Embryonal Rhabdomyosarcoma

Embryonal Rhabdomyosarcoma
Khin Thway, BSc, MBBS, FRCPath
Key Facts
Terminology
  • Malignant soft tissue tumor that shows variable differentiation toward skeletal muscle
  • Encompasses botryoid, spindle cell, and anaplastic variants
Clinical Issues
  • Most common rhabdomyosarcoma (RMS) subtype
    • Represents 60-70% of RMS
  • Generally affects younger population than alveolar RMS
    • Majority in children < 5 years
  • Head and neck
  • Genitourinary region
  • Other sites include retroperitoneum, pelvis, and biliary tract
Microscopic Pathology
  • Loose fascicles and sheets
    • Cytology, pattern, and cellularity can vary
  • Spindle, stellate, and ovoid cells
  • Rhabdomyoblasts in variable numbers and stages of differentiation
  • Botryoid RMS
    • Grows beneath epithelial surfaces
  • Spindle cell RMS
    • Spindle cells with elongated nuclei
    • May exhibit cross-striations
  • Anaplastic RMS
    • Atypical or bizarre tumor cells, present focally or more diffusely
  • Complex karyotypes
  • Associated with 11p15.5 loss of heterozygosity
Embryonal rhabdomyosarcoma of the bladder is seen under urothelium image. It is mostly of low cellularity and not easily identifiable at low power when the appearances can be mistaken for inflammation.
The tumor cells show a range of appearances. There are relatively undifferentiated ovoid cells image along with rhabdomyoblasts with prominent eosinophilic cytoplasm and eccentric nuclei image.
TERMINOLOGY
Abbreviations
  • Embryonal rhabdomyosarcoma (ERMS)
Definitions
  • Malignant soft tissue tumor that shows variable differentiation toward embryonic skeletal muscle
  • ERMS encompasses botryoid, spindle cell, and anaplastic variants
ETIOLOGY/PATHOGENESIS
Unknown
  • Cell of origin still unknown
    • Possible candidate cells include muscle stem cells and multipotent mesenchymal stem cells
  • Often occur in sites lacking skeletal muscle
    • e.g., bladder, prostate
CLINICAL ISSUES
Epidemiology
  • Incidence
    • Rhabdomyosarcomas (RMS) are most frequent soft tissue sarcomas in children and young adults
    • ERMS is most common RMS subtype
      • 4.3 cases per 1,000,000 children/year in USA
      • Represent 60-70% of RMS
  • Age
    • Children
    • ERMS generally affects younger population than alveolar RMS
    • Typically < 10 years of age
      • Majority in children < 5 years
      • Bimodal distribution with smaller peak in adolescence
      • Occasional cases congenital
    • Rarer in adults
      • Where pleomorphic subtype predominates
  • Gender
    • Slight male preponderance
    • M:F= 1.4:1
Site
  • Head and neck
    • Particularly orbital and parameningeal sites
  • Genitourinary region
    • Bladder
    • Prostate
    • Paratesticular soft tissue
  • Other sites
    • Retroperitoneum and pelvis
    • Biliary tract
  • Much less frequent involvement of trunk and limbs than alveolar rhabdomyosarcoma (ARMS)
  • Spindle cell RMS
    • Paratesticular region
  • Botryoid RMS
    • Arise beneath mucosal epithelial surfaces
      • Bladder
      • Vagina
      • Extrahepatic bile ducts
      • More rarely, auditory canal or conjunctiva
Presentation
  • Suddenly enlarging mass
    • Local symptoms pertaining to site of origin
      • e.g., deafness, proptosis (head and neck)
      • e.g., urinary retention (genitourinary sites)
Prognosis
  • Main prognostic parameters are histologic type, disease stage, and site
  • Favorable sites are head and neck (nonparameningeal), genitourinary (nonbladder, nonprostate), and bile duct
  • Botryoid and spindle cell variants (excluding aggressive adult spindle cell variant) have better prognosis
  • ERMS has significantly better prognosis than ARMS
  • 5-year overall survival approximately 73%
MACROSCOPIC FEATURES
General Features
  • Fleshy mass
  • Margins usually infiltrative
  • Tan to white
  • Rubbery cut surface
  • Hemorrhage
  • Necrosis
  • Cystic degeneration
  • Botryoid RMS
    • Exophytic, polypoid tumor
    • More circumscribed margins
    • Small nodules adjacent to mucosal surface
    • Masses may fill lumen of hollow viscus
    • Gelatinous cut surface
MICROSCOPIC PATHOLOGY
Histologic Features
  • Patterns variable
  • Loose fascicles and sheets
  • Variable cellularity
    • Can alternate between markedly cellular and looser myxoid zones
  • Spindle, stellate, and ovoid cells
    • In varying stages of myogenic differentiation
    • Ovoid and elongated nuclei
      • Nuclei hyperchromatic or vesicular
  • Rhabdomyoblasts
    • Cells with eccentric nuclei and variable amounts of eosinophilic cytoplasm
    • Cytoplasmic cross-striations may be visible
    • Variable numbers and stages of differentiation
    • Varying shapes
      • Strap cells
      • Tadpole cells
      • Spider cells
  • Myxoid stroma
  • Mitoses usually easily discernible
  • Necrosis
  • Tumor giant cells are rare, in contrast to alveolar RMS
  • Botryoid RMS
    • Polypoid
    • Cambium layer
      • Tightly packed cellular layer of tumor cells closely abutting epithelial surface
    • Loose myxoid stroma
    • Can be of relatively low cellularity
      • May be missed or mistaken for chronic inflammation
  • Spindle cell RMS
    • Spindle cells with elongated nuclei
    • May exhibit cross-striations
    • Tumors may have abundant collagenous stroma
  • Anaplastic RMS
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Jul 9, 2016 | Posted by in PATHOLOGY & LABORATORY MEDICINE | Comments Off on Embryonal Rhabdomyosarcoma

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