Myxofibrosarcoma is one of the commonest soft tissue sarcomas in older adults. It usually appears as a slowly enlarging lump in a limb (most often a leg), frequently just beneath the skin, and frequently painless.
It is worth a page of its own because it presents two specific problems that shape everything about its treatment, and neither is obvious from the outside. The first is that its edges are genuinely hard to find. The second is that removing it properly often means removing enough tissue that the wound cannot simply be closed. Understanding both makes the operation, and its scale, much easier to make sense of.
Who it affects
In a population-based analysis of 1,879 patients, the median age at diagnosis was 64 years, with a slight male predominance. The lower limb was the commonest site, accounting for 45.4%. Just over half (51.0%) had disease that was localised when it was found.
The name has changed over the years. Myxofibrosarcoma was formerly grouped under myxoid malignant fibrous histiocytoma, and it is sometimes still recorded as fibromyxosarcoma. Encountering different names for the same tumour in older letters or reports is common and does not mean anything has been misunderstood.
The defining problem: it grows along tissue planes
Most tumours a surgeon removes behave, roughly, like a ball. There is a mass, it has an edge, and the operation takes the mass along with a margin of healthy tissue around it.
Myxofibrosarcoma does not reliably do this. Alongside the main mass it sends fine strands of tumour outward along the fascial planes: the thin sheets of connective tissue that wrap muscles and lie beneath the skin. These extensions can run a considerable distance from what appears to be the boundary of the lump, and they are difficult to see. They do not feel different to normal tissue at operation, and they can be too fine to resolve clearly on a scan.
On MRI this appearance is called the tail sign: a thin curvilinear extension trailing away from the main tumour along a fascial plane. In one study of 40 patients, a tail sign was present in 55% before treatment. It is characteristic, but it is not proof, and benign conditions can produce something similar. In a comparison of 44 myxofibrosarcomas against 30 cases of nodular fasciitis, a benign condition, a fascial tail appeared in both; what separated them was the patient’s age, the length of the tail, and the swelling in the surrounding tissue, not its mere presence.
The consequence of this growth pattern is straightforward. The visible lump under-represents the tumour. An operation planned around what can be seen and felt will leave tumour behind. That is the reason these tumours are removed more widely than their size alone would suggest, and it is the reason they should not be removed at all until the diagnosis is known.
Why a lump should be diagnosed before it is removed
Myxofibrosarcoma is unusually well placed to be mistaken for something harmless. It occurs in older people, in whom lumps are common and usually benign. It often sits just beneath the skin, where a lipoma or a cyst would sit. It is frequently painless. Removed under the assumption that it is one of those things, it will be shelled out without an adequate margin, and for a tumour that infiltrates the way this one does, that leaves disease behind in the surrounding tissue.
The definitive operation then has to encompass not only the residual tumour but the whole field disturbed by the first procedure, which makes it substantially larger than it would have been.
This is a general principle in sarcoma rather than one confined to this tumour, and it is covered in the Journal Club entry why a lump is best diagnosed before it’s removed. Myxofibrosarcoma is simply one of the tumours where the cost of getting the sequence wrong is highest.
What the evidence says about margins
Here the literature is genuinely mixed, and it is more useful to say so than to pick the most convenient number.
Reported local recurrence rates vary widely between centres:
- In a series of 94 patients treated over 23 years, local recurrence occurred in 33%.
- In a series of 135 patients from a single European unit, it occurred in 10.4%.
- In a series of 112 patients with superficial tumours, 90% were free of local recurrence at ten years.
Those are very different figures, and the differences almost certainly reflect what kind of tumours each unit sees and how they were managed rather than any real disagreement about the disease.
What the studies do agree on is more useful than the headline rates:
Whether the margin is clear matters enormously. How wide it is beyond clear has not been shown to matter. The 94-patient series looked specifically for a critical margin width and found no threshold that survived proper statistical correction. The 135-patient series found no significant effect of margin width by either of the two classification systems it used, but a clearly significant effect where tumour was present microscopically at the edge. The 112-patient series found that a positive margin, whether detected during the operation or on the final pathology, raised the hazard of recurrence roughly seven- to eight-fold.
