A 50-year-old female with a known history of neurofibromatosis type 1 (NF1) presented with atraumatic pain and weakness of the left leg. She had previously had a focal neurofibroma excised from the left thigh. MRI imaging of the left femur and knee was performed to identify a cause for the pain.
MRI scan of the left knee (Figures 1, 2, and 3) demonstrated diffuse T2 bright and iso-hypotense T1 tissue infiltrating the subcutaneous and intermuscular planes at the posterior aspect of the knee and reaching the periosteal surface of the distal femur, causing bone dysplasia with widening of the posterior intercondylar notch. CT imaging also demonstrated the marked widening of the intercondylar notch (Figures 4 and 5).
Background
The intercondylar notch is a depression in the distal femur between the medial and lateral condyles; it provides attachments for the cruciate and meniscofemoral ligaments [1]. A widening of the intercondylar notch can be caused by haemophilic arthropathy, juvenile rheumatoid arthritis, tuberculous arthropathy, and psoriatic arthropathy [2].
Neurofibromatosis type 1 (NF1) is an autosomal dominant disorder caused by a mutation in the NF-1 gene located on chromosome 17. NF1 commonly manifests with the presence of café-au-lait macules and neurofibromas [3]. To our knowledge, there is no literature describing a widening of the intercondylar notch in NF1. This report suggests the potential for such an association.
Clinical and Imaging Perspective
Approximately 59.6% of individuals with NF1 develop some form of malignancy in their lifetime [4]. In our institution, we regularly perform MRI whole-body surveillance for patients with complex NF1. We have observed different types of neurogenic lesions in this population, including focal neurofibroma, diffuse neurofibromatosis tissue, plexiform neurofibroma and malignant peripheral nerve sheath tumour (MPNST):
Bone dysplasia in NF1 is seen in typical sites such as the sphenoid wing and distal third of the tibia with pseudoarthrosis. The pathogenesis of bone dysplasia in NF1 is attributed to NF tissue propagating along the periosteum, causing bone remodelling or mesodermal dysplasia (in the absence of any macroscopic lesion) [7].
Regarding potential differential diagnoses for this appearance, there were no low T2 signal nodular elements to indicate hemosiderin deposition to suggest this being pigmented villonodular synovitis (PVNS). PVNS would also not tend to extend outside the joint capsule. There was also no T1 hyperintensity to indicate fatty tissue to suggest this being lipoma arborescens. Intra-articular tumours are extremely rare, the most common potential soft tissue tumour around the knee would be a synovial sarcoma, which would not tend to have intra-articular extension as per this case.
We have provided a companion case from our complex NF1 service, shown in Figures 6a and 6b, with almost identical appearances with widening of the intercondylar notch due to infiltrative neurofibromatosis tissue.
Outcome
Given the patient’s history of NF1 and symptoms, a biopsy was performed to rule out potential malignancy. Histology showed a tumour of haphazard arrangements of bland spindle cells within a loose collagenous stroma. There was no significant atypia and no necrosis. Mitoses are inconspicuous. Immunohistochemistry showed strong positivity for S100 and SOX10. There was reticular staining for CD34 throughout and p16 was positive. MIB1 is very low. Appearances were of a benign peripheral nerve sheath tumour consistent with neurofibroma. No evidence of malignancy in the sampled material. The lesion was stable on sequential MRI scans over a 10-year period, indicating a benign process.
Take Home Message / Teaching Points
All patient data have been completely anonymised throughout the entire manuscript and related files.
Diffuse neurofibromatosis tissue in neurofibromatosis type 1
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Based on the provided MRI and other imaging, a noticeable “widening” or “expansion” is observed in the distal femur region of the knee joint (at the intercondylar notch between the medial and lateral femoral condyles). On MRI, the lesion shows high signal intensity on T2, with the soft tissue structure extending intra- and extra-articularly and exhibiting an infiltrative growth trend. Some degree of remodeling of the adjacent bone can be seen around the joint. The abnormal soft tissue signal appears to have partially ill-defined boundaries with the surrounding muscle fascia and joint capsule, suggesting possible growth along the peripheral tissue surfaces and bone surface. On certain slices, the tumor shows a relatively uniform or mildly lobulated shape, without notable bone destruction or signs of acute fracture.
Considering the patient’s history of NF1 and the imaging stability over several years, diffuse or focal neurofibroma stands out as the primary consideration.
Based on the patient’s established history (NF1) and this imaging presentation, and combined with subsequent pathological findings (no malignant cytologic evidence, S100 and SOX10 positive, consistent with a benign nerve sheath tumor), the most likely diagnosis is:
“A benign neurofibroma (diffuse neurofibromatous lesion) related to NF1 involving the knee joint.”
If there is rapid enlargement, increased pain, or imaging findings such as a necrotic area in the future, further evaluation is recommended to rule out malignant transformation (e.g., malignant peripheral nerve sheath tumor).
1) Treatment Strategy:
▪ If the patient’s symptoms are mild and the lesion is stable, regular imaging follow-up can be considered;
▪ If the lesion enlarges, causes marked pain or joint dysfunction, or if a malignant change is suspected, surgical excision or biopsy is warranted;
▪ Pharmacological treatment mainly focuses on symptomatic relief (e.g., NSAIDs for pain); there is no specific anti-tumor drug that can reverse neurofibroma growth.
2) Rehabilitation/Exercise Prescription:
For maintaining and improving knee joint function, taking into account the patient’s bone structure and neurofibromatous changes, a gradual, individualized exercise program is recommended:
This report provides referential medical analysis and recommendations and should not substitute for in-person consultation or a professional medical opinion. Patients should consult with a specialized medical team to develop and adjust their final treatment and rehabilitation plan according to their specific condition.
Diffuse neurofibromatosis tissue in neurofibromatosis type 1
