Soft-Tissue & Bone Tumours of the Foot & Ankle (Overview)

Why the rule exists. Malignant tumours of the foot and ankle are rare, and because they are rare they are frequently misdiagnosed as something benign: a "ganglion", a wart, a callus. The misdiagnosis leads to an inadequate, unplanned excision, and the unplanned excision leads to a poor outcome.

The trap. A small foot mass is excised as a ganglion, without margins, and proves to be a synovial sarcoma: the unplanned ('whoops') excision. There is now tumour in the wound and contaminated tissue planes, and ahead lies a far harder, often mutilating, re-resection. In 80 malignant foot and ankle tumours, unplanned initial surgery carried a local recurrence of about 50%, against about 22% after planned surgery, with worse outcomes, and the authors used it to reinforce referral to a specialist tumour centre.

Earning the benign label. Most foot lumps are benign, and the ganglion is the commonest of them. The benign label must still be earned by appropriate assessment, not assumed from the location or a soft history.

Benign lesions. These are common and are treated on their own merits once the diagnosis is secure. The calcaneus is a common site for bone cysts, and a plantar fibroma recurs after marginal excision.

Synovial sarcoma. The commonest soft-tissue sarcoma of the foot is small, can be relatively slow-growing, sits near joints and tendons and may calcify. That profile is exactly why it is the classic lesion to be mistaken for a ganglion and excised unplanned.

Malignant bone tumours. These are rare. Osteosarcoma is the commonest primary bone malignancy of the foot, with Ewing sarcoma and chondrosarcoma also occurring. Metastasis to the foot (acrometastasis) is rare as well.

Size thresholds do not transfer. Referral criteria for soft-tissue masses elsewhere in the limb lean heavily on size, and applying thresholds borrowed from the thigh or shoulder to the foot is dangerous, because the lesion that most often kills here does not obey them. The foot's confined compartments make a sarcoma symptomatic, and noticed, while it is still small, so a mass that fails a size threshold has not been reassured, it has merely been measured.

How to use size. A large mass is concerning, but a small one is not reassuring. Treat size as one feature among several, and decide on the qualitative red flags and on uncertainty, never on a centimetre count.

The evidence. In a series of 14 patients with synovial sarcoma of the foot and ankle, tumour size was not a prognostic variable. The rest of that series gives the foot synovial sarcoma a profile that should raise suspicion rather than lower it.

Longer symptoms, better outcome. One finding in that series is easy to misread: patients with a longer duration of symptoms before diagnosis had better outcomes. The authors' interpretation is that tumours which had been present for a long time without declaring themselves were biologically less aggressive in the first place, the indolent lesions being the ones that survive long enough to be diagnosed late. It is a statement about tumour biology, not a licence to wait.

The foot-mass trap is not only the subcutaneous sarcoma. The foot is the classic site for acral lentiginous melanoma and subungual melanoma, both repeatedly mistaken for benign skin or nail problems.

Acral lentiginous melanoma. It arises on the sole, heel and web spaces and is mistaken for a wart, callus, chronic ulcer or blood blister. It is the melanoma subtype least related to sun exposure, and it tends to present late.

Subungual melanoma. It is mistaken for a subungual haematoma, a fungal nail (onychomycosis) or paronychia. A pigmented nail streak that is new, widening or changing is melanoma until proven otherwise, especially with Hutchinson's sign, pigment spreading onto the nail fold and periungual skin. Amelanotic (non-pigmented) variants of either are especially deceptive.

The rule. Biopsy any non-healing acral ulcer, atypical pigmented lesion or suspicious nail-unit pigmentation rather than treating it repeatedly as a wart or fungal nail. Use the ABCDE features for skin lesions and the nail-unit ABCDEF (in the memory aids below) for the nail.

Squamous cell carcinoma. A non-healing wound or sinus can also harbour a squamous cell carcinoma (Marjolin's ulcer). Refer suspicious lesions for biopsy or excision by the appropriate skin-cancer or sarcoma service.

Referral Frameworks

Every major sarcoma framework converges on the same rule for an unexplained soft-tissue mass: refer before excising. NICE (suspected sarcoma pathway), ESMO-EURACAN and national bone-and-soft-tissue tumour networks all mandate specialist-centre assessment with MRI before any definitive surgery, and the biopsy performed by, or in direct discussion with, the unit that will resect. The Musculoskeletal Tumor Society data (Mankin) - a 2-to-12-fold penalty for biopsies done outside the treating centre - is the evidence underneath that rule.

Registry and Evidence Notes

• Soft-tissue sarcoma of the foot is too rare for dedicated registries; outcome data come from referral-centre series (Karaca, Scully) and pooled sarcoma collaboratives.
• No randomised trial will ever compare planned against unplanned excision - the ethical breach would be deliberate - so the retrospective recurrence and mortality difference (50% vs 22%, 10% vs 6%) is the strongest evidence the field will produce.

High- vs Limited-Resource Practice

• High-resource settings: MRI, image-guided core biopsy at the tumour unit, neoadjuvant radiotherapy, flap reconstruction, and planned amputation where margins demand it.
• Limited-resource settings: the single highest-value intervention is referral discipline - not excising the unknown mass - because the contaminated re-resection is exactly the operation a constrained system can least afford. Where MRI is unavailable, the red-flag history and examination still separate "excise locally" from "refer first".

Wide excision is far harder to deliver in the foot than elsewhere, which is exactly why preventing the unplanned excision matters so much.

The envelope. The foot has little subcutaneous cover, tightly packed compartments, and tendons and neurovascular bundles close to the skin, so a true wide margin around a sarcoma often cannot be achieved without sacrificing essential structures. The plantar weight-bearing surface tolerates resection and grafting poorly, which constrains limb salvage further.

Amputation. Adequate margins frequently mean a ray amputation, a partial-foot (transmetatarsal or midfoot) amputation or a below-knee amputation rather than a marginal local excision. A prior unplanned excision, by contaminating the planes, makes amputation or a much larger salvage resection more likely.

Adjuncts to salvage. Neoadjuvant or adjuvant radiotherapy is used for high-grade soft-tissue sarcoma and chemotherapy for osteosarcoma and Ewing sarcoma, as indicated. Soft-tissue flap reconstruction may be needed to close a limb-salvage defect.

Margins and staging. These are defined and applied within the multidisciplinary team. The Enneking surgical-staging system and the intralesional, marginal, wide and radical margin definitions are developed in Enneking Staging.