Plexiform Neurofibroma of the Wrist: Imaging Features and When to Suspect Malignancy

Plexiform neurofibromas are essentially pathognomonic for neurofibromatosis type 1 (NF1), occurring when there is diffuse involvement along a nerve segment and its branches. Transformation into a malignant peripheral nerve sheath tumour (MPNST) is a major cause of mortality in NF1 patients. These tumours are highly aggressive and particularly difficult to diagnose in NF1 patients due to the clinical overlap between benign and malignant lesions. We present a case of a plexiform neurofibroma and discuss the typical imaging characteristics on ultrasound, CT, and MRI, including the target sign and continuity with the parent nerve. Certain imaging features should raise suspicion for malignancy however, these modalities may not always reliably differentiate between benign and malignant lesions. Recent studies show a very high negative predictive value for FDG-PET making it quite useful in excluding malignancy. In positive scans, PET/CT aids in guiding biopsy to the most metabolically active area of the tumour. 1. Introduction Neurofibromatosis type 1 (NF1) is an autosomal-dominant disorder affecting 1 in 3500 individuals [1]. Plexiform neurofibromas are essentially pathognomonic for NF1 occurring with diffuse involvement along a nerve segment and its branches, giving a “bag of worms” appearance [2]. The estimated prevalence of transformation to a malignant peripheral nerve sheath tumour (MPNST) is approximately 5% [2]. Early diagnosis is crucial for effective treatment of MPNSTs but they are particularly difficult to diagnose in NF1 patients due to clinical overlap between benign and malignant lesions [3]. We present a case of a plexiform neurofibroma and discuss the features that should raise suspicion for malignant change. 2. Case History A teenage male patient presented with progressive swelling of the right wrist for three years and pain for two days following blunt minor trauma. He reported a four-year-old injury to the wrist with a ball and repeated blunt trauma to the wrist since. No significant past medical or surgical history was elicited. Physical examination revealed a swollen and slightly tender ulnar aspect of the wrist with slight decrease in range of motion. The ulnar nerve was intact clinically with normal sensation and grip strength. Baseline blood results were normal. X-ray (Figure 1) showed soft tissue swelling on the ulna aspect of the wrist with triquetrum remodeling and ulna subluxation. Figure 1: X-ray of the right wrist: (a) lateral, (b) oblique, and (c) AP—soft tissue swelling overlying ulnar aspect of wrist with remodeling of