Benign tumors
Rhabdomyomatous Mesenchymal Hamartoma
Definition
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A rare dermal or subcutaneous benign lesion composed of disordered mature elements, including skeletal muscle, adipose tissue, nerve bundles, blood vessels, adnexal tissue, and collagen
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Initially reported as striated muscle hamartoma
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Additional terms used in the past include congenital midline hamartoma and hamartoma of cutaneous adnexa and mesenchyme
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Could reflect abnormalities in the migration of mesoderm during embryogenesis
Clinical features
Epidemiology
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Slight male predominance
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Most cases are congenital or discovered during infancy or early childhood
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Rarely diagnosed in young adults
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Mostly sporadic but may occur as part of a syndrome such as Goldenhar (oculoauriculovertebral) and Delleman (oculocerebrocutaneous) syndromes
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Other associated conditions include persistent thyroglossal duct, amniotic band syndrome, spinal dysraphism, cleft lip, and cleft gum
Presentation
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Majority of cases discovered incidentally
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Solitary lesions by far predominate (over 80%)
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Size ranges from a few millimeters to 2 cm
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Skin-colored papule, nodule, pedunculated lesion, or mass, exceptionally as an atrophic plaque mimicking morphea “en coup de sabre”
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The head and neck area is the most common affected location, particularly the periorbital and perioral regions
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Rare cases described in perineal area, vagina, digits, oral cavity, and nares
Prognosis and treatment
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Benign lesion
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Cured by total surgical excision
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No recurrences documented
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An instance of complete spontaneous regression reported
Pathology
Histology
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Subcutaneous or dermal lesion with spared overlying epidermis
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Proliferation of mature skeletal muscles with regular cross-striations arranged in bands, single muscle fibers, or haphazard fashion
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Additional components in variable proportions include
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Mesodermal tissue: mature fat, elastic fibers, collagen, blood vessels, arrector pili muscles
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Ectodermal tissue: eccrine glands, pilosebaceous units, vellus hair follicles, nerves
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Calcification or ossification rarely seen
Immunohistochemistry/special stains
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The skeletal muscle component expresses desmin, myoglobin, and myogenin
Main differential diagnoses
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Accessory tragus
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Rhabdomyoma
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Rhabdomyosarcoma
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Nevus lipomatosis superficialis
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Fibrous hamartoma of infancy
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Infantile myofibromatosis
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Neuromuscular choristoma
Rhabdomyoma
Definition
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An uncommon benign tumor of either mature (adult type) or immature (fetal type) striated skeletal muscle arising in the dermis or subcutaneous tissue
Clinical features
Epidemiology
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Cutaneous lesions are divided into
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Adult type: more common; affects the head and neck of middle-aged or elderly males
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Fetal type: most commonly located at the periorbital, periauricular, and perioral areas of male infants
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Presentation
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Slowly growing, painless, solitary
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Small nodular or lobulated mass
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Periorbital lesions present with proptosis
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Size ranges from 0.5 to 10 cm
Prognosis and treatment
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Benign tumor
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Cured by complete surgical excision
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Recurs only if incompletely excised
Pathology
Histology
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Well-circumscribed, nonencapsulated, subcutaneous tumor composed of variable proportion of large polygonal cells with abundant eosinophilic cytoplasm and small, round or spindle-shaped cells
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Intracytoplasmic crystalline, rodlike inclusions may be seen
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Cytoplasm may be vacuolated (spider cells)
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Small, round vesicular nuclei with prominent nucleoli
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Adult rhabdomyoma
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Mainly large and polygonal cells with abundant eosinophilic cytoplasm
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A scant stroma
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Mitoses absent or rare
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Intracytoplasmic crystalline, rodlike inclusions (corresponding to the Z-lines) are commonly seen
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Fetal rhabdomyoma
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A predominance of immature, round to spindle-shaped rhabdomyoblasts with a wide spectrum of skeletal muscle differentiation
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Cross-striation in the cytoplasm of skeletal muscle fibers usually present at the periphery of the tumor, consistent with maturation of lesional cells
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A variably myxoid stroma
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Variation in cell size and shape without prominent atypia
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Mitoses can be frequent
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Necrosis is usually absent
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A cellular variant is recognized
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Immunohistochemistry/special stains
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Tumor cells express desmin, myogenin, and muscle specific actin
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Vimentin, S100 protein, and smooth muscle actin are less commonly positive
Ultrastructure
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The adult type
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Muscle cell differentiation with actin and myosin bundles
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Cytoplasmic glycogen and crystalline rod-shaped inclusions
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The fetal type
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Muscle cell differentiation with actin and myosin bundles
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Rod-shaped cytoplasmic inclusions are less common
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Undifferentiated spindle cells also seen
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Main differential diagnoses
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Rhabdomyosarcoma
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Granular cell tumor
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Hibernoma
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Infantile fibromatosis
