JPC SYSTEMIC PATHOLOGY
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 dense, polypoid neoplasm composed of spindle cells arranged in interlacing streams and bundles separated by scant eosinophilic fibrillar matrix. Neoplastic cells are most dense beneath the mucosa (cambium layer) and less dense at the deeper aspects with few neoplastic cells embedded in a myxomatous matrix. Neoplastic cells have variably distinct cell borders, moderate to abundant eosinophilic fibrillar cytoplasm, oval to elongate nuclei containing 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.
- Most common locations are laryngeal and cardiac:
- Twelve laryngeal rhabdomyomas/rhabdomyosarcomas have been reported in dogs, all but one in dogs older than 2 years
- It is generally believed that laryngeal rhabdomyomas/rhabdomyosarcomas are a distinct clinical entity of dogs, being locally invasive yet rarely metastatic
- The majority of cardiac muscle tumors, though rare in dogs, are RMS (6/12), 3 of which had metastasized
- In dogs, the botryoid variant of rhabdomyosarcoma occurs in the urinary bladder and urethra and is considered rare
- Occurs in young (less than 2 years of age)
- Large breed dogs; St. Bernards are overrepresented
- Females outnumber males 2:1
- Cutaneous rhabdomyomas are rarely reported
- Most common locations are laryngeal and cardiac:
- 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:
- The most aggressive of these neoplasms arise in juvenile dogs younger than 2 years and include alveolar RMS and embryonal RMS
- 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%)
- 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
- Botryoid (a distinct subtype of embryonal that affects the urinary bladder) Grape-like masses protruding from the mucosa that contain undifferentiated myoblast cells as well as multinucleated myotube cells within a myxomatous stroma
- Histologically, botryoid RMS of the urinary bladder is located in the submucosa, and neoplastic cells are separated from the mucosal epithelium by a “cambium” (ie, transitional) layer of connective tissue
- 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
- Classic alveolar pattern is characterized by extensive sloughing of neoplastic cells into a central open space with additional neoplastic cells lining the fibrous septa
- 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”
- Pleomorphic: often affects adults
- 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
- Lacks any of the features of alveolar or embryonal recognized in any part of the neoplasm
- 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
- 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 pro-liferation 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
Actin and myosin filaments, polyribosomes, Z-bands, and mitochondria are prominent in botryoid RMS, laryngeal muscle tumors (both rhabdomyomas and RMS), and RMS that is not further classified (RMS not otherwise specified [NOS])
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,
- Then later, during differentiation, they express the intermediate filament, desmin, which is common to skeletal muscle, cardiac muscle, smooth muscle, and myofibroblasts
- 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
- Cytoplasmic markers: vimentin, desmin, myoglobin, muscle specific actin, sarcomeric actin
- Phosphotungstic acid hematoxylin (PTAH): may accentuate intracytoplasmic cross-striations
- 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
- 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)
- In horses, botryoid rhabdomyosarcoma has been reported in the uterus of a yearling filly and in the urinary bladder of a 2-year-old filly; in a case series report and review of reported occurrences of rhabdomyosarcoma in horses the most common location was the tongue and limb muscles; most were classified as embryonal
- 15 cases of rhabdomyosarcoma has been reported in a cat
- Sheep: rhabdomyosarcoma has been reported relatively frequently
- One pleomorphic rhabdomyosarcoma was subjacent to nonhaired, nonpigmented, stratified squamous epithelium with prominent keratohyalin granules
- The second rhabdomyosarcoma was in the hard palate/maxilla and was classified as embryonal subtype
- Not commonly reported in wild type mice; the p16Ink4a/p19Arf-Mdm2-Trp53 and Patched1 pathways have been implicated as a cooperative effect for transgenic and xenotoxic mouse models
- A recent study of in vitro mouse mesenchymal stem cells demonstrated the ability of the PAX-FKHR translocation to produce alveolar RMS when coupled with loss of RB and p53 tumor suppressor gene function and constitutive H-RAS activation
- Pig: skeletal muscle RMS mentioned in pigs, cardiac rhabdomyomas are found in pigs more often than in other species
- Protein gene product 9.5 expression in two pigs suggest a Purkinje cell origin (Purkinjeoma)
- Cattle: appears histologically as cuboidal epithelioid cells arranged in glandular acini in a collagenous stroma
- Brook trout: report of a spontaneously arising solid alveolar rhabdomyosarcoma in soft tissues
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