Solitary popliteal neurofibroma: an unusual encounter

Abstract

Diagnosing a popliteal fossa mass can be intriguing at times, and are not always Baker’s cysts, as expected. We present an uncommon case of a young lady who presented with a palpable firm mass in the popliteal fossa with no neurological deficit or compromised distal pulsations. On surgical exploration, it was found to be a well defined, firm mass, closely abutting the neurovascular bundle, which on histopathological study revealed a solitary encapsulated neurofibroma.

Background

Peripheral nerve sheath tumours (PNSTs), though uncommon, are a possibility among the multitude of potential pathologies in the popliteal fossa. Neurofibroma are benign nerve sheath tumours that can arise as solitary or multiple masses in any nerve of the body and manifest variously according to the anatomical location and type of lesion. Initial history and clinical examination are aided by imaging to narrow down the diagnosis. An uncommon site of presentation and the dearth of plenitude of literature on this topic warrants this case to be reported.

Case presentation

A 29-year-old lady presented to the surgery outpatient department with a 2-month history of a gradually increasing swelling in the left popliteal fossa associated with intermittent pain while ambulating. She had no history of trauma, pre-existing joint disease, similar lump in the past or any other local or systemic symptoms. No family history of neurofibromatosis or malignancy was reported.

Examination revealed a longitudinally oblong 8×5 cm firm, immobile, non-pulsatile mass in the left lower popliteal fossa extending to the upper calf, with a smooth surface and an ill-defined margin. There were no inflammatory signs or skin changes over the swelling. No sensory or motor deficits were noted in the limb and distal pulsations were well felt.

Investigations

Doppler ultrasonography reported a well-defined oval mass with a hyperechoic, heterogenous echotexture, 12×5 cm containing both arterial and venous flow. MRI of the lesion revealed a well-defined heterogenous collection in the intermuscular plane suggestive of a neoplastic aetiology. There was effacement of the popliteal vein at the level of mid-popliteal fossa and mild dilation proximally. Joint space, articular surfaces and bones were normal on imaging.

Differential diagnosis

Popliteal fossa mass, as in any other part of the body, can arise from skin and subcutaneous tissue, muscles, bursae, nerves, vascular or lymphatic structures, bone or cartilage (table 1). Findings on clinical examination helped ruling out possibilities of sebaceous cyst, lipoma, haemangioma, muscular swelling or a bony pathology. Absence of trauma or pre-existing joint pathology and the relatively young age of the patient made a diagnosis of a secondary Baker’s cyst less likely.

Table 1.

Differential diagnosis of a popliteal fossa mass (in alphabetical order)

Bone and cartilage	Osteosarcoma
	Chondrosarcoma
	Osteochondroma
Bursal	Baker’s cyst
Lymphoid	Non-Hodgkins lymphoma
Muscle/tendon	Giant cell tumour of tendon sheath
	Leiomyosarcoma
Nerve	Peripheral nerve sheath tumour (benign)
	Peripheral nerve sheath tumour (malignant)
Soft tissue	Ganglion
	Lipoma
	Fibromatosis
	Fasciitis
	Epidermal inclusion cyst
	Heterotopic ossification/myositis ossificans
	Soft-tissue sarcoma
Synovial	Synovial chondromatosis
	Pigmented vilonodular synovitis
Vascular	Popliteal artery aneurysm
	Vascular malformations and haemangiomas
	Haemangiopericytoma
	Myopericytoma

Treatment

Surgical exploration and excision was performed under general anaesthesia in prone position through an oblique incision. Intraoperatively, it was a firm well-defined encapsulated mass under cover of gastrocnemius with the neurovascular bundle closely abutted to its ventral surface (figure 1). However, there was no evidence of invasion into the neural or vascular structures. After dissection, the mass was everted along its long axis to deliver it out preserving the neurovascular bundle (figure 2). No vascular component was identified.

Figure 1.

Intraoperative still of the mass closely abutting the neurovascular bundle, but not invading into it.

Figure 2.

Post excision image.

Outcome and follow-up

Patient was discharged on sixth postoperative day after removal of closed suction drain and confirming that the patient had no neurological deficit. Follow-up was uneventful.

Histopathological examination demonstrated features of solitary encapsulated neurofibroma with extensive myxoid changes and symplastic neural cells. No mitosis or necrosis was seen (figure 3).

Figure 3.

(A) Histopathology image showing an encapsulated paucicellular spindle cell lesion (H&E)—capsule marked with white arrow. (B) Histopathology image showing tumour cells of neural quality dispersed in a myxoid matrix without mitosis or necrosis (H&E)—neural elements marked with black arrow.

