---

Soft Tissue Tumors: Complete Overview

---

1. DEFINITION AND CLASSIFICATION PRINCIPLES

• Soft tissue sarcomas (STSs) are malignant mesenchymal lesions distinct from epithelial tumors, CNS tumors, hematopoietic, and lymphoid neoplasms.
• Tumors likely arise from pluripotent mesenchymal stem cells rather than mature differentiated cells (explaining why rhabdomyosarcoma occurs in anatomic sites with no skeletal muscle).
• WHO recognizes approximately 70 distinct subtypes of STS.
• Classification is based on: line of differentiation (what tissue the tumor resembles), behavior (benign, intermediate, malignant), histology, immunohistochemistry (IHC), and molecular/genetic signature.

---

2. EPIDEMIOLOGY

• STSs are rare: approximately 13,400 new cases and 5,140 deaths annually in the USA (2023 data), accounting for ~1% of all cancer incidence and ~2% of cancer deaths.
• Most common location: extremities, particularly the thigh (44% of cases).
• Age distribution: bimodal - rhabdomyosarcoma, hemangioma, neurofibroma, and alveolar soft parts sarcoma predominate in children/young adults; liposarcoma, leiomyosarcoma, and undifferentiated pleomorphic sarcoma (UPS) predominate in adults/elderly.
• The most common histologic subtype overall is liposarcoma, which accounts for 45% of all retroperitoneal sarcomas.

---

3. ETIOLOGY AND RISK FACTORS

Sporadic

Germline / Genetic Syndromes

Environmental / Iatrogenic

• Radiation exposure: post-radiation sarcomas (classically angiosarcoma after breast RT)
• Chemical carcinogens: vinyl chloride (hepatic angiosarcoma); thorotrast
• Chronic lymphedema: Stewart-Treves syndrome (angiosarcoma after axillary lymph node dissection)

---

4. COMMON SPECIFIC TUMOR TYPES

A. ADIPOSE TISSUE TUMORS

Lipoma (Benign)

• Most common soft tissue tumor overall
• Age 40-60 years; superficial extremity/trunk location
• Morphology: mature adipose tissue, encapsulated, no nodularity
• MRI features of benignity: <10 cm, <2 mm septa, >75% fat content, homogeneous
• Treatment: simple excision beyond the capsule

Liposarcoma (Malignant)

• Most common STS subtype; 45% of all retroperitoneal sarcomas
• Diagnostic hallmark: lipoblasts (signet ring-shaped cells)
• IHC: MDM2-positive (due to chr 12q13-15 amplification)
• Rarely occurs in subcutaneous tissues

---

B. FIBROUS TISSUE TUMORS

Nodular Fasciitis (Benign)

• Young adults (20-30 years); arm/forearm
• Histology: tissue culture-like growth with extravasated red cells
• Self-limiting; can mimic sarcoma

Deep Fibromatosis / Desmoid Tumor (Locally Aggressive)

• Age 30-40 years; abdominal wall
• Associated with FAP/Gardner syndrome (intraabdominal desmoids)
• Does not metastasize but is locally invasive and prone to recurrence
• Histology: dense collagen, long unidirectional fascicles
• First-line treatment: surgery; adjuncts include sorafenib, imatinib

Fibrosarcoma (Malignant)

• Age 40-60 years; deep extremity
• Histology: fasciculated growth of fusiform/spindle cells with herringbone pattern - intersecting fascicles where nuclei align transversely in one fascicle and longitudinally in the adjacent fascicle

---

C. SKELETAL MUSCLE TUMORS

Rhabdomyoma (Benign)

• Frequently associated with tuberous sclerosis
• Head/neck or cardiac location
• Histology: polygonal rhabdomyoblasts, "spider cells"

Rhabdomyosarcoma (Malignant)

• Most common STS of childhood and adolescence (before age 20)
• Paradoxically arises in sites with no skeletal muscle (head/neck, genitourinary tract) - supporting mesenchymal stem cell origin
• Subtypes:

• Embryonal: soft gray infiltrative mass; primitive round/spindle cells in myxoid stroma; rhabdomyoblasts with cross-striations. Sarcoma botryoides is a variant in hollow viscera (bladder, vagina)
• Alveolar: fibrous septa creating alveolar pattern; uniform round, discohesive cells
• Pleomorphic: large bizarre multinucleated eosinophilic cells; confirmed by IHC (myogenin)
• Spindle cell/sclerosing: fusiform cells in long fascicles or storiform pattern; dense collagenous stroma

