Soft tissue
Skeletal muscle
Embryonal rhabdomyosarcoma

Author: Michael Clay, M.D. (see Authors page)
Deputy Editor Review: Debra Zynger, M.D.

Revised: 27 July 2018, last major update May 2018

Copyright: (c) 2002-2018, PathologyOutlines.com, Inc.

PubMed Search: Embryonal rhabdomyosarcoma[TI] pathology free full text[sb]

Related topics: Rhabdomyosarcoma, NOS, spindle cell

Cite this page: Clay, M. Embryonal rhabdomyosarcoma. PathologyOutlines.com website. http://www.pathologyoutlines.com/topic/softtissueembryonalrhabdo.html. Accessed September 22nd, 2018.
Definition / general
  • Subtype of the rhabdomyosarcoma (RMS) soft tissue cancer family whose lineage derives from the undifferentiated mesoderm
  • Embryonal rhabdomyosarcoma (ERMS) is the most common RMS subtype
  • Several distinct and prognostic patterns of growth have been noted, including the botryoid and anaplastic variants
Essential features
  • Most common subtype of rhabdomyosarcoma in the pediatric and adolescent setting
  • Anaplasia is associated with TP53 mutations, p53 protein overexpression and worse overall prognosis (Cancer 2014;120:1068)
  • Displays a wide spectrum of morphologic features, including cases with spindled morphology
    • See Terminology section below regarding spindle cell rhabdomyosarcoma
  • Displays some degree of myogenic differentiation, including either MyoD1 or myogenin positivity
  • Not associated with recurrent gene fusions but displays some recurrent copy number changes
Terminology
  • Botryoid embryonal rhabdomyosarcoma ("sarcoma botryoides") only occurs in certain locations, specifically beneath a mucosal epithelial lined viscera, such as the bladder, biliary tract, vagina or upper respiratory tract
  • Anaplasia is an important prognostic feature and is defined similar to anaplastic Wilms tumor: significant nuclear variation (cells that are 3x larger than background tumor cells) and the presence of atypical multipolar mitotic figures
  • Spindle cell rhabdomyosarcoma:
    • Over the years, tumors with spindled morphology have been regarded as a variant of ERMS
    • These show a predilection for the paratesticular soft tissues and are now recognized as carriers of specific gene fusions, including NCOA2 and VGLL2 fusions or MYOD1 L122R point mutations (for an excellent recent review, see Am J Surg Pathol 2016;40:224)
    • These entities will be better defined in future editions of the WHO classification and have already been distanced from ERMS with the provisional classification of spindled / sclerosing rhabdomyosarcoma
ICD-10 coding
Epidemiology
  • Most common in children 0 - 4 years old with a maximum reported incidence of 4 cases per 1 million children but can be identified at any age (Cancer 2009;115:4218)
  • Represent approximately 60 - 70% of childhood cases and 20% of adult cases
  • Rare congenital cases have also been reported (Int J Surg Case Rep 2017;31:47)
Sites
  • Most common in the head and neck region, including the nasal and oral cavities, as well as the orbit and middle ear (J Clin Oncol 1995;13:610)
  • Also common in the paratesticular soft tissues and the genitourinary tract
  • Extremity involvement of the somatic soft tissues is less common (< 9%)
  • Metastasizes to soft tissue, serosa, lung, lymph nodes and bone marrow
Pathophysiology
  • Derived from the undifferentiated mesoderm and shows phenotypic and biologic features of primitive skeletal muscle
Etiology
Prognostic factors
  • Intergroup Rhabdomyosarcoma Study Group (IRSG) has subdivided rhabdomyosarcoma into low, intermediate and high risk groups for the purpose of protocol placement in pediatric patients (Pediatr Blood Cancer 2012;59:5)
    • Based on clinicopathologic variables including histologic type, extent of surgical control of local disease and primary tumor site
  • Favorable prognostic factors are age (between 1 and 9 years), botryoid variant, orbital and paratesticular location and absence of metastatic disease at the time of resection
  • Anaplasia is a negative prognostic factor, regardless of whether it is focal or diffuse (Cancer 2008;113:3242)
  • Extremity involvement is associated with more relapses and lower survival
Case reports
Treatment
  • Surgery is the preferred local therapy, with radiotherapy typically used only in the event of residual disease, nodal disease or poor response to combined chemotherapy
  • Intergroup Rhabdomyosarcoma Study Postsurgical Clinical Grouping System is recommended to plan treatment in pediatric cases (Pediatr Blood Cancer 2012;59:5)
  • American Joint Committee on Cancer (AJCC) TNM staging system remains appropriate for planning treatment in adult patients (Amin: AJCC Cancer Staging Manual, 8th Edition, 2018)
  • Anaplastic tumors often require more intensive treatment (Cancer 2008;113:3242)
Gross description
  • Poorly circumscribed mass, white, soft or firm, infiltrative
Gross images

Images hosted on PathOut server:

Contributed by Mark R. Wick, M.D.

