JPC SYSTEMIC PATHOLOGY

URINARY SYSTEM

December 2023

U-N03

 

Signalment (JPC #1667565): 1‑year‑old male Doberman pinscher

 

HISTORY: Hematuria for six months 

 

HISTOPATHOLOGIC DESCRIPTION: Urinary bladder: Infiltrating and replacing the subepithelial connective tissue and elevating the overlying markedly hyperplastic and multifocally ulcerated epithelium, is a 1 x 2 cm, unencapsulated, poorly circumscribed, infiltrative, multilobular, variably densely cellular, polypoid neoplasm composed of spindle cells arranged in interlacing streams and bundles separated by scant eosinophilic fibrillar matrix. Neoplastic cells are most dense within the submucosa and are separated from the mucosal epithelium by a less dense myxomatous layer of connective tissue. Neoplastic cells have variably distinct cell borders, moderate to abundant eosinophilic fibrillar cytoplasm often with intracytoplasmic cross-striations, oval to elongate nuclei containing finely to coarsely stippled chromatin, and small indistinct nucleoli. The mitotic rate averages 1 per 10 HPF’s. There are scattered, large, rectangular multinucleated neoplastic cells with tandem peripheralized nuclei and prominent cross striations (strap cells). The mucosa is diffusely hyperplastic with formation of papillary projections or has areas of ulceration and hemorrhage. There is multifocal submucosal edema with moderate numbers of neutrophils.

 

MORPHOLOGIC DIAGNOSIS: Urinary bladder: Botryoid rhabdomyosarcoma, Doberman pinscher, canine. 

 

GENERAL:  

• In dogs, the botryoid variant of rhabdomyosarcoma (RMS) occurs in the urinary bladder and urethra and is considered rare
  1. Occurs in young dogs (less than 2 years of age)
  2. Large breed dogs; St. Bernards are overrepresented
  3. Females outnumber males 2:1
• Most common locations of rhabdomyomas/rhabdomyosarcomas in dogs are laryngeal and cardiac:
  1. Twelve cases of laryngeal rhabdomyomas/rhabdomyosarcomas are reported in young dogs
  2. It is generally believed that laryngeal rhabdomyomas/rhabdomyosarcomas

are a distinct clinical entity of dogs, and are locally invasive yet rarely metastatic

1. The majority of cardiac muscle tumors, though rare in dogs, are RMS (6/12)
2. Cutaneous rhabdomyomas are rarely reported in dogs

• The origin of the botryoid rhabdomyosarcoma is thought to be pluripotent stem cells arising from an embryonic rest in the urogenital ridge
• There are four variants of rhabdomyosarcomas in dogs:
• The most aggressive are alveolar RMS and embryonal RMS which arise in juvenile dogs younger than 2 years 
• Pleomorphic RMS is the least common (n =2;3%), and botryoid RMS is the most common (n = 28; 43%), followed by embryonal RMS (n =15;23%)

1. Embryonal: Primitive myogenic cells with well-differentiated rhabdomyoblasts +/- cross striations that have rhabdomyoblastic, myotubular, and spindyloid variants

• The myotubular variant is dominated by multinucleated “strap” cells forming myotubes with frequent cross-striations (often only identified by histochemical staining with phosphotungstic acid hematoxylin [PTAH] stain, which highlights striations as dark blue/purple lines among paler blue/purple myofibers)
• The rhabdomyoblastic variant is dominated by round to polygonal cells with abundant eosinophilic cytoplasm with only rare PTAH positive cross-striations
• The spindyloid embryonal RMS is composed of thin spindyloid myoblast cells forming bundles within a myxoid stroma

1. Botryoid: A distinct subtype of embryonal that affects the urinary bladder
   1. Grape-like masses protruding from the mucosa that contain undifferentiated myoblast cells as well as multinucleated myotube cells within a myxomatous stroma 
   2. Histologically, botryoid RMS of the urinary bladder is located in the submucosa, and the dense layer of neoplastic cells (cambium layer) is separated from the mucosal epithelium by a myxoid less densely cellular transitional layer of connective tissue 
2. Alveolar: Small, poorly differentiated round cells with scant cytoplasm, arranged into packets of cells separated from each other by fibrous connective tissue, forming alveolar-like patterns
   1. Classic alveolar pattern is characterized by extensive sloughing of neoplastic cells into a central open space with additional neoplastic cells lining the fibrous septa
   2. Solid variant is characterized by sheets of small round cells closely packed together that may have a thin fibrous septa dividing nests of round cells similar to the classic “neuroendocrine pattern”
3. Pleomorphic: Often affects adults
   1. Composed exclusively of haphazardly arranged spindle cells with rare  multinucleated cells present and is recognized by the appearance of bizarre or multipolar mitotic figures and a high degree of cellular and nuclear; cells lack cross striations
   2. Lacks any of the features of alveolar or embryonal recognized in any part of the neoplasm
   3. Rapidly infiltrative and warrants a poor prognosis; metastasis occurs infrequently to abdominal and thoracic lymph nodes, uterine wall, lung, liver, and kidney; complete excision is difficult, and recurrence is expected  