---

D. SMOOTH MUSCLE TUMORS

Leiomyoma (Benign)

• Deep extremity; uniform plump eosinophilic cells in fascicles

Leiomyosarcoma (Malignant)

• Location: thigh or retroperitoneum; age 40-60
• Histology: fascicular growth pattern with spindle cells intersecting at right angles
• IHC: positive for smooth muscle actin (SMA) and desmin

---

E. VASCULAR TUMORS

Hemangioma (Benign)

• Most common in children/infants (age 0-10); head and neck
• Circumscribed mass of capillary or venous channels

Angiosarcoma (Malignant)

• Age 50-80; skin or deep lower extremity
• Highly malignant - lymph node and cutaneous metastases common
• Histology: cells resemble endothelium; infiltrating capillary channels
• Amputation may be necessary for local control due to infiltrative margins
• Part of the ESARC group (lymph node-metastasizing STS)

---

F. NERVE SHEATH TUMORS

Schwannoma (Benign)

• Head and neck; age 20-50
• Histology: encapsulated, fibrillar stroma, nuclear palisading (Antoni A/B areas)

Neurofibroma (Benign)

• Skin/subcutis; age 10-20+
• Myxoid, ropy collagen, loose fascicles, mast cells
• Associated with NF1 (Von Recklinghausen disease)

Malignant Peripheral Nerve Sheath Tumor (MPNST)

• Age 20-50; extremities and shoulder girdle
• Histology: tight fascicles, sweeping pattern, whorled appearance
• IHC: S-100 positive
• Can arise de novo OR in the setting of neurofibromatosis (NF1)

---

G. TUMORS OF UNCERTAIN DIFFERENTIATION

Synovial Sarcoma

• Despite the name, does NOT arise from synovium
• Translocation: t(X;18) → SS18::SSX fusion
• Histology: classic biphasic pattern - spindle cells + gland-like (epithelioid) components (Fig. 26.46 in Robbins)
• Part of the ESARC lymph node-metastasizing group
• Occurs in young adults; near joints of extremities

Undifferentiated Pleomorphic Sarcoma (UPS)

• Formerly called malignant fibrous histiocytoma (MFH)
• Arises in deep soft tissues of extremities (especially thigh) in middle-aged/older adults
• Histology: sheets of large anaplastic polygonal cells with hyperchromatic bizarre nuclei; storiform (cartwheel) pattern; slit-like vessels; abundant mitoses including atypical forms
• Size: 10-20 cm; necrosis and hemorrhage common
• Complex karyotype due to genomic instability
• Metastases occur in 17-50% of cases

GIST (Gastrointestinal Stromal Tumor)

• Most common GI mesenchymal tumor
• KIT or PDGFRA mutations
• Treated with imatinib (tyrosine kinase inhibitor)
• Associated with Carney-Stratakis syndrome (SDH mutations)

---

5. TUMORS WITH LYMPH NODE METASTASES (ESARC Mnemonic)

---

6. CLINICAL PRESENTATION

• Masses are deceptively painless and are commonly mistaken for lipomas
• Key rule: a mass that is >5 cm, growing, and deep to the superficial fascia should be presumed an STS until proven otherwise
• Some masses are small and present for a long time before recognized as malignant
• Delay in diagnosis is common
• Lung is the most common site of STS metastasis (hematogenous spread via IVC)
• Lymph node metastasis occurs with only 5% of STSs (except ESARC tumors)

---

7. IMAGING

• MRI provides: tumor size, grade, depth, anatomic location, and aids surgical planning
• CT chest is performed to evaluate for pulmonary metastases
• MRI features suggesting liposarcoma (vs benign lipoma): >10 cm, septa >2 mm, non-adipose areas, <75% fat content

---

8. BIOPSY

• Core needle biopsy is preferred over open biopsy
• Biopsy tract must be placed in a location that can be excised en bloc with the tumor during definitive surgery
• An unplanned (inadvertent) excision is the most common error in STS treatment - residual tumor may remain and repeat excision (often with RT) is required

---

9. STAGING

• Tumor size: <5 cm (T1) vs >5 cm (T2) vs >10 cm (T3) vs >15 cm (T4)
• Histologic grade: G1 (low) / G2 (intermediate) / G3 (high)
• Nodal status (N)
• Metastasis (M)