Orbital


Case of the Week #268:

Parotid mass



Images hosted on other servers:

White tumor mass with focal necroses

Incision of the oral mucosa

Pitfalls and tips
  • Anaplasia specifically requires multipolar mitotic figures
    • Make sure you are not overinterpreting degenerating or apoptotic cells
    • Look for organization of the division and exclude cells that appear to be "exploding"
    • Also try to avoid interpreting overlapping cells as one larger cell
  • Absence of a translocation does not result in the default diagnosis of embryonal rhabdomyosarcoma; tumor must adhere to the described histologic features
  • Some rhabdomyosarcomas defy classification (rhabdomyosarcoma, not otherwise specified); this designation is preferable to misclassification, especially in small or otherwise limited samples
  • Beware nonspecific staining (aberrant positivity for cytokeratin or S100 protein) vs. true staining (S100 protein as a component of malignant peripheral nerve sheath tumor)
  • Damaged skeletal muscle can express MyoD1 and myogenin; a different sarcoma that otherwise infiltrates skeletal muscle will very much mimic a rhabdomyosarcoma on immunohistochemistry staining
    • Look for the MyoD1 / myogenin to be positive in tumor cells in a linear pattern, which suggests the staining is of background muscle and not the tumor
Microscopic (histologic) description
  • Composed of primitive mesenchymal cells that show variable degrees of skeletal muscle differentiation
  • They are moderately cellular but in the typical pattern often contain both hypo- and hypercellular areas with a loose, myxoid stroma
  • Perivascular condensations of tumor cells in the less cellular regions are common
  • Sheets of small, stellate, spindled or round cells with scant or deeply eosinophilic cytoplasm and eccentric, small oval nuclei with a light chromatin pattern and inconspicuous nucleoli
  • Can occasionally identify tumor cells that contain generous amounts of eosinophilic cytoplasm, a feature of rhabdomyoblastic differentiation (so called "strap" cells)
    • These may become more prominent with chemotherapy (chemotherapeutic induced cytodifferentiation)
  • May have cells with elongated tails of cytoplasm (tadpole cells)
  • If densely cellular, may resemble solid alveolar rhabdomyosarcoma (Am J Clin Pathol 2013;140:82)
  • Botryoid variant frequently shows a "cambium layer," a hypercellular zone immediately beneath the epithelial surface
    • Deeper layers of the tumor are typically less cellular but overall conform to the histology of embryonal rhabdomyosarcoma (ERMS) with variation by region
  • Rare morphological patterns:
Microscopic (histologic) images

Images hosted on PathOut server:

Contributed by Erdener Özer, M.D., Ph.D.

Small spindled and round cells with variable eosinophilic cytoplasm; H&E and stains



Contributed by Mark R. Wick, M.D.

Orbital

Myogenin



Case of the Week #276 - tonsillar mass:

Various images


Actin

Desmin

Myogenin

MyoD1



Anaplastic rhabdomyosarcoma:

Extremely hyperchromatic nuclei are 3x larger than nuclei of adjacent tumor cells, mixed with smaller, undifferentiated, round and spindle cells

Bizarre mitotic figure (left side)

Rare cells suggest skeletal muscle differentiation



Images hosted on other servers:

Anaplastic rhabdomyosarcoma

Cytology description
Cytology images

Images hosted on PathOut server:

Contributed by Erdener Özer, M.D., Ph.D.