 

PATHOGENESIS

• The developing embryo, uncommitted mesodermal cells differentiate into myogenic progenitor cells under the control of transcription factors such as PAX3, PAX7, and the FOXO1 family of transcription factors
• Myogenic precursors then proliferate by asymmetric replication, forming immature myoblasts under the direction of myoblast determination protein 1 (MyoD1) and myf4
• An arrested stage of development could be related to the PAX-FKHR mutation  
• The fusion of the PAX and FKHR genes leads to uncontrolled cell growth signals, cell cycle progression, failure of tumor suppressor gene function, and myogenic differentiation
• Fusion of PAX-FKHR causes increased levels of MyoD1, resulting in cell proliferation and also increased levels of myogenin, leading to myogenic differentiation
• Increased levels of IGF-II also result in cell proliferation.
• Mutations in the oncogene FGFR4 have been found in 7.5% of human alveolar and embryonal RMS
• Interleukin-4 (IL-4) helps recruit myoblasts to form myotubes in normal mammalian myogenesis and has been associated with childhood and mouse RMS differentiation, mitogenesis, and metastasis

 

TYPICAL CLINICAL FINDINGS:

• Hematuria, dysuria, cystitis, weight loss and secondary anemia 
• Lameness and swelling of distal limbs which is a finding of hypertrophic osteopathy, a paraneoplastic syndrome reported in dogs with bladder tumors, including botryoid rhabdomyosarcoma 
• Other complications include cystitis, urinary obstruction (hydroureter, hydronephrosis), and renal failure  

 

TYPICAL GROSS FINDINGS:

• Mass usually located at the trigone of the bladder; also rarely reported in the urethra, uterus, vagina  
• Polypoid or multinodular (grape-like), pale to light tan, firm or soft and friable
  • Frequently there is mucosal ulceration, hemorrhage, and necrosis

 

TYPICAL LIGHT MICROSCOPIC FINDINGS:

• Tumors can be poorly differentiated, and thus the histopathologic diagnosis of rhabdomyosarcoma may be difficult
• Polypoid neoplasm growing into the lumen of the bladder that is densely cellular beneath the mucosa (cambium layer) and a sparsely cellular with a myxoid matrix in the deeper aspects
• Spindle to stellate to fusiform or pleomorphic cells with numerous multinucleate cells; +/- cytoplasmic cross striations
• The classic diagnostic criteria of cross striations in elongated cells (“strap cells”) is useful to diagnose rhabdomyosarcoma, but many confirmed rhabdomyosarcomas lack demonstrable cross striations

 

ULTRASTRUCTURAL FINDINGS: 

 

ADDITIONAL DIAGNOSTIC TESTS: 

• A potential diagnostic pitfall is the fact that all myogenic markers used to diagnose RMS will also be present in developing and regenerating skeletal muscle; it is necessary to first differentiate satellite cell proliferation and differentiation from a neoplastic proliferation 
• Immunohistochemistry: In dogs, rhabdomyosarcomas are consistently immunohistochemically positive using antibodies for vimentin, desmin, muscle specific actin, src actin, and myoglobin but because of their lack of cellular differentiation, myoglobin staining is variable; in dogs, embryonal and botryoid RMS are frequently positive for myoglobin compared with alveolar RMS 

• Cytoplasmic markers: 
  • Vimentin, desmin, myoglobin, muscle specific actin, sarcomeric actin
  • Myoblast cells first express the intermediate filament vimentin,

1. During differentiation, they express the intermediate filament, desmin, which is common to skeletal muscle, cardiac muscle, smooth muscle, and myofibroblasts
2. More specific to skeletal muscle is expression of proteins involved in sarcomere construction such as the α-actin isoform found in sarcomeres, identified by the antibody sarcomeric actin (src actin)
   • As skeletal myocytes differentiate further, they begin to accumulate sarcomeric actin, followed by myosin or fast myosin, and finally myoglobin, which is first seen in large amounts following myoblast fusion to form myotubes or strap cells

