Soft Tissue Tumours & Sarcomas

Soft tissue tumours arise from the mesenchymal tissues lying between the skin and the visceral organs — fat, muscle, fibrous tissue, blood vessels, nerves and synovium. The benign:malignant ratio is roughly 100:1, so the everyday challenge is distinguishing a harmless lipoma from a deadly sarcoma. This is a high-yield surgical-oncology topic that repeatedly tests histopathology, immunohistochemistry markers and the principles of wide excision.

Definition & classification

A soft tissue sarcoma (STS) is a malignant tumour of mesenchymal origin. They account for <1% of all adult malignancies but ~7–15% of paediatric cancers. They are named after the tissue of differentiation, not necessarily the tissue of origin.

Soft tissue tumours can be classified by tissue of origin:

Tissue of origin	Benign	Malignant
Adipose	Lipoma	Liposarcoma
Smooth muscle	Leiomyoma	Leiomyosarcoma
Striated muscle	Rhabdomyoma	Rhabdomyosarcoma
Fibrous tissue	Fibroma	Fibrosarcoma
Blood vessels	Haemangioma	Angiosarcoma
Lymphatics	Lymphangioma	Lymphangiosarcoma
Synovium	—	Synovial sarcoma
Peripheral nerve	Neurofibroma/Schwannoma	MPNST
GI mesenchyme (ICC of Cajal)	—	GIST
Fibrohistiocytic	Fibrous histiocytoma	Undifferentiated pleomorphic sarcoma (old "MFH")

High-yield: Undifferentiated pleomorphic sarcoma (formerly malignant fibrous histiocytoma, MFH) is the commonest adult soft tissue sarcoma overall; liposarcoma is the second commonest. In children, rhabdomyosarcoma is the commonest STS.

Sarcomas are also graded (FNCLCC system — differentiation, mitotic count, necrosis) into low- vs high-grade, which is the single most important predictor of metastasis.

Etiology & risk factors

Most are sporadic, but recognised associations are favourite exam fodder:

• Radiation exposure — post-radiotherapy sarcoma (angiosarcoma, UPS) after a latency of ~10 years.
• Chronic lymphoedema → lymphangiosarcoma. Post-mastectomy lymphoedema causing angiosarcoma = Stewart–Treves syndrome.
• Genetic syndromes: Li–Fraumeni (p53 mutation — sarcomas, breast, brain), neurofibromatosis type 1 (MPNST arising in a plexiform neurofibroma), familial retinoblastoma (RB1) → osteosarcoma & STS, Gardner syndrome → desmoid (aggressive fibromatosis).
• Viral: HHV-8 → Kaposi sarcoma.
• Chemicals: vinyl chloride, thorotrast, arsenic → hepatic angiosarcoma.

High-yield: A new sarcoma in a previously irradiated field, or angiosarcoma in a chronically lymphoedematous arm after mastectomy (Stewart–Treves), is a classic single-best-answer.

Pathophysiology & spread

Sarcomas grow centrifugally and compress surrounding tissue into a pseudocapsule infiltrated by microscopic tumour ("satellite lesions"). This pseudocapsule is the surgical trap: shelling the tumour out along this plane (marginal excision/enucleation) leaves residual disease and guarantees local recurrence. They also spread along fascial planes and longitudinally within a muscle compartment.

Metastasis is predominantly haematogenous to the LUNG — nodal spread is uncommon (<5%). The classic exceptions that do metastasise to lymph nodes are remembered by:

Mnemonic – "SCARE" nodes: Synovial sarcoma, Clear cell sarcoma, Angiosarcoma, Rhabdomyosarcoma, Epithelioid sarcoma. These warrant attention to regional nodes.

Clinical features

• A painless, progressively enlarging deep soft tissue mass is the cardinal presentation.
• Red-flag features mandating sarcoma work-up: size >5 cm, deep to deep fascia, rapidly growing, recurrence after previous excision, or any new lump in an adult that is firm and fixed.
• Pain is late and suggests neurovascular involvement.
• Site: ~50% in the lower limb (thigh most common), then trunk/retroperitoneum, upper limb, head & neck.
• Retroperitoneal sarcomas (often liposarcoma) present late with vague abdominal fullness, early satiety or mass effect because the retroperitoneum accommodates huge tumours silently.

Specific tumours worth memorising

Lipoma vs liposarcoma

Feature	Lipoma	Liposarcoma
Depth	Usually subcutaneous	Usually deep / retroperitoneal
Size	Small, stable	Large (>5 cm), growing
Consistency	Soft, slip sign positive	Firm, no slip sign
Common site	Back, neck, shoulder	Thigh, retroperitoneum
Imaging	Uniform fat density	Thick septa, nodular, non-fatty areas
Cytogenetics	—	MDM2/CDK4 amplification (well-diff/dedifferentiated); t(12;16) FUS-DDIT3 in myxoid type
Behaviour	Benign	Myxoid type metastasises to unusual soft tissue sites

High-yield: MDM2 and CDK4 amplification distinguishes well-differentiated/dedifferentiated liposarcoma from a benign lipoma — a frequently tested IHC/molecular fact. Myxoid liposarcoma carries the t(12;16) translocation and metastasises to fat-bearing areas and the spine rather than lung.

