Soft Tissue Pathology Basics
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Direct answer
Lineage is the organising principle of soft tissue pathology: every tumour is named for the mature tissue it mimics — adipocytic (lipoma, liposarcoma), fibroblastic and myofibroblastic (nodular fasciitis, dermatofibrosarcoma, desmoid), vascular (haemangioma, angiosarcoma), muscular (leiomyosarcoma, rhabdomyosarcoma), nerve sheath (schwannoma, malignant peripheral nerve sheath tumour) and a translocation-defined group of uncertain differentiation. Lipoma is the commonest benign soft tissue tumour of adults and rhabdomyosarcoma the commonest sarcoma of children, while liposarcoma, undifferentiated pleomorphic sarcoma and leiomyosarcoma lead in adults. Sarcomas spread haematogenously to the lung, and grading on differentiation, mitotic count and necrosis (the FNCLCC system) drives prognosis and staging more than histological type alone.
What you must remember
- The suspicion triad for sarcoma: a mass deep to the fascia, larger than 5 centimetres and enlarging or painful — investigate before excising; shelling out an undiagnosed deep lump (the "whoops procedure") compromises later wide excision.
- Commonest by age: lipoma in adults overall; rhabdomyosarcoma in children (embryonal, including the botryoid grape-like genitourinary variant); liposarcoma, undifferentiated pleomorphic sarcoma and leiomyosarcoma among adult sarcomas.
- FNCLCC grading: tumour differentiation score, mitotic count and necrosis — the three components; high grade prompts staging chest CT because the lung is the first metastatic station.
- Signature fusions: myxoid liposarcoma t(12;16); synovial sarcoma t(X;18) with SS18-SSX; alveolar rhabdomyosarcoma PAX3-FOXO1; Ewing sarcoma t(11;22) EWSR1-FLI1 with membranous CD99; dermatofibrosarcoma protuberans t(17;22) COL1A1-PDGFB.
- DFSP versus dermatofibroma: DFSP infiltrates subcutaneous fat in a storiform "honeycomb" pattern, recurs locally, rarely metastasises, and responds to imatinib because of its PDGFB fusion; dermatofibroma is a benign tethered nodule that dimples on pinching.
- Desmoid (fibromatosis): nuclear beta-catenin accumulation from WNT pathway activation; infiltrates and recurs locally but never metastasises — the classic "benign behaviour, malignant morphology" tumour.
- GIST is not a muscle tumour: it is defined by KIT (CD117) and DOG1 positivity, is commonest in the stomach, and risk stratification uses size and mitoses per 50 high-power fields; imatinib targets KIT.
- Nodal metastasis exceptions: most sarcomas go to lung, but epithelioid sarcoma, clear cell sarcoma, angiosarcoma and rhabdomyosarcoma also seed lymph nodes.
Working through a deep thigh mass
A 55-year-old presents with a three-month, painless 8-centimetre swelling of the medial thigh, firm and fixed, deep to the fascia on inspection of the MRI. Step one is refusing the intuitive move: no excision before diagnosis, because an unplanned excision cuts through the pseudocapsule sarcomas push ahead of themselves and seeds the field. Step two is imaging — MRI defines the compartment, relation to neurovascular structures and whether the mass is fat-density (pointing to a lipomatous tumour). Step three is a planned core-needle biopsy, its tract marked so it lies within the future excision specimen.
Step four is lineage. A lobulated fatty tumour in the thigh of this age group raises liposarcoma; the pathologist hunts for lipoblasts (vacuolated fat cells with scalloped, hyperchromatic nuclei) and subclassifies — myxoid (commonest, t(12;16), sensitive to radiotherapy) versus dedifferentiated (a low-grade lipogenic component juxtaposed with high-grade non-lipogenic sarcoma, retroperitoneal and aggressive). Step five is grade and staging: FNCLCC score plus chest CT, because even occult lung metastases change the entire plan. Step six is treatment — wide local excision with a cuff of normal tissue, with radiotherapy for large, high-grade extremity tumours.
How the exam frames it
One-liner pathology questions love the reverse diagnoses. Nodular fasciitis is the classic trap: a rapidly growing, tender subcutaneous nodule in a young adult whose biopsy shows plump myofibroblasts with mitoses — historically overcalled as sarcoma, it is reactive and self-limited, so the "mitosis equals malignancy" reflex fails here. The second favourite contrast is lipoma against liposarcoma: superficial, small, mobile and soft favours lipoma, while deep, retroperitoneal or limb-girdle, large and progressively enlarging favours liposarcoma, and the decisive microscopic finding is the lipoblast. Third, the nerve-sheath pair: a solitary encapsulated schwannoma with Antoni A and B patterns and Verocay bodies is cured by excision, whereas roughly half of malignant peripheral nerve sheath tumours arise in neurofibromatosis type 1 — an association examiners expect.
Frequently asked questions
Which is the commonest benign soft tissue tumour, and the commonest childhood sarcoma?
Lipoma in adults; rhabdomyosarcoma in children, chiefly the embryonal subtype including the botryoid variant of hollow viscera.
What three components constitute the FNCLCC grading system?
Tumour differentiation, mitotic activity and necrosis — together they assign grade, which drives prognosis and staging.
Which translocations define synovial sarcoma and Ewing sarcoma?
t(X;18) producing SS18-SSX in synovial sarcoma, and t(11;22) producing EWSR1-FLI1 with membranous CD99 positivity in Ewing sarcoma.
Which soft tissue tumours characteristically metastasise to lymph nodes?
Epithelioid sarcoma, clear cell sarcoma, angiosarcoma and rhabdomyosarcoma — the recognised exceptions to the haematogenous-to-lung rule.
Why is GIST separated from smooth-muscle tumours, and what drug targets it?
GIST expresses KIT (CD117) and DOG1, arises from the pacemaker interstitial cell of Cajal lineage, and responds to imatinib, a KIT tyrosine-kinase inhibitor.
What does nuclear beta-catenin staining indicate in a desmoid tumour?
Activation of the WNT signalling pathway (CTNNB1 mutation or APC loss), supporting deep fibromatosis — locally infiltrative and recurrent, but non-metastasising.