1. Tumor differentiation score
2. Mitotic count
3. Tumor necrosis

1. Presence of metastases
2. High histologic grade
3. Size >5 cm
4. Unplanned (inadvertent) excision

---

10. MOLECULAR GENETICS

---

11. TREATMENT

Surgery

• Wide local excision (limb-sparing surgery) is the standard of care
• Goal: R0 resection (negative margins)
• Amputation is reserved for cases where limb-sparing surgery cannot achieve adequate margins (e.g., some angiosarcomas)
• For retroperitoneal sarcomas, involved organ resection is often required (kidney, colon, etc.)

Radiation Therapy

• Standard adjunct to surgery for most intermediate-to-high grade tumors
• External beam radiation (EBRT) - preoperative or postoperative with equivalent oncologic outcomes:
  • Preoperative RT: smaller field, better tumor downstaging, but higher wound complication rate
  • Postoperative RT: larger field, lower wound complication rate
• Brachytherapy is an alternative for selected cases

Chemotherapy

• Doxorubicin (anthracycline) + ifosfamide is the standard first-line regimen for metastatic STS
• Gemcitabine + docetaxel: second-line for retroperitoneal and uterine leiomyosarcomas
• Imatinib: first-line for GIST (KIT/PDGFRA-mutant)
• Pembrolizumab (PD-1 inhibitor): promising responses in UPS and dedifferentiated liposarcoma
• Trabectedin, tyrosine kinase inhibitors, MDM2 antagonists, CDK4 antagonists: under investigation
• CTLA-4 + PD-1 combination showed poor response rates in metastatic STS

---

12. PROGNOSIS AND SURVEILLANCE

• Lung is the primary metastatic site (for most STSs)
• Overall 5-year survival for high-grade STS: approximately 50-60%
• Retroperitoneal sarcomas carry worse prognosis due to late presentation, difficulty achieving negative margins, and high local recurrence rate
• Surveillance: clinical exam + cross-sectional imaging every 3-6 months for high-grade tumors, gradually decreasing in frequency

---

13. SPECIAL SUBTYPES - QUICK PEARLS

---

14. ENTITIES THAT MIMIC STS (Differential Diagnosis)

• Hypertrophic scar
• Retroperitoneal lymphadenopathy (lymphoma, germ cell tumor, GI metastasis)
• Hematoma
• Myositis ossificans (most classic mimic - can appear malignant on imaging)
• Benign lipoma
• Cyst
• Abscess
• Cutaneous malignancies including melanoma

---

Sources

• Miller's Review of Orthopaedics, 9th Edition - STS subtypes, clinical pearls, ESARC
• Robbins and Cotran Pathologic Basis of Disease - Pathology, classification table, morphology
• Sabiston Textbook of Surgery - Epidemiology, risk factors, staging, surgical management
• Mulholland & Greenfield's Surgery - Nonrhabdomyosarcoma STS in pediatrics

---

IHC to Differentiate Soft Tissue Tumors

---

GENERAL PRINCIPLE

"Morphologic assessment can be supported by ancillary techniques, including immunohistochemistry and molecular genetic testing, useful for classifying soft tissue sarcoma subtypes." - Schwartz's Principles of Surgery, 11th Ed.

---

IHC BY TUMOR LINEAGE

---

1. ADIPOCYTIC TUMORS

Key rule: MDM2 + CDK4 nuclear positivity = liposarcoma (especially well-diff/dediff type). Benign lipoma is negative.

• Miller's Review of Orthopaedics; Henry's Clinical Diagnosis by Lab Methods

---

2. FIBROUS / FIBROBLASTIC TUMORS

CD34 is the signature marker for DFSP and solitary fibrous tumor (SFT). However, CD34 is also positive in vascular tumors - context matters.

---

3. SKELETAL MUSCLE TUMORS (RHABDOMYOSARCOMA)

Cross-striations (rhabdomyoblasts) may be seen on H&E but are rare. Myogenin is the most specific confirmatory marker for skeletal muscle lineage.

• Henry's Clinical Diagnosis by Lab Methods; Robbins Pathology

---

4. SMOOTH MUSCLE TUMORS

SMA + Desmin + h-caldesmon = smooth muscle lineage. The intersecting right-angle fascicles on H&E + SMA/desmin = leiomyosarcoma.

