1 Introduction

Soft-tissue sarcomas (STS) can affect any age group and are classically located within deep tissue compartments and are of significant size.¹ However, STS can present superficially, remain unattached to nearby structures and can be pain free. As such, misdiagnosis of these pathology is common.² Slow-growing STS include low-grade fibromyxoid sarcoma (LGFS), synovial sarcoma and alveolar soft part sarcoma.³ We describe a case of right lateral foot LGFS. Wide resection included part of the 5th metatarsus and cuboid and hence the peroneus brevis and peroneus tertius as well. There are many management options post-resection and hence we consider the pros and cons of each together with an alternative which we used for our patient and present the 5 year outcome.

2 Case report

This is a child in early childhood who presented with a painless right foot lump since 6 months of age. There was no history of trauma and he could walk and run without issues. However, shoeing was problematic as it caused discomfort.

3 Clinical findings

On clinical examination, there was a 1.5cm × 1.5cm firm lump over the lateral border of the right foot just lateral to the fifth metatarsal base. It did not transilluminate, was not tender and had no overlying skin changes. The lump was not mobile and Tinel's sign was negative. There was also no neurovascular impairment.

The child was referred to the paediatric foot and ankle clinic and was advised to further evaluate the lesion with magnetic resonance imaging. However, parents did not want further investigations at the time and was followed up a year later. By then, the lesion had grown to 2cm × 2cm and became firmer to palpation.

A paediatric orthopaedic oncologist consult was sought and a magnetic resonance imaging under general anaesthesia was thence arranged. This was followed by an incisional biopsy.

4 Diagnostic assessment

Plain radiographs of the right foot showed no cortical destruction, scalloping or periosteal reaction [Fig. 1].

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Fig. 1 Plain radiograph of the right foot showing soft tissue shadow of the mass over the lateral aspect of the 5th tarso-metatarsal joint with no cortical destruction, scalloping or periosteal reaction. Appropriate permission has been obtained.

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Ultrasound showed a 1.9cm × 1.2cm x 0.5cm circumscribed, subcutaneous hypoechoic lesion at the region of the fifth tarsometatarsal joint. It was mildly compressible and had minimal peripheral vascularity [Fig. 2].

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Fig. 2 Ultrasound showing a 1.9cm × 1.2cm x 0.5cm circumscribed, subcutaneous hypoechoic lesion. Appropriate permission has been obtained.

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Magnetic resonance imaging (MRI) of the right foot reported a well-circumscribed, slightly lobulated lesion measuring 2.4cm × 1.4cm x 1.6cm anterolateral to the base of the fifth metatarsus [Fig. 3]. It had 2 components with different signal characteristics. The more proximal component was of intermediate signal with punctate hypointense foci on both T1 and T2 weighted sequences [Fig. 4]. The more distal component had predominantly intermediate signal on T1 and was heterogeneously hyperintense on T2 sequences [Fig. 5].

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Fig. 3 T1 sequences on magnetic resonance imaging showing a well-circumscribed, slightly lobulated lesion measuring 2.4cm × 1.4cm x 1.6cm anterolateral to the base of the fifth metatarsus. Appropriate permission has been obtained.

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Fig. 4 Both T1 and T2 weight sequences on magnetic resonance imaging showing the more proximal component of intermediate signal with punctate hypointense foci. Appropriate permission has been obtained.

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Fig. 5 Magnetic resonance imaging showing intermediate signal on T1 and heterogeneously hyperintense on T2 sequences. Appropriate permission has been obtained.

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Histology of the incisional biopsy yielded a relatively hypocellular proliferation of spindle to stellate cells, set within a fibromyxoid background. The spindle and stellate cells showed a strong diffuse positivity for MUC4, patchy weak positivity for EMA, and focal S100 positivity. Desmin, CD34, and SMA were negative. Beta-catenin was negative. FUS/GREB3L2 fusion gene was positive. This favoured low-grade fibromyxoid sarcoma.

Computer tomographic imaging of the thorax and bone scan were unremarkable.

