Soft Tissue
Skeletal muscle
Fetal type rhabdomyoma

Author: Vijay Shankar, M.D. (see Authors page)

Revised: 3 November 2015, last major update November 2012

Copyright: (c) 2003-2015, PathologyOutlines.com, Inc.

PubMed Search: Rhabdomyoma [title] fetal
Definition / General
  • Rare benign tumor of immature skeletal muscle differentiation, usually head and neck
  • Extracardiac rhabdomyomas are divided into fetal, adult and genital histologic types
  • Extracardiac tumors are not associated with tuberous sclerosis
  • Retroauricular in ages 0-3 years
Epidemiology
  • 70% male, median age 4.5 years, range 3-58 years (Hum Pathol 1993;24:754)
  • Usually head and neck, post auricular region is the most common site
  • Slightly male predominance
  • Multiple cases of fetal rhabdomyoma have been reported in patients with nevoid basal cell carcinoma syndrome (Virchows Arch 2011;459:235)
Case Reports
Treatment
Gross Description
  • Median 3-5 cm
  • Solitary, well circumscribed mass of soft tissue or mucosa
  • Gray-white-tan-pink, soft with glistening cut surface
Micro Description
  • Circumscribed but not encapsulated
  • Myxoid or cellular
  • Myxoid:
    • Bundles or fascicles of immature slender skeletal muscle with delicate cytoplasmic cross striations and thin tapering eosinophilic processes, resembling myotubules at week 7-12 of gestation
    • Also undifferentiated round/oval or spindled mesenchymal cells
    • Stroma is myxoid or fibromyxoid
    • Skeletal muscle cells mature towards periphery, may have "pseudocambium" layer of plasma cells and lymphocytes under mucosal epithelium
  • Cellular:
    • Bundles or fascicles of cells in parallel or plexiform patterns
    • Sparse collagenous or myxoid stroma
    • Cells have variable skeletal muscle differentiation ranging from immature cells of myxoid pattern (but in larger numbers) to ganglion cell-like rhabdomyoblasts with prominent nucleoli, or strap cells with abundant basophilic or eosinophilic cytoplasm and prominent cross-striations
    • Infiltration of skeletal muscle may make margins difficult to determine
    • Variable glycogen containing vacuoles
    • No/rare mitotic figures
Micro Images
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Immature muscle cells and mesenchymal cells are often in central portion of tumor, with better differentiated skeletal muscle cells and mesenchymal cells at periphery

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Undifferentiated round mesenchymal cells and immature skeletal muscle cells within myxoid or edematous stroma

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Various images

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Pleomorphic, polygonal muscle cells


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Figure 1

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Figure 2


Fig 1: The bipolar, immature skeletal muscle cells have tapered eosinophilic cytoplasmic processes and closely resemble the myotubular stage of striated muscle development, and the undifferentiated cells have minimal cytoplasm and round or oval nuclei
Fig 2: Mucosal tumors have a pseudocambium layer of plasma cells and lymphocytes resembling embryonal rhabdomyosarcoma, but there is no atypia and no mitotic figures

Cellular fetal type rhabdomyoma:

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Tumor cells are arranged in fascicles with less stroma

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Immature skeletal muscle cells

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Ganglion cell-like rhabdomyoblasts

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Strap cells with abundant eosinophilic cytoplasm and cross striations

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Myxoid tumor of tongue (various images)

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PTAH shows cross striations

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Ganglion cell like rhabdomyoblasts or strap cells are arranged in patternless pattern with undifferentiated mesenchymal cells



Courtesy of Dr. Mark R. Wick:

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Base of tongue, juvenile

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Cellular variant



Cytology Description
Positive Stains
Negative Stains
Electron Microscopy Description
  • Hypertrophied Z band material, thick and thin filaments, numerous mitochondria, some with inclusions
Differential Diagnosis