Embryonal Rhabdomyosarcoma
Definition
- Primitive myoblastic neoplasm most commonly found in hollow visceral organs, genitourinary tract and the head and neck region
Diagnostic Criteria
- Most common type of rhabdomyosarcoma, (68%)
- Considered a favorable histologic type
- 5-year failure free survival rate: 82%
- Alternating cellular and myxoid areas
- Foci of immature cartilage or bone are occasionally present
- Hyperchromatic histologically undifferentiated
small cell population usually predominates
- Immature cells showing muscle differentiation frequently scattered, rarely predominate
- Round cells with abundant, usually eccentric eosinophilic cytoplasm
- Cross striations not typically present
- Nuclei frequently vesicular with prominent nucleolus
- Spindle cell myoblasts with prominent tapered fibrillar eosinophilic cytoplasm
- May form tadpole or strap cells
- Nuclei may be multiple
- Cross striations may be present
- Anaplastic cellular features may be seen in approximately 13% of all subtypes of rhabdomyosarcoma.
- Anaplasia is defined as neoplastic nuclei at least 3 times the size of their neoplastic neighbors and/or atypical mitotic figures.
- If present, the focal or diffuse nature of the anaplasia should also be described.
- Focal anaplasia refers to anaplastic cells loosely scattered among non-anaplastic tumor cells.
- Diffuse anaplasia refers to anaplastic cells arranged in multiple clusters or diffuse sheets.
- The presence of anaplasia confers a worse prognosis (see Clinical), especially when the anaplasia is diffuse
- Three histologic subtypes
- Botryoid
- Requires by the presence of a condensed layer of neoplastic cells beneath intact epithelium (cambium layer)
- Typically separated from mucosa by a myxoid, hypocellular zone
- Primarily occurs in mucosal lined sites
- Grape like gross appearance is typical but not required
- Spindled
- Spindled morphology
- Primarily found in the paratesticular and orbital regions
- Not otherwise specified (NOS) is most common
- No PAX-FOXO1 translocations
- Common sites of involvement:
- Genitourinary system (infants and young children)
- Orbit
- Head and neck
- Population: 42% children < 4 years of age
