Renal adenoma in a 5-year-old Labrador retriever: Big is not always bad

Abstract

A 5-year-old Labrador retriever was presented with anorexia, hematuria, and a 3-week history of mild lethargy, periodic inappetance, and weight loss. A firm mass in the cranial abdomen was discovered on physical examination. Following clinical work-up the owners elected euthanasia. On postmortem examination, histopathology determined that the mass was a benign renal adenoma.

A 5-year-old, neutered male, Labrador retriever dog was presented to Mill Bay Veterinary Hospital with a 3-week history of mild lethargy, periodic inappetance, and an increase in “lip smacking.” Within this 3-week period the dog had also lost approximately 13% of its body weight.

Case description

There were no abnormal findings on physical examination. Complete blood (cell) count and biochemistry (IDEXX Laboratories, Delta, British Columbia) were normal except for mildly elevated chloride and amylase levels. Subsequently, a serum Spec cPL (canine pancreas-specific lipase) and Cryptococcus titer (by latex agglutination) (IDEXX Laboratories) were determined and the results of both were within normal limits.

Four days after initial presentation, the dog was re-admitted due to anorexia and hematuria of 24-hour duration. On physical examination, a firm mass was palpated in the ventral abdomen immediately caudal to the last rib; palpation of the mass appeared to elicit pain. A catheterized urine sample (in-house urinalysis) showed significant protein (3+ dipstick reaction; 5 g/L) and red blood cells (4+ dipstick reaction; ca. 250 erythrocytes/μL; under the microscope too numerous to count), many uniform transitional epithelial cells, and moderate numbers of white blood cells (2–3/high power field). In-house urine culture was negative for bacterial growth after 48 h. Single right lateral and ventro-dorsal radiographs of the thorax yielded no significant findings. On the single right lateral radiograph of the abdomen, a mass was visualized immediately caudal to the last rib and dorsal to the spleen. There appeared to be a mass effect on the single ventro-dorsal view of the abdomen, on the left side, just caudal to the last rib. An abdominal ultrasound revealed a poorly defined, 9.4 × 7.4 cm, left-sided cranial abdominal vascular retroperitoneal mass with left kidney invasion, no evidence of gross metastasis or great vessel invasion and an unremarkable aorta and vena cava (Radiology Vet Consulting, Trenton, Ontario). The main differentials at this point included renal adenocarcinoma, transitional cell carcinoma, and retroperitoneal hemangiosarcoma. A referral for advanced imaging with computed tomography (CT) or magnetic resonance imaging (MRI) was recommended, but due to the cost and clinical findings suggesting a poor prognosis, the owners opted to have the dog euthanized.

A postmortem examination revealed a 10.0 × 9.0 cm mass primarily affecting the cortical tissue of the cranial pole, as well as the renal pelvis, of the left kidney. The mass affected approximately 30% of the overall parenchyma (Figure 1). There was extensive hemorrhage within the retroperitoneal space surrounding the mass and extending from the left to the right side and over the caudal pole of the right kidney. The right kidney appeared grossly normal as did the other abdominal organs, lungs, and great vessels.

Figure 1.

Top: left kidney and mass in-situ. Bottom left, clockwise: left kidney with mass and left adrenal, normal urinary bladder/ureters, normal right adrenal, normal right kidney; Bottom right, left kidney and mass on cut section, normal right kidney on cut section.

The entire left kidney and associated mass as well as samples from the right kidney, urinary bladder, right and left ureters, urethra, lung, spleen, and liver were submitted for histopathology (Histovet Surgical Pathology, Guelph, Ontario). Multiple sections from the enlarged left kidney revealed the following histopathologic findings: an abrupt transition from normal kidney into a diffuse collection of coalescing tubules lined by orderly cuboidal epithelial cells showing minimal nuclear variation, no gigantism, no multinucleation, and virtually no mitotic figures. These cells were separated from the surrounding kidney by a very thin capsule of fibrous tissue. None of the sections taken from the junction between the tumor and the surrounding kidney displayed any significant invasive growth. In some areas there was significant tumor necrosis resulting in hemorrhage and fibrin deposition, which could explain the hematuria noted clinically. Sections of the left kidney taken at least 1 cm from the gross mass showed histologically normal renal architecture. The histopathological diagnosis of the sections taken from the left kidney and associated mass was as follows: a large but very well-differentiated renal papillary and tubular adenoma with a purely expansile growth habit. All other organs sampled were histologically normal.

Discussion

Primary renal tumors are an uncommon diagnosis in dogs (1–3). Over half of the primary renal tumors are epithelial in origin and most have been found to be malignant (1–4). Classification of malignancy is most often based on light microscopy, or gross size, but the distinction between benign and malignant is somewhat subjective and, as such, may have led to overclassification of carcinomas (2). Besides the standard criteria of malignancy such as poor degree of differentiation, infiltrative growth, lack of capsule, cellular atypia, hyperchromasia, high nuclear to cytoplasm ratio, number of mitotic figures and, evidence of metastases, there are no ironclad histopathologic features that definitively distinguish between benign and malignant primary renal neoplasms such as adenoma versus adenocarcinoma (2,5). Therefore, individual features such as those mentioned may be variably emphasized by different pathologists, resulting in inconsistent diagnoses in terms of benign versus malignant. This becomes most salient in cases that may be less clear-cut than the one outlined in this report, as in cases where the neoplasm may have some features that suggest malignancy but lack others. To further complicate matters, one text cites that some pathologists use an arbitrary cut-off of 2 cm in differentiating adenoma (< 2 cm) from carcinoma (> 2 cm) and that this may be a useful strategy in dogs and cats (2). Using such a strategy in this particular case, in which the mass was significantly larger than 2 cm, may have led to a false diagnosis of carcinoma. This highlights an important point to consider when reviewing primary literature that combines diagnoses of renal tumors from multiple institutions/pathologists. Although the consolidation of information on diagnosed cases of primary renal neoplasia is useful, it must still be interpreted by readers with the understanding that there is some subjectivity in the diagnostic process for each individual case. Furthermore, it should be noted that these primary studies are the ones most commonly cited in the “renal neoplasm” chapter of many popular texts (1,2).

Although it has been reported that renal adenomas are clinically silent and usually detected as incidental findings, these reports are largely from retrospective studies of cattle and horses and not from dogs (2). Common clinical signs associated with renal neoplasms, while nonspecific in nature, can include anorexia, depression, weight loss, and hematuria (1,3). Furthermore, palpation can reveal an abdominal mass that may elicit a painful response (1,3). The dog in this report had a benign renal mass that was not clinically silent. Therefore, the presence of clinical signs with a renal neoplasm should not immediately warrant a suspicion of malignancy.

Nephrectomy is the recommended treatment for unilateral renal neoplasms that have not metastasized (1). For malignant renal neoplams, surgical excision is primarily a palliative treatment that has been successful in extending survival times in affected dogs (1,3,4,6). In contrast, removal of the affected kidney has been reported as a curative procedure for some benign renal tumors (7).

Although most of the renal tumors causing clinical signs will be malignant, it is nevertheless crucial for readers to remain critical when using information from textbooks and primary literature sources in guiding their clinical decisions. The focus of this report is to highlight the fact that benign renal tumors do occur, they are not always clinically silent, and they should always remain on a list of differentials for a well-defined renal mass. As well, surgical excision should be considered for both benign and malignant primary renal neoplasms (1,3,4,6,7).