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Soft tissue tumors

Skeletal muscle

Fetal type rhabdomyoma

Reviewer: Vijay Shankar, M.D. (see Reviewers page)
Revised: 21 February 2013, last major update November 2012
Copyright: (c) 2003-2013, PathologyOutlines.com, Inc.

Definition
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● Rare benign tumor of immature skeletal muscle differentiation, usually head and neck
● Extracardiac rhabdomyomas are divided into fetal (below), adult and genital histologic types
● Extracardiac tumors are not associated with tuberous sclerosis
● Retroauricular in ages 0-3 years

Epidemiology
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● 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
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● 1 month old infant girl with swelling in submandibular region (Indian Pediatr 2004;41:839)
● 1 year old girl with skin tumor (Am J Surg Pathol 2008;32:485)
● 9 year old boy with cellular fetal rhabdomyoma of thigh (Pediatr Blood Cancer 2009;52:881)
● Congenital tumor with recurrence (Int J Pediatr Otorhinolaryngol 2006;70:1115)

Treatment
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● Complete excision
● Only rare recurrences (Pediatr Pathol Lab Med 1996;16:673)
● No metastases

Gross description
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● Median 3-5 cm
● Solitary, well circumscribed mass of soft tissue or mucosa
● Gray-white-tan-pink, soft with glistening cut surface

Micro description
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● 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

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

Various images

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

The fetal rhabdomyoma is comprised of less mature, somewhat pleomorphic, polygonal muscle cells admixed with spindle-shaped cells

Mucosal tumors have a pseudocambium layer of plasma cells and lymphocytes resembling embryonal rhabdomyosarcoma, but there is no atypia and no mitotic figures

Tumor cells are arranged in fascicles with less stroma	Immature skeletal muscle cells	Ganglion cell-like rhabdomyoblasts
Strap cells with abundant eosinophilic cytoplasm and cross striations	Myxoid tumor of tongue (various images)	PTAH shows cross striations

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

Cytology description
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● Spindle cells and rhabdomyoblasts with abundant eosinophilic cytoplasm (Singapore Med J 2009;50:e138)

Positive stains
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● Muscle specific actin, desmin and myoglobin (100%), GFAP (40%)

Negative stains
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● Keratin, EMA, CD68

Electron microscopy description
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● Hypertrophied Z band material, thick and thin filaments, numerous mitochondria, some with inclusions

Differential diagnosis
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● Botyroid variant of embryonal rhabdomyosarcoma: resembles myxoid variant of fetal rhabdomyoma but has deep location, true cambium layer, atypia, numerous mitotic figures, tumor cell necrosis, infiltrative margins, no maturation of cells at periphery
● Infantile fibromatosis: deep location, fascicles of spindle cells, no cross striations, no undifferentiated cells
● Neuromuscular hamartoma: S100+ nerve fibers and skeletal muscle in same perimysial sheath
● Spindle cell variant of embryonal rhabdomyosarcoma: resembles cellular variant of fetal rhabdomyoma but has cellular pleomorphism and tumor cell necrosis

End of Soft tissue tumors > Skeletal muscle > Fetal type rhabdomyoma

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