Discussion

Popliteal fossa mass can be a manifestation of an exhaustive list of pathologies ranging from benign to malignant lesions. Of these, the lesion most common is a popliteal synovial cyst, or secondary Baker’s cyst, usually found in older adults.¹

PNSTs constitute roughly for 5% of soft-tissue tumours. Histologically, they are classified as schwannomas or neurofibromas. PNSTs of the common peroneal nerve make knee, the most common location in the lower extremity.²

Neurofibromas are slowly growing benign tumours of peripheral nerve made up of of Schwann cells, fibroblasts and perineural-like cells. Discrete localised mass is the usual presentation—most commonly as a cutaneous neurofibroma, or in peripheral nerve as a solitary neurofibroma, or as an infiltrative lesion growing within and expanding a peripheral nerve (plexiform neurofibroma). Malignant transformation in solitary neurofibromas is rare, and are mostly of concern due to pain or cosmesis.³ On the contrary, plexiform neurofibroma may result in significant neurologic deficits and carry significant potential for malignant transformation.⁴

Majority of neurofibroma occur as single, sporadic tumours presenting as well circumscribed lesions with very low risk of malignant transformation, while the rest are associated with NF-1 or 2.

Neurofibromatosis type I is an autosomal dominant disorder, manifesting as numerous benign peripheral nerve neurofibromas, optic nerve gliomas, Lisch nodules, café au lait spots, in individuals who inherit one mutant allele of the NF1 gene.

Bilateral acoustic schwannomas and multiple meningiomas are seen in neurofibromatosis type 2, an autosomal dominant disorder, predisposed by germline mutations in NF2 gene (neurofibromin 2 or merlin).⁵

A neurofibroma is a firm, gray–white unencapsulated mass measuring grossly from millimetres to centimetres. Solitary neurofibromas are well encapsulated and rubbery. Plexiform neurofibromas appear as multifocal myxoid lesions that are often described as a ‘bag of worms’.⁶

On microscopic examination, cutaneous neurofibroma is paucicellular, containing bland Schwann cells admixed with stromal cells. Fibrous stands in the lesion stain positively for reticulin, collagen, glycosaminoglycans with markedly increased cholinesterase activity. Markers of Schwann cells on immunohistochemistry are S-100, vimentin and myelin basic protein.

Solitary neurofibromas can grow in any nerve of the body and manifest as an asymptomatic mass or as pain, weakness, tingling and numbness.

USG comes as a tool in the armamentarium for the diagnosis of a soft tissue mass. A linear probe with a frequency of 5–12 or 8–15 MHz is used for assessment.^{7 8} Neurofibroma appears as a homogenous, well-defined, hypoechoic mass. Presence of intrinsic vascularity on Doppler and nerve continuity, demonstrated by entry and exit of the nerve, are other hallmark features. Sonography cannot distinguish among schwannomas, neurofibromas and malignant PNSTs.⁹

On T2-weighted MRI images, neurofibroma lesions are hyperintense and are characterised by classic target sign appearance. The dense central collagenous stroma with low signal intensity surrounded by high intensity myxoid material results in this phenomenon. T1-weighted images show hypointense lesion and with gadolinium contrast may show reverse target sign or heterogenous enhancement.¹⁰

Electrodiagnostic studies like nerve conduction velocity and electromyography help assess muscle weakness, wasting and sensory symptoms associated with the lesion. These studies are helpful in the assessment of the prognosis and consequential follow-up of the recovery after surgical excision.

Our patient presented with a gradually increasing firm, immobile popliteal swelling but no sensorimotor deficit. Absence of history of trauma and normal coagulation profile ruled out a haematoma. Being non-pulsatile and without a bruit, a vascular malformation seemed unlikely. The young age and no prior problems in the knee joint made a secondary popliteal cyst a less likely diagnosis. A musculoskeletal ultrasonography was performed to gain more information about the nature of the swelling. The lesion was successfully removed in toto without sacrificing the neurovascular bundle.

A provisional diagnosis can be made for similar cases with history, examination and USG but a more definite diagnosis warrants an MRI followed by excisional biopsy. Surgical excision remains the mainstay of treatment to alleviate symptoms, for cosmesis, neurological deterioration and compression or when it is suspicious of having undergone malignant transformation.

Patient’s perspective.

I am happy to have been cured of the disease by the efforts of my doctors. I have no swelling over the site since then and can walk comfortably.

Learning points.

• Peripheral nerve sheath tumour must be kept in mind while evaluating a popliteal fossa mass even in the absence of neurological deficit.

• Everting the mass and turning it inside out along the axis of the mass could help preserve the neurovascular bundle during the surgery.

• USG, though not reliable for diagnosing neurofibroma and may not differentiate between neurofibroma and schwannoma, can point towards a peripheral nerve sheath tumour. Neurofibroma has typical characteristics on an MRI.