Periosteal plexiform neurofibroma of the femur

A 37-year-old man consulted for discomfort with months of evolution in the posterior aspect of the left knee, especially on flexion. In addition, clinical exam revealed a doubtful positive meniscal manoeuvre. The patient has neurofibromatosis type 1 (NF1). Knee radiographs were requested.

Radiographs showed sclerosis and scalloping of the posterior cortex of the femur (Figures 1a and 1b).

MRI demonstrated a soft tissue and polylobulated mass located next to the posterior aspect of the femur, with intermediate to low signal intensity on T1WI (Figures 2a, 2b and 2c), hyperintense and heterogeneous on PD-FSE-WI (Figures 3a, 3b and 3c) and intense enhancement after GdDPTA contrast administration (Figures 4a, 4b and 4c). DWI demonstrated an intermediate value on ADC (1.67x10^-3 mm²/s) (Figures 5a and 5b).

The lesion conditioned polylobulated scalloping of the femur, without cortical breakthrough or bone marrow oedema, raising the doubt of its origin, whether it was dependent on the periosteum or the adjacent soft tissues.

CT scan was performed to assess bone involvement, showing thickening of the cortex with associated sclerosis, confirming the chronic behaviour of the lesion (Figures 6a and 6b).

Imaging findings together with the history of NF1 allowed us the diagnosis of a periosteal plexiform neurofibroma (PPN) of the femur. As the lesion had benign behaviour and it was clinically stable, conservative management with follow-up by MRI was decided.

Background

Plexiform neurofibroma (PN) is one of the types of peripheral nerve sheath tumour (PNST). It is a specific tumour for NF1. Although it is a benign lesion, it may become malignant in 10% of cases [1–4].

The periosteum is a highly innervated tissue, so it is a target organ for PN. However, there are few cases of periosteal origin of PN described in the literature, and most of the cases are smaller, making our case interesting [5,6].

Clinical Perspective

The clinical manifestations are varied, ranging from asymptomatic patients to others with severe functional deficits. It is also described as presenting pain due to bleeding [2–6].

Imaging Perspective

MRI is considered the imaging method of choice to study PN [2–4]. MRI for PPN shows the typical characteristics of a PN: a soft tissue lesion of polylobulated morphology, with intermediate-low signal in T1WI, hyperintense in T2WI and with an intense enhancement after contrast administration [2–6]. Nevertheless, there are some particularities, such as its location (closely related to the periosteum) and the reactive thickening of the adjacent bone cortex in a lobulated and smooth way, as we demonstrated in our case [5,6].

Its imaging features may be similar to other PNSTs, and therefore, it is essential to know the patient’s clinical context to make a confident diagnosis by imaging [2,5]. Due to its location, the differential diagnosis should also include benign cortical bone lesions such as non-ossifying fibroma, juxtacortical chondroid tumours, and even surface osteosarcoma [7,8].

Follow-up has been accepted as a good treatment alternative unless clinical symptoms are predominant. Imaging features changes may suggest malignant degeneration, such as invasion of the cortex, necrosis or poorly differentiated borders [1,9–11]. However, sometimes, it can be difficult to differentiate a benign finding from malignant degeneration on standard MRI. In these cases, lower ADC values have been proven to be more associated with malignant degeneration than intermediate values. For this reason, ADC values determined by DWI combined with T1WI and T2WI, and also dynamic contrast-enhanced (DCE) sequences, have great sensitivity and specificity to differentiate malignant degeneration from a benign PN [1,9–11]. FDG-PET is useful in this situation too, but its cut-off value is lower than DWI and DCE, showing up to a 5% false positive rate in patients with PN [1,9–12].

Biopsy is the true gold standard. Nevertheless, given the confidence of MRI in typical cases, it is usually relegated to atypical cases or those with suspected malignant degeneration [1].

Take Home Message / Teaching Points

• Although the periosteum is a target organ for PN, it is an uncommon location.
• MRI is the imaging method of choice for diagnosis.
• Clinical context is essential to diagnose PN by imaging.

All patient data have been completely anonymised throughout the entire manuscript and related files.