Malignant peripheral nerve sheath tumor of the brachial plexus

A 45-year-old patient without any significant medical or family history was referred to our radiology department by the plastic surgeon due to paraesthesia in her left arm and rapidly growing palpable painful mass in her left axillary region during last two months.

Ultrasound (US) examination showed a round, hypoechoic lesion with increased vascularity due to feeding arteries arising directly from the posteriorly positioned axillar artery (Fig.1).

Magnetic Resonance Imaging (MRI) of the left axillary region showed expansile mass lesion attached to the medial cord of the brachial plexus that was hyperintense on T1-TIRM images (Fig.2), heterogeneous on T2-weighted images (Fig.3) and showed restricted diffusion on diffusion-weighted imaging (DWI) (Fig.4). After intravenous contrast administration, the lesion showed heterogeneous postcontrast enhancement (Fig.5).

Surgical extirpation of the tumour was performed. Immunohistochemically, the lesion was definitely characterised as a malignant peripheral nerve sheath tumour (MPNST).

MPNSTs are malignant tumours derived from Schwann cells or Schwann cell precursors surrounding peripheral nerves that account for 5-10% of all soft tissue sarcomas with an incidence rate of 0,001% in the general population [1,2]. MPNSTs may emerge from neurofibroma, therefore making neurofibromatosis type 1 (NF1) one of the main risk factors for developing MPNSTs [3]. However, many cases of MPNSTs arise sporadically or due to radiation exposure [4]. The location is most commonly in extremities and their most common metastatic sites are lungs [2,5].

The clinical manifestation of MPNT is classical of soft tissue sarcomas with symptoms such as enlarging mass, pain, skeletal muscle weakness, paraesthesia, and neurologic deficits [6].

Computed tomography (CT) and US have a limited role in the diagnostic work-up, therefore making MRI the main modality to characterise the topographic relationship of the tumour to the adjacent tissues, such as blood vessels and other vital structures, so radiologist’s report should note whether tumour infiltrates any of these structures or not [7, 8]. MRI characteristics of MPNSTs are varyingly present. Imaging features highly suggestive for malignancy include larger size, an ill-defined margin of the mass, lack of contiguity with adjacent specific nerves, and MR signal changes in adjacent soft tissue [9]. However, because the MRI cannot effectively confirm malignant transformation of lesions, histopathologic examination remains the gold standard for the diagnosis of MPNTs [6]. Nevertheless, conventional MRI may obviate the need for biopsies in presence of imaging features suggestive of a benign lesion, such as the T2-hypointensity in the central area of the mass and T2-hyperintense rim, so-called “target sign”, and absence of ill-defined margins or perilesional oedema. However, “target sign” is not sufficiently specific or sensitive as it is also encountered in some malignant lesions. Therefore, cystic changes, heterogeneity on T1-weighted images and intratumoral lobulation are also supposed to be taken into consideration [3, 10]. Surgical removal remains the main treatment for MPNST, whilst adjuvant therapies are recommended for patients with MPNST, specifically for those with positive margins [11]. MPNST has the highest local recurrence rate among sarcomas and 5-year survival remains poor in general [11].

Written informed patient consent for publication has been obtained.