Alveolar Rhabdomyosarcoma
Definition
- Primitive myoblastic neoplasm found most commonly in the extremities, paranasal sinuses and parameningeal region
Diagnostic Criteria
- Second most common type of rhabdomyosarcoma, comprises 31% of RMS
- Considered an unfavorable histologic type
- 5-year failure free survival rate: 65%
- Sheets of uniform cells, frequently discohesive, broken up by fibrous septae
- Generally round to oval nuclei
- Hyperchromatic with small nucleoli
- Occasional rhabdomyoblasts seen in 30% of cases
- Fibrillar cytoplasm but only rare cross striations
- Usually round
- Sheets broken up by fibrous septae
- Vessels contained in septae
- Two histologic subtypes
- Classical
- Nests of neoplastic cells arranged in alveolar spaces
- Cells adhere to the periphery of the alveoli
- Hobnail or tombstone appearance
- May look like a non-cohesive papillary pattern
- Non-cohesive cells appear to float in the center
- Multinucleated giant cell forms may be seen
- Nuclei usually peripheral, wreath-like
- Normal muscle fibers may be entrapped
- Solid
- Sheets of neoplastic cells
- Nests separated by thin fibrovascular septae but alveoli are not seen
- PAX-FOXO1 translocations aid in diagnosis and determination of prognosis
- Considered diagnostic if present (see Supplemental Studies)
- PAX3-FOXO1 t(2;13)(q35;q14) – 60%
- PAX7-FOXO1 t(1;13)(p36;q14) – 20% (better prognosis)
- Fusion negative ~ 15%
- 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
- Common sites of involvement:
- Population: most >10 years of age
