Malignant transformation of plexiform neurofibroma

Case contributed by Dr Francesco Buemi

Presentation

Shooting pains in lower back, radiating into the buttocks and lower limbs. Surgery one year ago. Worsening of symptoms.

Patient Data

Age: 35 years
Gender: Female

Lumbar spine:

There are post-surgical appearances secondary to partial resection of a right lumbosacral plexiform neurofibroma. Root stumps of the resected right L4, L5, S1, S2 and S3 nerves, with a swollen right L5 nerve root stump are noted. 

Compared to the previous MRI (not shown and performed in another center), there was an increase in size of the neoplastic tissue occupying the spinal canal from L2 to S1, particularly the L5 nerve root. 

Thickening and enlargement of the residual roots and branches of lumbosacral plexus, in particular the right L2 and L3 nerve roots and left L4, L5, S1, S2 and S3 roots (previously not involved).

Thickened and swollen the pelvic tract of the right femoral nerve.

Pelvis:

Post surgical changes in the right pelvis and adjacent the right proximal femur secondary to surgical resection of the right sciatic nerve, gluteus maximus, hamstrings and external rotators of the right hip. There is a fluid collection around the right hip.

Marked reduced trophism and altered signal of the right residual gluteal muscles, adductors, obturators, piriformis and the anterior compartment muscles of the right thigh due to chronic denervation. The right paraspinal muscles are also hyperintense on T2 sequences secondary to subacute denervation 

In the left ovary there is a thick walls cystic lesion with strong peripheric enhancement likely a corpus luteum.

 

F18-FDG PET/CT

Nuclear medicine

There is moderately increased FDG uptake (SUVmax 5.34) of the right L5 nerve root stump. Further mildly increased FDG uptake along the spinal canal from L2 to S1 (focal higher uptake is seen at the L2-L3 level - SUV max 3.31) and nerve roots of the lumbosacral plexus.

Diffuse non-specific uptake is found at the site of surgery, in the soft tissue surrounding the right hip, in right obturator and gluteal muscles, and along the surgical scar in keeping with inflammation.

High cutaneous-subcutaneous FDG uptake is seen in the posterior aspect of the right thigh (reported burn injury during the surgery).

Case Discussion

After her initial surgery, the patient was diagnosed with malignant nerve sheath tumor (MNST) involving the right sciatic nerve which was resected. Due to the histological finding of the tumor extending more proximally, the patient underwent a second operation with resection of the L4, L5, S1, S2 and S3 nerve roots. When the patient underwent MRI she complained of worsening of her chronic pain and limited mobility. She was lost to follow-up after the PET scan. 

Based on the MRI, the enlarging size of the neoplastic tissue in the spinal canal, the increased involvement of the roots-branches of the lumbosacral plexus and the appearance of the L5 right nerve root stump were suspicious for malignant transformation.

Focal moderately increased uptake at the right L5 nerve root stump raises suspicion for MPNST, however there is overlap in imaging features with benign neurofibromas. 

About 40–50% of MNSTs occur in patients with neurofibromatosis type 1 (NF1)1. The remaing cases are sporadic, with 10–13% occurring at sites of previous radiation therapy2, 3, 4. Some cases of sporadic plexiform neurofibroma have been described in patients without other features of NF1, caused by somatic NF1 mosaicism 5, 6 . Approximately 10% of patients with NF1 develop MPNST, usually from plexiform neurofibroma7.

This patient did not have history of previous radiotherapy and is not known to be affected by NF1 (she does not have clinical features of NF1, but results from molecular genetic testing of NF1 gene are unknown).

Thank you to Dr Salvatore Pignata for contributing this case with me.

 

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