• Nuclear markers: 
  • Myogenin and MyoD1 are muscle specific nuclear transcription factors expressed early in skeletal muscle development
  • MyoD1 expression is associated with immature rhabdomyoblasts that have a higher proliferative capacity
  • Myogenin is expressed during myoblast fusion and is associated with exit from the cell cycle
  • Rhabdomyosarcomas that are composed of relatively undifferentiated cells should be expected to express less desmin, actin, myosin, and myoglobin and more MyoD1 and myogenin
  • Positivity often limited to a small fraction of neoplastic cells and well differentiated neoplastic cells may be negative
  • Myogenin expression is also seen in nonneoplastic atrophy or regenerating skeletal muscle while mature normal skeletal muscle is negative
  • Mature skeletal muscle typically has little vimentin expression, whereas immature skeletal muscle and undifferentiated rhabdomyosarcomas typically have rare expression of myoglobin and myosin

• Phosphotungstic acid hematoxylin (PTAH): May accentuate intracytoplasmic cross-striations 
• Has been associated with radiographic osteolytic lesions, and may be confused with osteosarcoma

 

DIFFERENTIAL DIAGNOSIS:

• Transitional cell carcinoma: Usually cytokeratin5, and uroplakin III positive and vimentin negative; polygonal epithelial cells that form trabeculae, islands, and occasional acini
• Polypoid cystitis
• Leiomyosarcoma: Lack cross-striations; usually negative for myoglobin and positive for smooth muscle actin
• Papillomas
• Fibrosarcoma: Collagenous matrix (trichrome stains); lack cross-striations; negative immunohistochemically for muscle markers
• Hemangiosarcomas: Endothelial origin; factor VIII-related antigen immunopositivity
• Liposarcoma, rhabdomyosarcoma, and hibernoma share some overlapping histologic and IHC features (desmin and myogenin)

 

COMPARATIVE PATHOLOGY:

• Cats: 15 case reports of rhabdomyosarcoma 
• Sheep: Rhabdomyosarcoma has been reported relatively frequently
• Goats: 2 reported cases of rhabdomyosarcoma  
• Pig: Cardiac rhabdomyomas are found in pigs more often than in other species
  1. Protein gene product 9.5 expression in two pigs suggest a Purkinje cell origin (Purkinjeoma)
• Musk turtle: One case report of RMS in the forelimb (diagnosis confirmed with TEM)

 

REFERENCES: 

1. Agnew DW, MacLachlan J. Tumors of the genital systems. In: Meuten DJ ed. Tumors in Domestic Animals. 5^th ed. Ames, Iowa: Iowa State Press; 2017:693.
2. Barger, AM. Musculoskeletal System. In: Raskin RE, Meyer DM. Canine and Feline Cytology, 4^th Ed. St. Louis, MO:Elsevier. 2023:498-499.
3. Caserto BG.  A comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol. 2013; 55(5):806-826. 
4. Cianciolo RE and Mohr FC.  Urinary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed.Vol 2. Philadelphia, PA: Elsevier Ltd; 2016:462-464.
5. Cooper BJ, Valentine BA. Muscle and tendon. In: Maxie MG ed. Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals. 6th ed., Vol. 1. Saunders Elsevier, Philadelphia, PA, 2016:241-243.
6. Cooper BJ, Valentine BA: Tumors of muscle. In: Meuten DJ, ed. Tumors in Domestic Animals. 5^th ed. Ames, Iowa: Iowa State Press; 2017:444-466.
7. LaDouceur EB, Stevens SE, et al. Immunoreactivity of canine liposarcoma to muscle and brown adipose antigens. Vet Pathol. 2017; 54(6); 885-891.
8. Schwarz S, Mathes K, Wohlsein P. Rhabdomyosarcoma on the Forelimb of a Common Musk Turtle (Sternotherus odoratus). J Comp Pathol. 2021;186:73-76.
9. Shi J, Gao R, Zhang J, et al. Invasive spindle-cell rhabdomyosarcoma with osteolysis in a dog: case report and literature review. J Vet Diagn Invest. 2023;35(2):168-172.
10. Sula, MM and Lane, LV. The urinary system. In: Zachary JF, McGavin MD, eds. Pathologic Basis of Veterinary Disease. 7th ed. St. Louis, MO: Elsevier; 2022:669. 
11. Tuohy JL, Byer BJ, Royer S, et al. Evaluation of Myogenin and MyoD1 as Immunohistochemical Markers of Canine Rhabdomyosarcoma. Vet Pathol. 2021;58(3):516-526.

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