Definition/General
Introduction:
Rhabdomyoma is a rare benign tumor showing skeletal muscle differentiation
It represents <1% of soft tissue tumors
Two main types: cardiac and extracardiac
FNAC shows mature skeletal muscle cells.
Origin:
Arises from skeletal muscle cells
Cardiac type: Associated with tuberous sclerosis
Extracardiac type: Head/neck region most common
Originates from primitive mesenchymal cells with myogenic differentiation.
Classification:
WHO classification: Benign skeletal muscle tumor
Types: Cardiac rhabdomyoma (infancy)
Adult rhabdomyoma (head/neck)
Fetal rhabdomyoma (rare)
Genital rhabdomyoma (rare).
Epidemiology:
Cardiac type: Infancy and childhood
Adult type: 4th-7th decades
Male predominance (3:1) for adult type
Associated with tuberous sclerosis (cardiac)
Very rare soft tissue tumor.
Clinical Features
Presentation:
Slow-growing mass
Painless in most cases
Head and neck location (adult type)
Well-circumscribed mass
Mobile consistency
Heart involvement (cardiac type).
Symptoms:
Usually asymptomatic
Cardiac arrhythmias (cardiac type)
Heart failure (large cardiac tumors)
Mass effect symptoms
Dysphagia (pharyngeal)
Voice changes (laryngeal).
Risk Factors:
Tuberous sclerosis complex (cardiac type)
TSC1/TSC2 mutations
Male gender (adult type)
Age >40 years (adult type)
Head/neck location
Previous radiation (rare).
Screening:
Echocardiography (cardiac type)
Clinical examination for masses
Family history of tuberous sclerosis
FNAC/Biopsy for diagnosis
Imaging studies for extent
Genetic counseling if indicated.
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Gross Description
Appearance:
Well-circumscribed mass
Soft to firm consistency
Gray-red cut surface
Homogeneous appearance
Size varies 1-10 cm
No necrosis typically.
Characteristics:
Encapsulated or well-demarcated
Solid consistency
Red-brown color
Muscle-like appearance
Vascular cut surface
Homogeneous texture.
Size Location:
Head and neck (70% adult type)
Larynx and pharynx common
Heart (cardiac type)
Tongue and soft palate
Size: 2-8 cm average
Multiple tumors (cardiac type).
Multifocality:
Solitary (adult type)
Multiple lesions (cardiac type)
Bilateral in tuberous sclerosis
Spontaneous regression (cardiac type)
Unilateral predominance (extracardiac).
Microscopic Description
Histological Features:
FNAC shows mature skeletal muscle cells
Large polygonal cells
Abundant eosinophilic cytoplasm
Cross-striations visible
Multiple peripheral nuclei
No cellular atypia.
Cellular Characteristics:
Large, mature myocytes
Polygonal cell shape
Abundant cytoplasm
Multiple nuclei at periphery
Cross-striations in cytoplasm
Well-defined cell borders.
Architectural Patterns:
Individual large cells
Loose tissue fragments
Background of muscle cells
Syncytial appearance
Multinucleated cells
Clean background.
Grading Criteria:
Benign features
No grading applicable
Assessment: Skeletal muscle differentiation
Maturity of cells
Absence of atypia
Cross-striations present.
Immunohistochemistry
Positive Markers:
Desmin - strongly positive
MyoD1 - positive (nuclear)
Myogenin - positive (nuclear)
Muscle-specific actin - positive
Sarcomeric actin - positive
Myosin - positive.
Negative Markers:
S-100 protein - negative
Cytokeratin - negative
EMA - negative
CD34 - negative
Smooth muscle actin - negative
CD68 - negative.
Diagnostic Utility:
MyoD1 and myogenin confirm skeletal muscle differentiation
Desmin supports muscle origin
Distinguishes from smooth muscle tumors
Ki-67 very low (<1%)
Cross-striations diagnostic.
Molecular Subtypes:
Adult type: Standard skeletal muscle markers
Cardiac type: Same markers, cardiac location
Fetal type: Immature muscle markers
All types show myogenic transcription factors.
Molecular/Genetic
Genetic Mutations:
TSC1/TSC2 mutations (cardiac type with tuberous sclerosis)
mTOR pathway alterations
Simple karyotype (adult type)
Hamartin/Tuberin complex disruption
No specific mutations (adult type).
Molecular Markers:
mTOR pathway activation (cardiac type)
Myogenic transcription factors
Low proliferation markers
Intact tumor suppressors
Normal p53
Stable genome.
Prognostic Significance:
Excellent prognosis
No malignant potential
Local recurrence rare after excision
Spontaneous regression (cardiac type)
No metastatic risk
Long-term cure.
Therapeutic Targets:
Complete surgical excision
mTOR inhibitors (cardiac type with TSC)
Observation (small cardiac lesions)
No adjuvant therapy required
Symptomatic treatment.
Differential Diagnosis
Similar Entities:
Rhabdomyosarcoma
Granular cell tumor
Hibernoma
Oncocytoma
Adult granular cell tumor
Skeletal muscle (normal).
Distinguishing Features:
Rhabdomyoma: Mature skeletal muscle, no atypia, MyoD1+
Rhabdomyosarcoma: Immature cells, atypia, high Ki-67
Granular cell tumor: S-100+, granular cytoplasm
Hibernoma: Multivacuolated, UCP1+
Normal muscle: Fascicular arrangement, clinical correlation.
Diagnostic Challenges:
Distinguishing from rhabdomyosarcoma (especially embryonal)
Normal skeletal muscle contamination
Granular cell tumor confusion
Site-specific considerations
Sampling adequacy.
Rare Variants:
Fetal rhabdomyoma
Genital rhabdomyoma
Intermediate type
Rhabdomyoma with atypia
Multifocal rhabdomyoma.
Sample Pathology Report
Template Format
Sample Pathology Report
Complete Report: This is an example of how the final pathology report should be structured for this condition.
Specimen Information
Fine needle aspiration from [site], [size] cm soft tissue mass
Clinical History
[age]/[sex] with [duration] history of slow-growing mass at [site]
Microscopic Examination
Smears show large, mature skeletal muscle cells with abundant eosinophilic cytoplasm, multiple peripheral nuclei, and visible cross-striations. No cellular atypia.
Immunocytochemistry
MyoD1: Positive (nuclear), Myogenin: Positive (nuclear), Desmin: Positive, S-100: Negative
Cytological Diagnosis
Cytological features consistent with RHABDOMYOMA
Comments
Benign skeletal muscle tumor. Complete excision recommended. Excellent prognosis. Rule out tuberous sclerosis if cardiac location.