GIST (Gastrointestinal Stromal Tumour)

• Most common mesenchymal tumour of the GI tract; arises from the interstitial cells of Cajal (the GI pacemaker cells).
• Commonest site = stomach (~60%), then small bowel.
• Driven by gain-of-function mutation in c-KIT (CD117) — present in ~95%; KIT-negative GISTs may carry PDGFRA mutations (often gastric, epithelioid). DOG1 is another sensitive marker.
• Presents with GI bleed, mass or is found incidentally; may show central umbilication/ulceration on endoscopy.
• Risk of malignancy assessed by size + mitotic rate + site (Fletcher/Miettinen criteria): tumours >5 cm and/or >5 mitoses per 50 HPF are higher risk.

High-yield: CD117 (c-KIT) positive → drug of choice is imatinib (tyrosine kinase inhibitor). Surgery = complete resection with negative margins; lymphadenectomy is NOT required (GISTs don't spread to nodes). Imatinib is used both as adjuvant (≥3 yr for high-risk) and neoadjuvant to downsize. Second-line = sunitinib; third-line = regorafenib. The exon 11 KIT mutation responds best to imatinib; PDGFRA D842V is imatinib-resistant.

Rhabdomyosarcoma

• Commonest STS of childhood; from striated muscle precursors.
• Embryonal subtype (commonest, better prognosis) — head & neck, genitourinary (sarcoma botryoides = grape-like cluster in vagina/bladder). Alveolar subtype (adolescents, extremities, worse prognosis) carries t(2;13) PAX3-FOXO1.
• IHC: desmin, myogenin (MYF4), MyoD1 positive.
• Treatment is multimodal — chemotherapy (VAC: vincristine, actinomycin-D, cyclophosphamide) is central, plus surgery ± radiotherapy.

Synovial sarcoma

• Young adults, around (not within) joints, especially the knee. Despite the name it does NOT arise from synovium.
• Biphasic histology (epithelial + spindle cells); carries t(X;18) SS18-SSX. May calcify on X-ray.
• Can spread to lymph nodes (see SCARE).

Leiomyosarcoma

• From smooth muscle — uterus, GI tract, retroperitoneum, walls of large vessels (IVC). IHC: SMA, desmin positive. Behaves aggressively.

Desmoid tumour (aggressive fibromatosis)

• Locally infiltrative, does not metastasise but recurs relentlessly. Associated with Gardner/FAP (APC mutation) and abdominal wall in young women post-partum.

Diagnosis & investigation of choice

The diagnostic pathway is a classic stepwise question:

Clinical red flags → MRI of the primary (local staging) → core-needle/incisional biopsy (tissue diagnosis) → CECT chest (metastatic staging) → definitive surgery.

1. MRI is the imaging investigation of choice for an extremity/trunk soft tissue mass — best for local extent, compartment and neurovascular relations. CT is preferred for retroperitoneal/abdominal sarcoma and GIST.
2. Biopsy: core-needle biopsy is the preferred first-line tissue diagnosis. If inadequate, an incisional biopsy is done — with the incision placed longitudinally along the limb and in line with the planned definitive excision so the biopsy tract can be excised en bloc.

High-yield – FNAC limitations: FNAC cannot reliably grade or subtype a sarcoma and cannot assess architecture; it is acceptable mainly for confirming recurrence or metastasis, not for primary diagnosis. Core or incisional biopsy is required. Never do an excisional biopsy or "shell-out" of an undiagnosed deep mass — it contaminates planes and compromises later wide excision.

3. Staging CECT chest is mandatory (lung is the main metastatic site). PET-CT helps grade and detect distant disease in selected cases.
4. Immunohistochemistry clinches the cell of origin (table below).

Tumour	Key marker(s)	Cytogenetic clue
GIST	CD117 (c-KIT), DOG1, CD34	KIT/PDGFRA mutation
Rhabdomyosarcoma	Desmin, myogenin, MyoD1	t(2;13) alveolar
Synovial sarcoma	EMA, cytokeratin (biphasic)	t(X;18)
Leiomyosarcoma	SMA, desmin, h-caldesmon	complex
Liposarcoma	S-100, MDM2, CDK4	t(12;16) myxoid
Ewing/PNET	CD99 (MIC2)	t(11;22) EWS-FLI1
Angiosarcoma	CD31, CD34, ERG, factor VIII	—
Clear cell sarcoma	S-100, HMB-45	t(12;22)

Management & drug of choice

Surgery is the cornerstone and the principle examined most often:

High-yield: The aim is wide local excision with histologically negative margins (R0 resection) — removing the tumour with a cuff of normal tissue (ideally ~1–2 cm or an intact fascial barrier), excising the biopsy tract en bloc. Limb-sparing surgery + radiotherapy has replaced amputation as standard for extremity sarcoma, with equivalent survival.

Margin terminology (commonly tested):

• Intralesional – through tumour (worst).
• Marginal – through the pseudocapsule (leaves satellites).
• Wide – cuff of normal tissue, within the compartment.
• Radical – entire compartment removed.