Malignant tumors
Embryonal Rhabdomyosarcoma
Definition
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A malignant mesenchymal tumor with predominant primitive cell morphology and evidenced skeletal muscle differentiation
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Primary cutaneous tumors that arise in the dermis or subcutaneous tissue and lack deep soft tissue involvement are extremely rare
Clinical features
Epidemiology
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Strong predilection for children and young adults
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Rarely affects older individuals
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Both sexes approximately equally affected
Presentation
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Small, painless, solitary dermal or subcutaneous nodule
Prognosis and treatment
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Limited information about prognosis, and clinical behavior is incompletely defined
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Generally aggressive type of sarcoma despite small size and superficial location
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Recent series of 11 primary cutaneous rhabdomyosarcomas revealed the presence of regional lymph node or distant metastases in over 50% of the patients and overall mortality of 36% in the mean follow-up period of 38 months
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A combination of surgical excision, chemotherapy, and radiotherapy is employed
Pathology
Histology
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Sheets of small, round to ovoid, uniform primitive cells with indistinct cell borders predominate
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Well-differentiated muscle cells exhibiting cytoplasmic cross-striations may be focally seen
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Typical cells include (not all cell types necessary are present)
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Ribbon- or strap-shaped cells with elongated nuclei and bipolar cytoplasmic processes
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“Tadpole” cells with eccentric nuclei and a single cytoplasmic process
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“Spider-web” epithelioid cells with paranuclear periodic acid–Schiff (PAS)–positive intracytoplasmic vacuoles and threadlike cytoplasm
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Brisk mitotic activity
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Perivascular accentuation/condensation of tumor cells is typical
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A variably myxoid stroma
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Three histological variants exist
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Botryoid rhabdomyosarcoma: characterized by dense subepithelial “cambium” layer
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Anaplastic rhabdomyosarcoma: exhibiting marked atypia, hyperchromasia, and pleomorphism
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Spindle cell rhabdomyosarcoma: composed almost entirely of spindle-shaped cells; currently recognized as a separate subtype of rhabdomyosarcoma (see Spindle cell/sclerosing rhabdomyosarcoma )
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Immunohistochemistry/special stains
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Tumor cells express desmin, myogenin, and myoD1
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Aberrant cytokeratin expression can be seen
Genetic profile
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More structural (but not recurrent) and copy number alternations from alveolar rhabdomyosarcomas, including gains of chromosomes 2, 8, 12, 13, and 20, and regional losses of 11p
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MYOD1 mutations that may cooperate with PI3K-AKT pathway mutations in a subset of more aggressive cases
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Intragenic deletions of DMD, which encodes dystrophin
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The botryoid form can be seen in the familial cancer susceptibility syndrome of DICER1 -related disorders
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Association with neurofibromatosis type 1 ( NF1 mutations) and Costello syndrome ( HRAS mutations)
Main differential diagnoses
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Alveolar rhabdomyosarcoma
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Ewing sarcoma
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Neuroblastoma
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Cutaneous lymphoma
Alveolar Rhabdomyosarcoma
Definition
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A malignant mesenchymal tumor often with a distinct alveolar-like growth pattern and skeletal muscle differentiation
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Primary cutaneous tumors that arise in the dermis or subcutaneous tissue and lack deep soft tissue involvement are extremely rare
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PAX3 or PAX7 – FOXO1 fusion gene is characteristically seen in vast majority of cases
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Previously, tumors with combined embryonal and alveolar features were considered variants of alveolar rhabdomyosarcoma. Most such tumors lack the specific PAX – FOXO1 fusion gene and tend to behave more like embryonal tumors
Clinical features
Epidemiology
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Strong predilection for children and young adults
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Rarely affects older individuals
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Both sexes equally affected
Presentation
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Small, painless solitary dermal or subcutaneous nodule
Prognosis and treatment
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Limited information about prognosis, and clinical behavior is incompletely defined
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Generally aggressive type of sarcoma despite small size and superficial location
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Recurrence, distant metastasis and lymphnode involvement reported
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A combination of surgical excision, chemotherapy, and radiotherapy is employed
Pathology
Histology
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Nests of large rounded cells with readily recognizable rhabdomyoblastic differentiation separated by fibrous septa
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Alveolar growth pattern with central cellular dissociation and necrosis
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Nuclei are hyperchromatic and larger than nuclei in embryonal type
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Multinucleated giant cells with wreathlike nuclei are common
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Abundant mitoses
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Epidermotropism reported in a single case
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A solid variant lacking cellular discohesion is recognized
Immunohistochemistry/special stains
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Tumor cells express desmin, myogenin, and myoD1
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Aberrant cytokeratin expression can be seen
Ultrastructure
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Skeletal muscle differentiation with actin and myosin cytoplasmic filaments
Genetic profile
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t(2;13)(q35;q14) resulting in PAX3-FOXO1 fusion gene
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t(1;13)(p36;q14) resulting in PAX7-FOXO1 fusion gene is less common
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An estimated 15% to 20% of tumors lack PAX-FOXO1 fusion; in some other variant translocations are seen
Main differential diagnoses
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Metastatic rhabdomyosarcoma
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Embryonal rhabdomyosarcoma
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Triton tumor (malignant peripheral nerve sheath tumor with heterologous rhabdomyoblastic component)
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Ewing sarcoma
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Neuroblastoma
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Cutaneous lymphoma
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