Skeletal Muscle Tumors

Slight male predominance

Most cases are congenital or discovered during infancy or early childhood

Rarely diagnosed in young adults

Mostly sporadic but may occur as part of a syndrome such as Goldenhar (oculoauriculovertebral) and Delleman (oculocerebrocutaneous) syndromes

Other associated conditions include persistent thyroglossal duct, amniotic band syndrome, spinal dysraphism, cleft lip, and cleft gum

Majority of cases discovered incidentally

Solitary lesions by far predominate (over 80%)

Size ranges from a few millimeters to 2 cm

Skin-colored papule, nodule, pedunculated lesion, or mass, exceptionally as an atrophic plaque mimicking morphea “en coup de sabre”

The head and neck area is the most common affected location, particularly the periorbital and perioral regions

Rare cases described in perineal area, vagina, digits, oral cavity, and nares

Benign lesion

Cured by total surgical excision

No recurrences documented

An instance of complete spontaneous regression reported

Subcutaneous or dermal lesion with spared overlying epidermis

Proliferation of mature skeletal muscles with regular cross-striations arranged in bands, single muscle fibers, or haphazard fashion

Additional components in variable proportions include

• Mesodermal tissue: mature fat, elastic fibers, collagen, blood vessels, arrector pili muscles

• Ectodermal tissue: eccrine glands, pilosebaceous units, vellus hair follicles, nerves

Calcification or ossification rarely seen

The skeletal muscle component expresses desmin, myoglobin, and myogenin

Accessory tragus

Rhabdomyoma

Rhabdomyosarcoma

Nevus lipomatosis superficialis

Fibrous hamartoma of infancy

Infantile myofibromatosis

Neuromuscular choristoma

Cutaneous lesions are divided into

• Adult type: more common; affects the head and neck of middle-aged or elderly males

• Fetal type: most commonly located at the periorbital, periauricular, and perioral areas of male infants

Slowly growing, painless, solitary

Small nodular or lobulated mass

Periorbital lesions present with proptosis

Size ranges from 0.5 to 10 cm

Benign tumor

Cured by complete surgical excision

Recurs only if incompletely excised

Well-circumscribed, nonencapsulated, subcutaneous tumor composed of variable proportion of large polygonal cells with abundant eosinophilic cytoplasm and small, round or spindle-shaped cells

Intracytoplasmic crystalline, rodlike inclusions may be seen

Cytoplasm may be vacuolated (spider cells)

Small, round vesicular nuclei with prominent nucleoli

Adult rhabdomyoma

• Mainly large and polygonal cells with abundant eosinophilic cytoplasm

• A scant stroma

• Mitoses absent or rare

• Intracytoplasmic crystalline, rodlike inclusions (corresponding to the Z-lines) are commonly seen

Fetal rhabdomyoma

• A predominance of immature, round to spindle-shaped rhabdomyoblasts with a wide spectrum of skeletal muscle differentiation

• Cross-striation in the cytoplasm of skeletal muscle fibers usually present at the periphery of the tumor, consistent with maturation of lesional cells

• A variably myxoid stroma

• Variation in cell size and shape without prominent atypia

• Mitoses can be frequent

• Necrosis is usually absent

• A cellular variant is recognized

Tumor cells express desmin, myogenin, and muscle specific actin

Vimentin, S100 protein, and smooth muscle actin are less commonly positive

The adult type

• Muscle cell differentiation with actin and myosin bundles

• Cytoplasmic glycogen and crystalline rod-shaped inclusions

The fetal type

• Muscle cell differentiation with actin and myosin bundles

• Rod-shaped cytoplasmic inclusions are less common

• Undifferentiated spindle cells also seen

Rhabdomyosarcoma

Granular cell tumor

Hibernoma

Infantile fibromatosis