JPC SYSTEMIC PATHOLOGY

URINARY SYSTEM

December 2023

U-N05 (NP)

 

SIGNALMENT (JPC #2018633): 12-year-old male German Shepherd Dog

 

HISTORY: Incidental finding at necropsy

 

HISTOPATHOLOGIC DESCRIPTION: Kidney (2 sections): Focally expanding the corticomedullary junction, widely separating and often replacing tubules and glomeruli, and compressing adjacent renal parenchyma, is a round, 5 mm diameter, unencapsulated, well-demarcated, sparsely-cellular neoplasm composed spindle to stellate shaped cells arranged in interlacing streams and bundles separated by abundant dense collagenous matrix. Neoplastic cells have indistinct cell borders, scant amount of eosinophilic fibrillar cytoplasm, and an oval to elongate nucleus with finely-stippled chromatin and a small nucleolus. Anisocytosis and Anisokaryosis is mild. No mitoses are observed. Frequently surrounding the neoplasm, and multifocally expanding the interstitium of the remaining kidney, there are small aggregates of plasma cells and lymphocytes, and variable amounts of fibrosis. Multifocally, remaining glomeruli exhibit one or more of the following changes: thickened Bowman’s capsule with hypertrophy and hyperplasia of the visceral epithelial lining and occasional attachment to the glomerular tuft (synechiae); ectatic uriniferous space containing proteinaceous fluid; rare glomerular sclerosis and obsolescence; and periglomerular fibrosis. Multifocally, tubules are ectatic and contain proteinaceous fluid, and tubular epithelial cells are occasionally hypertrophied with vacuolated cytoplasm (degenerate). Focally there is a depression in the renal capsule, which is subtended by a wedge-shaped subcapsular area composed of moderate amounts of fibrous connective tissue that separates, surrounds, and replaces glomeruli and tubules (infarct). Remaining glomeruli in the infarcted area are variably sclerotic to obsolescent.

 

MORPHOLOGIC DIAGNOSIS:  

1.  Kidney, corticomedullary junction: Renal interstitial cell tumor (fibroma), German Shepherd Dog, canine.

2.  Kidney: Nephritis, interstitial, lymphoplasmacytic, multifocal, mild, with multifocal glomerulosclerosis and obsolescence.

3.  Kidney, cortex: Infarct, chronic, focal.

 

CONDITION: Renal interstitial cell tumor

 

GENERAL DISCUSSION:  

• Uncommon primary mesenchymal tumor of the kidney that occurs near the corticomedullary junction in dogs >10 years old 
• Usually incidental finding at necropsy
• Originates from fibroblast-like interstitial cells of renal stroma that are distinct from interstitial fibroblasts
• Renal interstitial cells contain prostaglandin, arachidonic acid, and a neutral antihypertensive lipid that may lower arterial blood pressure

 

TYPICAL GROSS FINDINGS:  

• Often bilateral, multiple, and located in cortex near corticomedullary junction
• Well-circumscribed, firm, grey-white nodules, ranging in size from 2–20 mm
• May be mineralized or compressive

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:  

• Unencapsulated; blend into the surrounding interstitium
• May entrap glomeruli and tubules
• Small spindle to stellate cells with loose fibrillar to dense eosinophilic hyalinized matrix containing variable amounts of collagen; cells may contain lipid droplets
• Low mitotic rate, though some reports of malignant renal interstitial tumors with high mitotic rates and/or systemic involvement have recently been recorded in the veterinary literature (Cho J Vet Diagn Invest 2020; Rissi J Vet Diagn Invest 2020)
  1. This may reflect variability within the tumor as less differentiated (anaplastic) regions had higher mitotic rates and decreased intracytoplasmic lipid vacuoles (Cho J Vet Diagn Invest 2020).

 

ULTRASTRUCTURAL FINDINGS:

• Presence of intracytoplasmic lipid droplets differentiates renal interstitial cell tumors from renal fibromas; can potentially be seen on H&E with oil red O stain if available
• Cells have irregular cell membranes, elongate and branched cytoplasmic processes, moderate amounts of rough endoplasmic reticulum, and cytoplasmic electron-dense osmiophilic membrane-bound vacuoles
• Collagen fibrils fill extracellular spaces

 

ADDITIONAL DIAGNOSTIC TESTS:  

• Renal interstitial cell tumors stain positively with Alcian blue, PAS, and Toluidine blue; renal fibromas are negative with these stains
• Tumor cells are immunoreactive for vimentin and COX-2 and immunonegative for pancytokeratin (Cho SH, J Vet Diagn Invest; 2020)

 

DIFFERENTIAL DIAGNOSIS:

• Macroscopically and microscopically indistinguishable from renal fibromas (electron microscopy and special stain to differentiate)
• Other primary renal mesenchymal tumors include hemangioma, benign cortical fibroma, neurofibroma, malignant fibrous histiocytoma and oncocytoma

 

COMPARATIVE PATHOLOGY:  

 

REFERENCES:  

1.  Cho SH, Seung BJ, Kim SH, Lim HY, Lee GS, Chae, MS, Sur JH. Renal interstitial cell tumor in a dog, clinicopathologic, imaging, and histologic features. J Vet Diagn Invest, 2020:32(1): 124-127

2.  Cianciolo RE, Mohr FC. Urinary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. Vol 2, 6th ed. St. Louis, MO: Elsevier; 2016:447.

3.  Diters RW, Wells M: Renal interstitial cell tumors in the dog. Vet Pathol. 1986;23:74-76.

4.  Meuten DJ, Meuten TLK: Tumors of the urinary system. In: Meuten DJ. Ed. Tumors in Domestic Animals. 5th ed. Ames, IA: Iowa State Press; 2017:650-651.

5.  Picut CA, Valentine BA: Renal fibroma in four dogs. Vet Pathol. 1985:22:422-423.

6.  Rissi DR, Kill-Okubo JA: Metastatic renal interstitial cell tumor in a dog. J Vet Diagn Invest. 2020; 32(6):957-961.

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