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PA32.2-3 | Bone & Soft Tissue Tumors — SDL Guide (Part 4)

Rhabdomyosarcoma, Leiomyosarcoma, Fibrosarcoma

Rhabdomyosarcoma (RMS) — most common soft tissue sarcoma in children and adolescents.
- Origin: skeletal muscle (striated) precursors.
- Sites: head and neck (orbit, nasopharynx), genitourinary tract (bladder, vagina — "botryoid" variant), and extremities.
- Embryonal RMS (most common subtype): children <10 years; spindle cells + rhabdomyoblasts (strap cells); botryoid variant has grape-like polypoid growth pattern; favorable prognosis.
- Alveolar RMS: adolescents and young adults; defining molecular marker: t(2;13) PAX3-FOXO1 or t(1;13) PAX7-FOXO1; cells arranged in alveolar spaces (like lung); aggressive, worse prognosis.
- Pleomorphic RMS: rare, adults; highly malignant.
- Histology: cells with deeply eosinophilic cytoplasm, cross-striations (visible in well-differentiated areas — confirm with desmin, myogenin, myoD1 immunostains).
- Spread: both hematogenous (lung) AND lymphatic (unlike most other sarcomas).

Embryonal Rhabdomyosarcoma — H&E and Desmin IHC (400×)

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Leiomyosarcoma (LMS) — malignant smooth muscle tumor; one of the most common soft tissue sarcomas in adults.
- Sites: retroperitoneum, uterus (most common soft tissue sarcoma in women), inferior vena cava, and deep extremities.
- Histology: interlacing bundles of elongated spindle cells with blunt-ended ("cigar-shaped") nuclei and eosinophilic fibrillary cytoplasm. Mitotic figures are key to diagnosis (≥5 mitoses/10 HPF for uterine LMS).
- IHC: smooth muscle actin (SMA) positive, desmin positive, CD117 negative (distinguishes from GI stromal tumor).
- Prognosis: variable; retroperitoneal LMS has poor prognosis due to late detection and large size.

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Fibrosarcoma — malignant tumor of fibroblasts.
- Rare in pure form today (most tumors previously called fibrosarcoma are now reclassified with molecular tools).
- Histology: herringbone (fascicular) pattern — interlacing bundles of atypical spindle cells in a chevron arrangement, with scant collagen between cells. No osteoid, no chondroid, no cross-striations.
- Prognosis: intermediate; treatment is wide surgical excision.

Fibrosarcoma Histology — Herringbone Pattern, Spindle Cell Atypia, and Mitosis (H&E)

Synovial Sarcoma

Synovial sarcoma is a high-grade soft tissue sarcoma that deserves separate emphasis because it has a distinctive age group, translocation, and morphology.

Key facts
- Despite the name, it does not arise from synovial cells but from an unspecified mesenchymal precursor that differentiates along synovioblastic lines.
- Age: adolescents and young adults (15–35 years).
- Site: typically around large joints — knee, ankle, and foot; also found in the thigh, paravertebral region, and pleuropulmonary locations.
- Does not arise within the joint cavity — it arises near joints in periarticular soft tissues.

Molecular hallmark: t(X;18)(p11;q11) — SYT-SSX fusion gene; present in >95% of cases; diagnostic by FISH or RT-PCR.

Radiology: soft tissue mass near a joint, often with calcification (30% of cases — a distinguishing feature from most other sarcomas of this age group).

Histology
- Biphasic pattern (classic): two components:
1. Epithelial-like cells — form glands or tubules, resemble carcinoma.
2. Spindle cells — sarcomatous component.
- Monophasic pattern (more common): only spindle cells, no glandular differentiation.
- IHC: TLE1 positive (highly sensitive marker), EMA and CK positive (epithelial markers — unusual for a sarcoma), CD34 negative.

Biphasic Synovial Sarcoma — H&E, 200×: Epithelial and Spindle Cell Components

Spread and prognosis
- Hematogenous spread to lungs; lymph node metastasis in ~20% (unusual for sarcomas).
- 5-year survival ~50–70% for localized disease; poor if metastatic.
- Treatment: wide resection + adjuvant radiation ± chemotherapy (ifosfamide-based).