Rhabdomyosarcoma

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Rhabdomyosarcoma, often abbreviated RMS, is a malignant tumour of skeletal muscle.

General

Classification

Histologic

  1. Alveolar rhabdomyosarcoma.
    • Usually young adults/adolescents.
    • Early mets common.
    • Usu. arises in regions with skeletal muscle.
  2. Embryonal rhabdomyosarcoma.
    • Usual <10 years old.
    • Typically locally invasive.
    • Usu. arises in regions without skeletal muscle.

Less common types:[2]

  1. Undifferentiated rhabdomyosarcoma.
  2. Botryoid - may be considered a subtype of embryonal RMS.
  3. Spindle cell - may be considered a subtype of embryonal RMS.
  4. Anaplastic.

Molecular and histologic

  1. Translocation-positive alveolar RMS.
  2. Translocation-negative alveolar RMS.
  3. Embryonal RMS.

Notes:

  • Translocation-negative alveolar RMS shares gene expression prolifing characteristics with embryonal RMS -- suggesting these can be grouped together.
  • Several classification of RMS exist - see: Classifications of Rhabdomyosarcoma.[3]

Microscopic

Alveolar rhabdomyosarcoma

Features:[1]

  • Alveolus-like pattern -- key low-power feature.
    • Fibrous septae lined by tumour cells.
      • Cells may "fall-off" the septa, i.e. be detached/scattered in the alveolus-like space.
      • Space between fibrous sepate may be filled with tumour = solid variant of alveolar rhabdomyosarcoma.
  • Rhabdomyoblasts - essentially diagnostic.
    • Eccentric nucleus.
    • Moderate amount of intensly eosinophilic cytoplasm.
    • Striations -- if you're really lucky; these are not common.

Other features:

  • Nuclear pleomorphism - common.
  • Mitoses - common.

Notes:

  • Well-differentiated rhabdomyoblasts are uncommon in alveolar RMS.

Embryonal rhabdomyosarcoma

Features:[1]

  • Randomly arranged small cells.
  • Myxoid matrix.
  • Strap cells:
    • Tadpole-like morphology.
  • Rhabdomyoblasts - essentially diagnostic.
    • Eccentric nucleus.
    • Moderate amount of intensly eosinophilic cytoplasm.
    • Striations -- if you're really lucky; these are not common.

Subtypes of embryonal RMS

There are two common subtypes of embryonal RMS. Both of them have a better prognosis that embryonal RMS not otherwise specified (NOS).

Common subtypes:

  1. Botryoid subtype (AKA sarcoma botryoides):
    • Gross: Grape-like morphology.
    • Microscopic: Non-proliferating layer deep to the surface ("Cambium layer").
  2. Spindle cell subtype.
    • General: may mimic leiomyosarcoma (complete with vesicular pattern) -- which is not common in the pediatric population.
    • Microscopic: vesicular growth pattern, spindle cells.

Notes:

  • Cambium layer = cellular region deep to epithelial component.[4]

Anaplasia

Criteria:

  1. Hyperchromatic nuclei with size variation greater or equal to 3x.
  2. Multipolar (atypical) mitotic figures.

Subclassification:

  1. Focal - a few cells.
  2. Diffuse - cluster or sheets of anaplasia.

Notes:

  • Not subtle - can identify at low power.
  • Seen in 10-15% of RMS.
    • More common in older individuals.
  • Poorer prognosis in embryonal RMS.
    • No change in prognosis in alveolar RMS.

IHC

Panel of muscle markers -- DAM:

  • Desmin (best marker).
  • Actin.
  • Myogenin.

Subtyping via IHC

PST proposes[1] the following (presumably based on Makawitz et al.[5]):

IHC Translocation positive
alveolar RMS 		Embryonal RMS Translocation negative
alveolar RMS
myogenin +ve -- diffuse +ve -- focal +ve -- diffuse
EGFR -ve +ve -ve
P-cadherin +ve -ve -ve
IGF2 -ve +ve +ve

A paper by Wachtel at al.[6] proposes the use of:

  • AP2beta and P-cadherin +ve in translocation positive alveolar RMS, and
  • EGFR and fibrillin-2 +ve in embryonal RMS and translocation negative alveolar RMS.

Electron microscopy

Features:

  • Sarcomeric like structures - usu. in "bent" cells; cells that are U-shaped.

Molecular diagnostics

Alveolar rhabdomyosarcoma

Common translocations (~85% of cases):

  • t(1,13).
    • PAX7/FKHR fusion gene.
    • Seen in approx. 15% of cases.
  • t(2,13).[7]
    • PAX3/FKHR fusion gene.
    • Seen in approx. 70% of cases.

Notes:

  • t(1,13) vs. t(2,13) -- t(1,13) usually: younger age, extremity lesion, localized disease, better survival.
  • Several uncommon translocations exist.

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 PST. 14 February 2011.
  2. ↑ Hicks, J.; Flaitz, C. (Jul 2002). "Rhabdomyosarcoma of the head and neck in children.". Oral Oncol 38 (5): 450-9. PMID 12110339.
  3. ↑ Parham, DM. (May 2001). "Pathologic classification of rhabdomyosarcomas and correlations with molecular studies.". Mod Pathol 14 (5): 506-14. doi:10.1038/modpathol.3880339. PMID 11353062.
  4. ↑ URL: http://www.medilexicon.com/medicaldictionary.php?t=48297. Accessed on: 9 August 2011.
  5. ↑ Makawita S, Ho M, Durbin AD, Thorner PS, Malkin D, Somers GR (2009). "Expression of insulin-like growth factor pathway proteins in rhabdomyosarcoma: IGF-2 expression is associated with translocation-negative tumors". Pediatr. Dev. Pathol. 12 (2): 127–35. doi:10.2350/08-05-0477.1. PMID 18788888.
  6. ↑ Wachtel M, Runge T, Leuschner I, et al. (February 2006). "Subtype and prognostic classification of rhabdomyosarcoma by immunohistochemistry". J. Clin. Oncol. 24 (5): 816–22. doi:10.1200/JCO.2005.03.4934. PMID 16391296.
  7. ↑ URL: http://www.ncbi.nlm.nih.gov/omim/606597. Accessed on: 18 August 2010.
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