• Miller's Review of Orthopaedics

---

5. VASCULAR TUMORS

CD31 is the most specific vascular endothelial marker. ERG (nuclear) is also highly specific for endothelial differentiation and is very sensitive.

---

6. NERVE SHEATH TUMORS

Key distinction: Schwannoma = S100 diffuse and strong. MPNST = S100 focal/weak (or negative) + H3K27me3 loss (due to PRC2 complex inactivation - most specific marker for high-grade MPNST).

• Miller's Review of Orthopaedics (S100); Henry's Lab Methods

---

7. SYNOVIAL SARCOMA

"TLE1 is the most useful marker for distinguishing synovial sarcoma from other soft-tissue malignancies on immunohistochemistry." - Henry's Clinical Diagnosis by Lab Methods

Confirm with FISH or RT-PCR for t(X;18) / SS18::SSX fusion.

---

8. GASTROINTESTINAL STROMAL TUMOR (GIST)

CD117 + DOG1 = GIST. For KIT-negative GISTs (PDGFRA-mutant), DOG1 remains positive and is diagnostically critical.

---

9. UNDIFFERENTIATED PLEOMORPHIC SARCOMA (UPS / MFH)

UPS is a diagnosis of exclusion. All specific lineage markers are absent by definition. The storiform/cartwheel pattern + anaplastic pleomorphic cells + no lineage markers = UPS.

---

10. EWING SARCOMA / PNET

Confirm with FISH/RT-PCR for t(11;22) EWSR1::FLI1 (most common, ~85%) or other EWSR1 rearrangements.

Important: CD99 is also positive in synovial sarcoma, lymphoblastic lymphoma, and other small blue cell tumors - it is sensitive but NOT specific alone. Always combine with NKX2.2 and molecular testing.

---

11. CLEAR CELL SARCOMA ("Melanoma of Soft Parts")

IHC profile is virtually identical to melanoma. Differentiation requires:

• Clinical context (deep soft tissue vs. skin primary)
• Molecular: EWSR1::ATF1 or EWSR1::CREB1 fusion (by FISH/RT-PCR) = clear cell sarcoma

---

12. ALVEOLAR SOFT PART SARCOMA (ASPS)

---

13. EPITHELIOID SARCOMA

INI1/SMARCB1 loss is the diagnostic hallmark of epithelioid sarcoma (especially proximal type). Also seen in malignant rhabdoid tumor.

---

14. DERMATOFIBROSARCOMA PROTUBERANS (DFSP)

CD34+/Factor XIIIa- = DFSP vs. CD34-/Factor XIIIa+ = Dermatofibroma (benign). This is a critical distinction.

• Henry's Clinical Diagnosis by Lab Methods

---

MASTER SUMMARY TABLE

---

DIAGNOSTIC APPROACH TO AN UNKNOWN SARCOMA

Spindle cell sarcoma
├── S100+ → Nerve sheath tumor
│     ├── Diffuse S100 → Schwannoma
│     ├── Focal S100 + H3K27me3 loss → MPNST
│     └── S100+ + HMB45+ → Clear cell sarcoma (do EWSR1 FISH)
├── SMA + Desmin → Smooth muscle (leiomyosarcoma)
├── CD31/ERG+ → Angiosarcoma
├── CD34 (diffuse) → DFSP or Solitary Fibrous Tumor
│     └── STAT6 nuclear → Solitary Fibrous Tumor
├── TLE1+ → Synovial sarcoma (confirm SS18::SSX)
├── MDM2/CDK4+ → Liposarcoma
└── All negative → UPS (diagnosis of exclusion)

Round cell sarcoma
├── CD99 (strong membranous) → Ewing sarcoma (confirm EWSR1)
├── Myogenin+ → Alveolar RMS (confirm PAX::FOXO1)
├── CD117/DOG1+ → GIST
└── TFE3 (nuclear) → ASPS

---

• Henry's Clinical Diagnosis and Management by Laboratory Methods - molecular IHC for sarcomas
• Miller's Review of Orthopaedics, 9th Ed. - SMA, desmin, S100, MDM2 per subtype
• Robbins and Cotran Pathologic Basis of Disease - myogenin, MyoD1 in RMS; morphology-IHC correlation
• Schwartz's Principles of Surgery, 11th Ed. - IHC as ancillary technique in STS subtyping