Radiotherapy: adjuvant RT is added for high-grade, large (>5 cm), deep tumours or where margins are close/positive, to reduce local recurrence. Can be pre-operative (smaller field, lower dose, but more wound complications) or post-operative.

Chemotherapy: doxorubicin ± ifosfamide is the mainstay of systemic therapy, of greatest benefit in chemosensitive tumours (rhabdomyosarcoma, Ewing, synovial sarcoma) and metastatic disease. Most adult STS are relatively chemoresistant.

Targeted therapy: imatinib for GIST (CD117+) is the prototype molecular-targeted drug in solid tumours and a guaranteed exam point. Pazopanib is approved for advanced non-adipocytic STS.

Stepwise summary of a high-grade extremity sarcoma: MRI + core biopsy → CECT chest staging → MDT planning → limb-sparing wide excision (R0) → adjuvant radiotherapy ± chemotherapy → surveillance.

Complications

• Local recurrence — the commonest problem, driven by inadequate margins or "whoops" enucleation by an unwary surgeon.
• Pulmonary metastasis — leading cause of death; isolated lung mets may be resected (metastasectomy).
• Surgical: wound dehiscence and delayed healing (worse with pre-op RT), nerve/vessel injury, lymphoedema.
• Radiation-induced second sarcoma; fibrosis and joint stiffness.
• GIST-specific: bleeding, rupture (seeding), imatinib resistance over time.

Key differentials

• Lipoma (benign) — the single most important "do-not-miss-but-don't-overtreat" mimic of liposarcoma.
• Haematoma / organising haematoma — can mimic a mass; history of trauma.
• Abscess — inflammatory signs, but a "cold" deep mass should never be assumed benign.
• Nodular fasciitis — benign, rapidly growing, self-limiting; classic pseudosarcoma that is over-diagnosed.
• Desmoid tumour — locally aggressive, non-metastasising.
• Myositis ossificans — peripheral calcification (zonal pattern) after trauma, opposite of osteosarcoma's central ossification.
• Lymph node / metastatic carcinoma — especially head & neck masses.

High-yield: Any soft tissue lump that is >5 cm, deep to fascia, or enlarging should be treated as a sarcoma until proven otherwise and referred to a sarcoma unit before biopsy/excision.

Recently asked / exam angle

• GIST → CD117 (c-KIT) positive → imatinib (and arises from interstitial cells of Cajal, stomach commonest, no nodal spread/lymphadenectomy). The most repeated STS fact in NEET PG.
• Commonest adult STS — undifferentiated pleomorphic sarcoma; commonest in children — rhabdomyosarcoma; sarcoma botryoides site (vagina/bladder).
• Investigation of choice = MRI for extremity; biopsy of choice = core/incisional, not FNAC/excisional; FNAC only for recurrence/metastasis.
• Metastasis is haematogenous to lung; nodal-spreading sarcomas = "SCARE".
• Stewart–Treves syndrome (lymphangiosarcoma/angiosarcoma in post-mastectomy lymphoedema); Li–Fraumeni (p53).
• MDM2/CDK4 amplification = well-differentiated liposarcoma vs lipoma; t(X;18) = synovial sarcoma; t(2;13) = alveolar rhabdomyosarcoma; t(11;22) = Ewing/PNET.
• Treatment principle: limb-sparing wide excision + adjuvant RT has replaced amputation; biopsy tract must be excised en bloc and oriented along the limb.

Rapid revision

1. Commonest adult soft tissue sarcoma = undifferentiated pleomorphic sarcoma (old MFH); second = liposarcoma.
2. Commonest childhood STS = rhabdomyosarcoma; embryonal subtype best prognosis; sarcoma botryoides in vagina/bladder.
3. MRI is the imaging investigation of choice for limb/trunk sarcoma; CT for retroperitoneum & GIST.
4. Core-needle (or incisional) biopsy for diagnosis; biopsy tract oriented longitudinally and excised en bloc. FNAC only for recurrence/metastasis.
5. Staging CECT chest is mandatory — lung is the main metastatic site; spread is haematogenous, NOT nodal (except SCARE tumours).
6. GIST: interstitial cells of Cajal, stomach commonest, CD117/DOG1 positive, drug of choice imatinib, no lymphadenectomy.
7. MDM2 & CDK4 amplification distinguishes well-differentiated liposarcoma from benign lipoma.
8. Stewart–Treves = angio/lymphangiosarcoma in chronic post-mastectomy lymphoedema; Li–Fraumeni = p53 mutation.
9. Surgical aim = wide excision with R0 (negative) margins; never enucleate along the pseudocapsule.
10. Limb-sparing surgery + adjuvant radiotherapy has replaced amputation with equal survival; RT for high-grade, >5 cm, deep, or close-margin tumours.
11. Systemic chemo = doxorubicin ± ifosfamide; most adult STS are chemoresistant, paediatric/Ewing/synovial are chemosensitive.
12. Translocations: synovial t(X;18), alveolar RMS t(2;13), myxoid liposarcoma t(12;16), Ewing t(11;22).