Small round cell tumor

Positive stains
Electron microscopy description
  • Developing striated muscle, thick and thin filaments
Molecular / cytogenetics description
  • 11p15.5 loss of heterozygosity or loss of imprinting is a frequent finding
  • Chromosomal aneuploidies are very common, especially with gains of chromosome 8 (seen in 90% of patients)
    • Additional gains frequently seen include chromosomes 2, 11, 12, 13 and 20
  • No diagnostic translocation found to date (see Terminology regarding spindled rhabdomyosarcoma)
  • Inactivating mutations of TP53 and CDKN2A and activating mutations of RAS family genes (Genes Chromosomes Cancer 2011;50:397)
  • Frequency of ALK dysregulation is debatable, as are any significant associated clinical features (Mod Pathol 2013;26:772)
Differential diagnosis
  • Alveolar rhabdomyosarcoma (ARMS):
    • Diffuse and strong nuclear staining for myogenin and MyoD1; molecular studies (including FISH) show PAX-FOXO1 fusion gene product in approximately 85% of cases (Am J Clin Pathol 2013;140:82)
    • Histologically, ARMS are more uniform and show more predictable patterns of growth (a finding which correlates with their consistent driver fusion event - they look like a "translocation" sarcoma)
  • Desmoplastic small round cell tumor:
    • Tumor nodules on serosal surfaces, strongly keratin+ and EMA+, may be desmin+ but muscle specific actin-
    • Associated with desmoplasia and histologically more closely mimic alveolar rather than embryonal rhabdomyosarcoma (ERMS)
    • Will not display myogenin or MyoD1 positivity; beware false positivity if they infiltrate background skeletal muscle
    • See Pitfalls and tips above
  • Ewing / PNET:
    • Another round cell tumor, often displays Homer Wright rosettes
    • Nuclei are far more uniform and pale, not dense and hyperchromatic; CD99+, desmin-, MyoD1-, myogenin-
    • Look for characteristic rearrangements including t(11;22) or t(12;22)
    • In general, although there can be histologic overlap, if you sample the tumor enough you will find more atypia and cell to cell variability in rhabdomyosarcoma
  • Large cell lymphoma:
  • Monophasic synovial sarcoma:
    • Negative for muscle markers, usually far more spindled and organized (long fascicles) when compared to ERMS
    • Cytokeratin and EMA expression will show scattered and focal positivity in the spindled cells, the characteristic pattern seen in nearly all of these lesions
    • Molecular or FISH testing for the t(X;18) fusion, if needed
  • Myxoid liposarcoma (MLS):
    • Although MLS typically is very easy to distinguish from ERMS, it can have a very similar background matrix that is loose and lightly basophilic
    • Cells can focally resemble one another as well
    • Look for signet ring lipoblasts in MLS, more bland histology and more uniformity when compared to ERMS
    • Immunostaining and molecular features are also easily utilized to differentiate the two entities
  • Neuroblastoma:
    • Undifferentiated neuroblastoma can be very difficult to distinguish histologically; you may need to rely on the clinical presentation and the immunoprofile
    • Elevated urinary catecholamines, rosettes, granular chromatin, neuropil
    • S100+ (often), chromogranin+, synaptophysin+, GFAP+
    • Absence of myogenic markers
  • Pleomorphic rhabdomyosarcoma:
    • Exclusively adults, usually in their 60s - 70s
    • Usually deep soft tissue of the extremity and remarkable for its universal diffuse cytologic atypia
    • Uniformly pleomorphic and does not contain elements of embryonal rhabdomyosarcoma
    • Anaplastic, as opposed to ERMS which occasionally displays anaplasia in a background of classic EMRS
  • Wilms tumor:
    • Can have rhabdomyoblastic differentiation, especially in the setting of chemotherapy
    • Beware any tumor in the kidney or any metastatic site in a patient with a previous Wilms tumor
    • Epithelial and blastemal components can be very focal following chemotherapy
    • If a Wilms tumor displays this pattern, it usually shows abundant cytodifferentiation, as opposed to classic rhabdomyosarcoma, which is usually focal
Board review question #1
Which FISH testing may assist in making distinction between the dense pattern of embryonal rhabdomyosarcoma and the alveolar subtype of rhabdomyosarcoma?

  1. ETV6-NTRK3
  2. EWS-FLI1
  3. PAX3-FOXO1
  4. SYT-SSX1
  5. TPM3-ALK
Board review answer #1
C. PAX3-FOXO1. This fusion gene product occurs in approximately 80% of alveolar rhabdomyosarcoma cases. Additionally, PAX7-FOXO1 rearrangements are present in a minority of cases.
Board review question #2
Which histologic features in embryonal rhabdomyosarcoma are needed to diagnose anaplasia?
  1. 3x nuclear size variability and tripolar mitotic figures
  2. Alternating hypercellular and hypocellular areas
  3. Coagulative necrosis
  4. More than 20 mitoses per 10 high power fields
  5. Solid growth pattern
Board review answer #2
A. Multipolar mitotic figures (including tripolar mitoses) and nuclear anaplasia (3x variation in nuclear size) are required to diagnosis anaplasia in embryonal rhabdomyosarcoma (ERMS). Solid growth pattern is irrelevant and necrosis can be seen in any subtype of rhabdomyosarcoma. ERMS typically displays the alternating pattern of hypo and hypercellularity (also a buzzword for high grade malignant peripheral nerve sheath tumors). There is no requirement for a specific mitotic rate.