January 2015



Signalment (AFIP # 2020925):  An 18-month-old Sprague Dawley rat


HISTORY:  A CRCD Sprague Dawley rat with a renal mass


HISTOPATHOLOGIC DESCRIPTION:  Kidney:  Affecting over 40% of the section, effacing the renal pelvis, infiltrating the adjacent medulla and cortex, separating, surrounding and replacing tubules and glomeruli, and extending through the capsule is an unencapsulated, poorly demarcated, densely cellular, infiltrative neoplasm composed of pleomorphic polygonal to spindle cells that are densely packed or arranged in short streams on a fine fibrovascular stroma.  Neoplastic cells have variably distinct cell borders and moderate amounts of pale eosinophilic cytoplasm that contain either: multiple variably-sized clear vacuoles (lipoblast); one large vacuole that peripheralizes the nucleus (lipocyte); or are unvacuolated (undifferentiated mesynchymal cell).  Nuclei are round to oval to elongate with coarsely stippled chromatin and 1-2 distinct nucleoli.  Mitotic figures average less than one per ten high-power fields.  Multifocally within the neoplasm are linear to round foci of proliferating transitional epithelial cells that often form lumina.  There is focally extensive necrosis with abundant eosinophilic cellular and karyorrhectic debris admixed with large numbers of degenerate neutrophils and abundant hemorrhage, fibrin, and edema.  Scattered throughout the neoplasm are aggregates of few lymphocytes and neutrophils.  Multifocally within the cortex and medulla, there are occasional dilated tubules that contain eosinophilic homogeneous proteinaceous material (proteinosis).  Occasionally within the interstitium there are low numbers of lymphocytes and plasma cells.  Rarely glomeruli are expanded by eosinophilic homogenous material.   


MORPHOLOGIC DIAGNOSIS: Kidney:  Liposarcoma, CRCD S/D rat, rodent.



·         Lipomatous renal neoplasms are generally uncommon;  however they are the most common primary renal neoplasm of the aging Fischer 344 and Osborne-Mendel rat strains (highly malignant in Osborne-Mendel strain and frequently metastasize to the lungs)

·         Cell of origin and etiology have not been determined; interstitial growth suggests intertubular mesenchymal origin



·         Usually an incidental finding at necropsy






·         Lipomas: 

o   Usually solid and unilateral

o   Generally <5mm in diameter

o   Well demarcated, pale to white lesions

o   Commonly located within the outer medulla at the corticomedullary junction


·         Liposarcomas: 

o   Usually single and unilateral

o   Generally >5mm in diameter

o   Poorly demarcated with irregular outline, substantial loss of the renal parenchyma and often distortion of the kidney

o   Often located at a pole of the kidney, may grow large enough to obliterate the kidney



·         Lipomas: 

o   Monomorphic population of mature adipocytes or lipocytes within the interstitum

o   May surround and separate tubules

o   No capsular reaction, hemorrhage or necrosis

·         Liposarcomas

o   Pleomorphic cell types, including:  mature adipocytes; lipoblasts, large cells with poorly defined cytoplasmic vacuoles or a single large vacuole that peripheralizes the nucleus (signet-ring cell); and stellate or spindled undifferentiated mesenchymal cells (spindle cells lacking lipid vacuoles)

o   May entrap preexisting tubules

o   Areas of hemorrhage and necrosis in large tumors   

o   A characteristic feature of liposarcomas is the presence of islands or dense aggregates of poorly differentiated mesenchymal cells within sheets of mature adipocytes



·         Mature adipocytes characterized by a large eccentrically placed nucleus, large non membrane bound lipid droplets, few profiles of endoplasmic reticulum, free ribosomes, and micropinocytotic vesicles



·         Lipoma vs. Liposarcoma: Size, cellular constituents and tissue invasiveness differentiate lipomas from liposarcomas

·         Renal mesenchymal tumor: Generally in young rats (<12 months old) and do not contain lipid

·         Fatty infiltration: Mature adipocytes with infiltrative pattern, may be difficult to differentiate from renal lipoma




·         Uncommon in most species

·         Dog – Predilection for subcutis and deeper soft tissues (axial most common); viscera also reported

·         Human – Predilection for deeper soft tissues including gluteal region, thigh, lower extremities, and retroperitoneum



1.  Baez JL, Hendrick MJ, Shofer FS, Goldkamp C, Sorenmo KU. Liposarcomas in dogs: 56 cases (1989-2000). JAVMA. 2004;224:887-890.

2.  Hard GC. Lipomatous Tumors, Kidney, Rat. In: Jones TC, Mohr U, Hunt RD, eds. Monographs on Pathology of Laboratory Animals: Urinary System. Berlin, Germany: Springer-Verlag; 1986: 80-7.

3.  Meuten DJ. Tumors in Domestic Animals. 4th ed., Ames, IA: Iowa State University Press;2002:96-9.

4.  Mohr U. Urinary System. In: Mohr U, ed. International Classification of Rodent Tumors, Part 1-The Rat. New York, NY: Oxford University Press; 1992:10-11.


Hinton 2015

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