The practical conclusion is that the goal of the operation is a completely clear margin, and the effort goes into confirming that rather than into adding millimetres for their own sake.
One approach to confirming it is to have the pathologist examine the margins during the operation, on frozen sections, so that more tissue can be taken in the same anaesthetic if any edge is involved. In the 112-patient series, margins were clear on that intra-operative assessment in 92% of cases, and only one of those proved positive on the final analysis. That series was published specifically to argue that the wound can be closed and reconstructed in a single operation rather than left open while the pathology is awaited, an alternative practice that delays treatment and adds cost.
Soft tissue coverage: why a plastic surgeon may be involved
This follows directly from everything above. Clearing a superficial, infiltrating tumour properly can mean taking the overlying skin along with a broad area of underlying tissue. What is left is often a defect that cannot simply be pulled together and stitched.
Restoring cover is a planned part of the operation, not a complication of it, and the options are:
- A skin graft: a thin layer of skin taken from elsewhere, usually the thigh, laid over the defect. Suitable where the underlying bed is healthy tissue with a good blood supply.
- A local or pedicled flap: tissue moved from an adjacent area while remaining attached to its own blood supply, and rotated into the defect.
- A free flap: tissue taken from a distant site with its blood vessels, transferred to the defect and reconnected there under a microscope.
Which of these is appropriate depends on the size and site of the defect, whether critical structures such as bone, tendon, nerves or vessels are exposed, and whether the area has been irradiated. In one series of 56 patients having musculoskeletal tumours removed from the lower limb, pelvis and sacrum, 41% required a pedicled or free flap. That series covered tumours of all types, not myxofibrosarcoma alone, but it gives a fair sense of how often this arises in practice.
The important part is the sequencing. Planning the reconstruction in advance (often jointly with a plastic surgical colleague, an approach sometimes called orthoplastic) means the decision about how much tissue to remove is never constrained by the question of how the wound will be closed. Durable cover also matters for what comes next: a wound that heals reliably allows any radiotherapy or other treatment to start on time.
Radiotherapy
Radiotherapy is commonly part of the plan, often given before surgery. In the population-based series, 48.9% of patients received radiotherapy at some point.
The logic is a good fit for this tumour: radiotherapy treats a volume of tissue rather than an object, so it can address the infiltrating edge that surgery finds hard to define. In the MRI study above, the authors describe pre-operative radiotherapy followed by surgery as the standard of care for this tumour, precisely because of its growth pattern, while also observing that local recurrence rates remain high despite it.
The honest position is that the specific size of the benefit in myxofibrosarcoma has not been settled by a randomised trial. One of the series above found recurrence numerically lower with radiotherapy but not statistically so, and noted the comparison was vulnerable to the fact that the tumours selected for radiotherapy differ from those that are not. Whether to use it, and when, is a multidisciplinary team decision made for each patient.
Outcomes
From the population-based analysis of 1,879 patients:
| 5 years | |
|---|---|
| Overall survival | 74.3% (95% CI 72.2–76.5) |
| Cancer-specific survival | 85.2% (95% CI 83.4–87.0) |
The gap between those two figures is worth explaining rather than glossing over. Cancer-specific survival counts only deaths from the sarcoma; overall survival counts all deaths. In a group whose median age is 64, a meaningful proportion of deaths within five years are from other causes. Age was the strongest independent prognostic factor in that analysis by a wide margin.
For the individual patient, the factors that matter are the grade of the tumour, its size and depth, whether it has spread, and whether it can be removed with a clear margin.
What this means in practice
- A new or enlarging lump in a limb in an older adult deserves imaging before anything else. Most such lumps are harmless. The ones that are not are best identified before an operation, not during one.
- The operation is usually wider than the lump looks, and that is a considered decision about the way this tumour grows, not excess caution.
- Reconstruction is planned from the outset, so that clearing the tumour and closing the wound are not in competition.
- Follow-up matters, because local recurrence is the characteristic pattern of failure with this tumour and is most treatable when found early.
The route in is set out in tests and diagnosis, and the way a plan is agreed in how treatment is planned. If you are a GP with a patient you are concerned about, early referral before biopsy is the thing that makes the most difference.