MRI of the right foot done 6 months post-operatively showed no residual mass. There was deformity of the 5th metatarsal base, soft tissue volume loss, susceptibility foci and some mild enhancement around it in keeping with post-operative changes.

Plain radiograph of the right foot at 2 weeks post-operation showing partial resection of the 5th metatarsal and cuboid [Fig. 6].

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Fig. 6 Plain radiograph of the right foot at 2 weeks post-operation showing partial resection of the 5th metatarsus and cuboid. Appropriate permission has been obtained.

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Plain radiograph of the right foot at 5 years post-operation showing remodelling of the 5th metatarsus and cuboid. No abnormal soft-tissue shadow [Fig. 7].

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Fig. 7 Plain radiograph of the right foot at 5 years post-operation showing remodelling of the 5th metatarsus and cuboid. No abnormal soft-tissue shadow. Appropriate permission has been obtained.

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5 Therapeutic intervention

The child underwent wide resection of the lesion. Using the Midas Rex® B1 Legend burr, the 5th metatarsus and cuboid were partially resected en masse with the insertion of the peroneus tertius and peroneus brevis tendons. These tendon ends were tagged with a non-absorbable monofilament suture; prolene 3-0. A temporary vacuum assisted closure device was applied to the defect.

A week later, histology reported clear margins; closest was 0.35mm from the superior margin and the child underwent reconstruction. A bony tunnel was made in the proximal end of the residual 5th metatarsus and cuboid each using a 2mm drill. Leaving 2cm of the distal end intact, 10 cm of the distal peroneus brevis tendon was longitudinally split in the middle in estimation of the length required to create a loop through the bony tunnels [Fig. 8]. A braided non-absorbable suture, ethibond 5-0, was used to whipstitch the distal 2cm end. The split tendon was then turned down to lengthen it by approximately 8cm. This was looped through the 5th metatarsus tunnel entering from the plantar end and then through the cuboid tunnel. Finally, this was tensioned with the foot in full eversion and dorsiflexion. The end was then whipstitched to the proximal limb of the loop. The subcutaneous layer was closed with interrupted vicryl 2-0 and skin with prolene 3-0 interrupted. A tight compression dressing with crepe bandage and Plaster of Paris backslab holding the foot in full dorsiflexion and eversion was applied. No drain was inserted.

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Fig. 8 Schematic showing the peroneus brevis ‘turn down lengthening’. Drawn by the first author.

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6 Follow-up and outcomes

The backslab and dressings were only removed on post-operative day 14. The wound healed untowardly, and full removal of stitches were done. A below knee full cast was applied with the foot in full dorsiflexion and eversion for a month.

At 6 weeks post-operatively, the full cast was converted to removable backslab for another 6 weeks. The child was allowed to remove the backslab only for hygiene purposes.

At 3 months post-operatively, the child was allowed to walk unprotected and to gradually resume normal activities.

A baseline MRI was done 6 months post-operatively. The child was reviewed 3 monthly for the first year then 6 monthly for the second year and subsequently annually. Plain radiographs of the right foot and chest were done at each visit.

He is currently 5 years post-operation and can walk, run and jump with no issues. He plays football and basketball without trouble. There is also no clinical recurrence thus far [Fig. 9].

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Fig. 9 Clinical photograph showing the right foot at 5 years post-operation. Appropriate permission has been obtained.

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7 Discussion

Most sarcomas are located deep within muscle and are larger than 5cm.¹ As such, many medical professionals have assumed that painless, slow growing masses cannot be malignant. On the contrary, this cannot be further than the truth. These lesions may even be superficial and mobile.² In our case, not only did the child present 3 years after onset of symptoms, the initial treating physician assumed that the lesion was benign. Other slow growing sarcomas include synovial sarcoma and alveolar soft part sarcoma.³

