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
All patient data have been completely anonymised throughout the entire manuscript and related files.
Periosteal plexiform neurofibroma
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1. From the anteroposterior and lateral X-ray views of the knee joint, the left knee joint structure is essentially aligned, with no obvious destructive lesions on the articular surface. However, a slight contour elevation is visible in the distal femur near the posterior soft tissue region. No clear cortical erosion is noted, and there may be mild reactive periosteal thickening or linear changes.
2. MRI in the axial, sagittal, and coronal planes shows a lobulated soft tissue mass in the distal femur posterior area near the articular surface. On T1-weighted images, the lesion mainly appears isointense or slightly hypointense, while on T2-weighted images, it shows relatively high signal. Areas of enhancement can be seen in distinctly defined regions, and the lesion tightly abuts the posterior femoral periosteum. Smooth cortical thickening is observed locally, with no clear signs of invasive destruction.
3. On diffusion-weighted imaging (DWI) and ADC maps, the lesion shows a certain degree of restricted diffusion. Contrast-enhanced scans demonstrate pronounced enhancement, suggesting abundant vascular or cellular components.
4. The adjacent articular cartilage and joint space are largely preserved. Posterior soft tissue shows a mass effect, potentially closely related to the joint capsule and popliteal structures. No overt signs of bone or cartilage destruction are observed.
Based on the patient's past medical history of Neurofibromatosis Type 1 (NF1) and the imaging findings, the following should be considered:
In summary, given the patient’s NF1 background, the multilobulated appearance of the lesion, its close proximity to the periosteum with localized smooth cortical thickening, and its characteristic soft tissue signals on MRI, the most likely diagnosis is:
Plexiform Neurofibroma
If malignancy is suspected or the diagnosis remains uncertain, further MRI functional imaging (DCE, DWI), PET-CT, or biopsy may be considered to clarify the nature of the lesion.
1. Treatment Plan:
(1) Follow-up Observation: If the lesion is asymptomatic and remains stable on imaging, periodic check-ups (MRI or ultrasound) can be performed to monitor changes in size. Further intervention should be considered if there is rapid enlargement, persistent pain, or suspicion of malignant transformation (including bone destruction, unclear margins, necrotic areas, etc.).
(2) Surgical Treatment: If there are significant symptoms affecting mobility or a strong suspicion of malignant change, surgical resection can be considered. For plexiform neurofibroma, complete resection of the lesion with preservation of key neurovascular structures should be ensured.
(3) Adjuvant Therapy: In cases of malignant transformation or unclear margins, postoperative radiotherapy or chemotherapy may be considered. However, most benign lesions, if fully resected, typically do not require routine radiotherapy or chemotherapy.
2. Rehabilitation/Exercise Prescription (Based on FITT-VP Principles):
(1) Early Stage (Conservative Functional Phase):
Disclaimer: This report is for medical reference only and does not replace in-person consultations or professional medical advice. If you have any concerns or worsening symptoms, please visit a regular medical institution for evaluation.
Periosteal plexiform neurofibroma
