell carcinoma is most common2 and renal transitional cell carcinoma, although reported,4 is rare.
Figure 9. Histologic examination of lung tissue obtained at necropsy revealed numerous small metastases in the subserosa (arrow)
(hematoxylin-eosin stain; 10X [inset 20X]).
Feline renal transitional cell carcinoma and feline renal cell carcinoma exhibit distinct biologic behaviors. Renal carcinomas
are less likely to metastasize, with only five cases of confirmed metastasis noted in 13 cats with the disease,1 so surgical excision of a unilateral tumor may prolong survival. Like the cat described here, all previously reported cases
of feline renal transitional cell carcinoma have developed distant metastases. Sites of metastasis include the sublumbar or
tracheobronchial lymph nodes, contralateral kidney, liver, spleen, adrenal glands, omentum, mesentery, lungs, eyes, skeletal
muscles, and meninges.3
This cat displayed some of the most common signs of a feline primary renal tumor: decreased appetite, weight loss, and a palpable
abdominal mass. Less common signs include hematuria, abdominal pain, abdominal distention, dyspnea, systemic hypertension,
and blindness or other neurologic signs.1,3 Although pyuria has not been specifically reported in cases of feline primary renal tumors, it was a prominent feature in
this cat's initial presentation and persisted until nephrectomy. In the absence of a urinary tract infection, the differential
diagnoses for pyuria in a sample obtained by cystocentesis are noninfectious inflammatory conditions of the urinary tract,
including urolithiasis, trauma, chemical irritation, and neoplasia.5 In this cat, multiple negative bacterial culture results ruled out a urinary tract infection, and the pyuria was considered
to be due to inflammation caused by the tumor.
A feline renal tumor can be diagnosed with a physical examination along with an abdominal radiographic examination, ultrasonographic
examination, or both. Cytologic examination can help determine whether neoplasia is present and often indicates a general
tumor type, but histologic examination is needed to make a definitive diagnosis. Because primary renal epithelial tumors are
rare in cats and the cytologic appearance can be similar, pathologists may have difficulty in distinguishing among specific
types. Two of the three tumors diagnosed as renal transitional cell carcinoma on final histologic review in a recent study
were initially identified as other renal epithelial tumors.1 Overall, eight of 19 feline primary renal tumors were reclassified after histologic verification in that study.1 As the metastatic potential for renal transitional cell carcinoma is higher than for other renal epithelial tumors, histologic
confirmation of tumor type should be sought after definitive surgery. In this cat, the initial cytologic diagnosis suggested
a renal cell carcinoma, but histologic evaluation of both the surgical biopsy and necropsy samples confirmed renal transitional
Before determining a treatment plan, an overall health assessment and complete staging of feline primary renal tumors should
include thoracic radiography (three-view), a CBC, a serum chemistry profile, a total T4 measurement, urinalysis, and feline leukemia virus and feline immunodeficiency virus serology.