Fibrosarcoma

Malignant fibroblastic tumour producing collagen; true adult-type fibrosarcoma is now very rare (often a diagnosis of exclusion after modern IHC/molecular reclassification).

Clinical

• Usually painless, enlarging deep soft-tissue mass (often extremities; thigh common)
• “Haematoma” history is a trap: sarcomas can bleed; lack of clear trauma or failure to regress is suspicious

Imaging features

MRI

• T1: low–intermediate (≈ muscle)
• T2: variable (cellular tumours can be high T2; low T2/band-like low signal reflects collagen and is not specific)
• Enhancement: solid, often heterogeneous; nodular internal enhancement = red flag vs haematoma
• Margins/behaviour: infiltrative or lobulated; may cross fascial planes; assess neurovascular relationship
• Haemorrhage/necrosis: can be present (don’t let fluid–fluid levels seduce you into “benign haematoma”)

US / CT / radiograph

• Non-specific solid mass; CT mainly for extent, compartments, and staging work-up pathways
• Calcification/ossification is uncommon (if present, think alternative diagnosis or post-treatment change)

Differential diagnoses

practically, the “modern reclassified” list

• Myxofibrosarcoma (often superficial, “tail sign” along fascia; frequently T2 bright myxoid areas)
• Undifferentiated pleomorphic sarcoma (UPS) (very heterogeneous, necrosis; diagnosis of exclusion)
• Low-grade fibromyxoid sarcoma (LGFMS) / sclerosing epithelioid fibrosarcoma (SEF) (often deceptively bland; may show low-signal collagenous components)
• Desmoid-type fibromatosis (benign but infiltrative; tends to encase rather than destroy; no metastases)
• Chronic haematoma (should evolve/regress; lacks solid nodular enhancement; GRE/SWI haemosiderin rim may support blood products)

Pearls

• “Fibrosarcoma” used to be a common label; now most are reclassified → be cautious naming it on imaging alone
• Separate entity: infantile fibrosarcoma (paeds; often extremity; classically ETV6–NTRK3 fusion)