Tumors of skeletal and smooth muscles

1. Skeletal Muscle Tumors

A) Rhabdomyoma (Benign)

• Rare benign tumor of skeletal muscle.
• Different types:
  • Adult type: in the oral cavity.
  • Fetal form: heart + head/neck.
  • Genital type: vulvovaginal region.
• Cardiac rhabdomyoma is associated with tuberous sclerosis.
• Histology: well-differentiated, large rounded/polygonal cells with abundant acidophilic cytoplasm + cross-striations.

B) Rhabdomyosarcoma (Malignant)

• Malignant skeletal muscle tumor of mesenchymal origin.
• #1 soft tissue sarcoma of childhood; usually before age 20.
• Sites: head + neck, urogenital tract, retroperitoneum, limbs.
  • Embryonal subtype = sarcoma botryoides (grape-like polypoid mass in vagina/bladder of young girls).

Pathogenesis

• Chromosomal translocation: t(2;13) → PAX3-FKHR fusion protein → dysregulates muscle differentiation.

Morphology / Subtypes

Subtype	Age	Prognosis
Embryonal	Most common; children	Good
Alveolar	Adolescents + young adults	Worst prognosis
Spindle cell	Variable	Variable
Pleomorphic	Older adults; no cross-striations	Poor

• Rhabdomyoblast = cell seen in all types; granular eosinophilic cytoplasm + cross-striations (“strap cells”).
• IHC: desmin⁺, muscle-specific actin⁺, myogenin⁺, MyoD1⁺.

Treatment

• Surgery + chemotherapy + radiation.
• Curable in children; less likely in adults.

2. Smooth Muscle Tumors

A) Leiomyoma (Benign — “Uterine Fibroid”)

• Benign tumor arising from smooth muscle of myometrium.
• Most common benign tumor in females — affects 30–50 % of women of reproductive age.
• Estrogen + OCPs stimulate growth → enlarges during pregnancy, shrinks postmenopausally.
• Genetic influence likely.

Morphology

• Sharply circumscribed, firm, gray-white masses.
• Whorled cut surface (characteristic).
• Locations:
  • Intramural — within myometrium.
  • Submucosal — beneath endometrium (causes bleeding).
  • Subserosal — beneath serosa; may develop attenuated stalk → pedunculated.
• Larger lesions → ischemic necrosis + hemorrhage + cystic softening.
• Postmenopausal → collagenous + calcified.
• Histology: whorled (fascicular) pattern of SM bundles duplicating normal myometrium.

Clinical Features

• Menorrhagia (heavy + prolonged menses).
• Uterine bleeding at irregular intervals.
• Pelvic pain, mass effect, infertility.
• Rarely transforms into leiomyosarcoma (most leiomyosarcomas arise de novo).

B) Leiomyosarcoma (Malignant)

• Malignant smooth muscle neoplasm.
• Arises de novo from mesenchymal cells of myometrium, NOT from preexisting leiomyomas.
• Almost always solitary (vs leiomyomas, which are usually multiple).
• Older women (postmenopausal).
• Recurrences + lung metastases frequent; 5-yr survival ~40 %.

Morphology

• Patterns: bulky mass infiltrating uterine wall, polypoid lesion into uterine cavity, or resembling benign leiomyoma.
• Soft, hemorrhagic, necrotic.
• Histology: spindle cells with cigar-shaped nuclei in fascicles.

Diagnostic Triad

• Necrosis + cytological atypia + mitotic activity (> 10 mitoses / 10 HPF).

3. Summary Table

Tumor	B/M	Origin	Key features
Rhabdomyoma	B	Skeletal muscle	Cardiac form → tuberous sclerosis; vulvovaginal/oral/H&N
Rhabdomyosarcoma	M	Skeletal muscle	#1 soft tissue sarcoma in kids; t(2;13) PAX3-FKHR; embryonal (botryoides) vs alveolar (worst); desmin/myogenin⁺
Leiomyoma	B	Smooth muscle (myometrium)	Most common benign tumor in females; estrogen-sensitive; whorled; menorrhagia
Leiomyosarcoma	M	Smooth muscle	De novo (not from leiomyoma), solitary, postmenopausal, lung mets; necrosis + atypia + mitoses

💡 High-yield: Rhabdomyoma = rare benign skeletal muscle; cardiac rhabdomyoma = tuberous sclerosis. Rhabdomyosarcoma = #1 soft tissue sarcoma of childhood (< 20 yr); t(2;13) PAX3-FKHR fusion; subtypes: embryonal (#1, sarcoma botryoides), alveolar (worst), spindle, pleomorphic; desmin + myogenin + MyoD1 positive; rhabdomyoblasts with cross-striations. Leiomyoma (fibroid) = #1 benign tumor in females, 30–50 % of reproductive women, estrogen-sensitive (↑ pregnancy, ↓ menopause), whorled pattern, menorrhagia, rarely transforms. Leiomyosarcoma = arises de novo, solitary, postmenopausal, necrosis + atypia + mitoses, lung mets, 5-yr survival ~40 %.