Typical treatment for LGFS is wide resection without adjuvant therapy as they are poorly chemo and radiosensitive.⁴ However, they have been reported to display a relatively higher and atypical metastatic potential.^5,6 This tendency was higher in lesions of axial locations such as the pelvis and gluteal region with metastasis to non-lung regions such as lymph nodes, brachium, thigh and liver; even up to 17.5 years after index operation.⁷ Two case series have suggested that superficial lesions in children tend to be less aggressive with better prognosis in terms of local recurrence and metastasis.⁸ This has implications for the duration and modality of follow up. Positron emission tomography might be negative in this disease and hence whole-body computer tomographic or magnetic resonance imaging has been recommended by some authors.⁷ Our patient is currently disease-free at 5 years. As the LGFS was in a peripheral location and in a child, we consider the risk of recurrence low. Whether the lesion is considered superficial or deep in the foot is up for discussion. Nevertheless, clinical examination is performed annually to assess for local recurrence. We feel that the subcutaneous depth allows for adequate local assessment. Plain radiographs of the chest are done to assess for lung metastasis and that of the right foot to look for progressive deformity in a growing child secondary to resection and reconstruction which has been uncomplicated thus far. Magnetic resonance imaging of the right foot was done at 6 months post-operatively. This serves as a baseline to compare with future scans should one be done for suspicion of clinical recurrence. We have found that 6 months is sufficient time to allow post-operative oedema and distortion to settle on imaging.

For soft tissue sarcomas, we advocate using temporizing negative pressure wound therapy (NPWT) whilst awaiting margin status especially where margins are particularly at high risk of being positive and chemo or radiosensitivity is low.^9,10 Anecdotally, we have not found this to increase the risk of infection, wound complications, and tumour recurrence. In fact, it reduces the need of a drain during the reconstructive surgery as we find that the chance of a seroma developing is lower. Should there be a positive margin, this allows reresection of the bed without take down or compromising a reconstruction. Finally, we have found that is allows ‘training’ and confirms viability of wound edges. We feel that NPWT allows a tension-free temporary closure while allowing the skin edges to adjust and revascularize following devascularisation post-resection. Necrotic and unhealthy edges would have declared themselves by then and would be resected to healthy bleeding tissue. This reduces the risk of subsequent skin necrosis and wound dehiscence.

Many authors have proposed the use of tendon transfers to address unsalvageable peroneus brevis pathology to maintain hindfoot balance and normal gait. Either the flexor digitorum longus (FDL) or flexor hallucis longus (FHL) can be donors. Reconstruction be done as a single or staged procedure. Though both tendons are in-phase donors and share similar excursion potential, the FHL transfer has demonstrated better strength and mean American Orthopaedic Foot and Ankle Society scores.¹¹ Mook et al. reported the use of tendon allografts. However, this is costly and bears the risk of disease transmission, inferior mechanical properties, and incorporation due to the sterilization process.¹² Rapley et al. introduced the use of acellular dermal matrix but is again costly and of a foreign material which has poor potential for healing.¹³ By performing a peroneus brevis tendon turn down to lengthen it, we had hoped to avoid the above issues. Nevertheless, there was a risk of tendon rupture by splitting it. We hypothesized that the risk would be controlled as the peroneus longus was still competent and that the tendon would hypertrophy with growth of the child. Unfortunately, the peroneus tertius did not have sufficient tendon length left post resection to allow proper reconstruction.

In conclusion, fibromyxoid sarcomas are rare tumours. In a growing child, the reconstruction of the peroneus brevis tendon by ‘turn down lengthening’ is a possible option to restore normal gait and maintain hindfoot balance.

Ethical statement

We declare that this has not been published nor presented elsewhere. We have obtained written from both patient and parent. All data has been deidentified.

Financial support and sponsorship

The authors declare that no funds, grants, or other support were received during the preparation of this manuscript. The authors have no relevant financial or non-financial interests to disclose.

CRediT authorship contribution statement

Kenneth Pak Leung Wong: Conceptualization, Methodology, Validation, Investigation, Writing – original draft, Writing – review & editing, Visualization, Project administration. Mohammad Ashik bin Zainuddin: Conceptualization, Methodology, Writing – review & editing, Visualization. Arjandas Mahadev: Conceptualization, Methodology, Writing – review & editing, Visualization.