False pelvic lymphadenopathy on oncologic patient. Low-grade malignant sheath tumour of sciatic nerve

A 74-year-old male patient with advanced prostate cancer in treatment with hormone therapy. First CT-control is requested.

On the enhanced CT, a pelvic mass is identified compatible with metastatic lymphadenopathy. However, from reviewing the images, the mass seems to be in close contact with the left sciatic nerve and it shows a tapered lateral end (Fig. 1).

An MRI was requested to complete the study. On T1-WI and T2-WI on different planes (Fig. 2a, b, c, d), a large mass which arises from the left sciatic nerve was identified at the level of the lesser sciatic foramen. On sagittal and coronal planes the lesion is slightly fusiform-shaped with tapered ends. On T2-WI the mass shows central and round hyperintense areas, probably related with cystic and necrotic changes. On T1-WI fat saturation enhanced with gadolinium (Fig. 2e), the mass has intense contrast uptake, except in central necrotic areas.

Eventually, a CT-guided biopsy with an 18-G needle was performed (Fig. 3) and samples sent for pathological study.

Malignant peripheral nerve sheath tumours are undifferentiated neoplasms originating in the nerve sheath. It is preferred to call them this way because often it is not known from which cell line they originate (Schwannoma, neurofibroma...) [1]. These tumours most commonly occur in the extremities (52%), followed by the trunk (25%), and head and neck (22%) [2].

It is a rare soft tissue tumour that affects patients 20-50 years of age without sex predilection. Most of them are associated with neurofibromatosis type I (50%), while approximately 40% occur sporadically. Eventually, in the other 10% of these tumours there is antecedent of previous radiotherapy, with a latent period that exceeds 10 years. [1,3]
These neoplasms are extremely rare in the general population (incidence< 0.001%). That's why they are not usually included in the differential diagnosis of low back pain [5]. Accordingly, in the case presented the initial diagnosis of the lesion on the CT control was a metastatic lymph node in a patient with advanced prostate cancer who also did not initially present neurological symptoms. In most cases, pain is the initial symptom in patients with malignant sheath tumour of the sciatic nerve [4].

MRI is the method of choice for evaluation of these tumours, either malignant or benign. The diagnostic key is to identify a fusiform mass with conical ends arising from a known peripheral nerve. Another diagnostic pearl is to observe muscle fat atrophy in T1-WI of the muscles innervated [1].
Unfortunately MRI cannot determine if a tumour is benign or malignant. However, there are several signs that suggest malignancy: Wasa et al. demonstrated markedly hyperintense central areas in the T2-WI, as well as the intense and nodular enhancement with gadolinium with absence of central enhancement, indicating intratumoral cystic-necrotic changes. In this sense, large masses (> 50 mm, as in our case) also suggest malignancy and also the presence of perilesional oedema. On the contrary, the presence of well-defined contours does not exclude malignancy. Anyway, if there is any clinical or radiological feature that raises the possibility of malignancy, a carefully planned biopsy should be obtained to confirm the diagnosis [1, 6].

The basic treatment is radical surgery with wide margins. Unfortunately, it is a tumour with a high tendency to local recurrence and to metastasise at a distance also (mostly to the lungs).
Some of the variables that imply a worse prognosis are: size> 100 mm, history of previous radiation and tumour necrosis> 25%. [5]