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]
Low-grade malignant sheath tumour of sciatic nerve.
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1. A relatively well-defined soft tissue mass is seen in the pelvic region (particularly near the path of the sciatic nerve), spindle-shaped, with mild fat demarcation observed locally.
2. On the plain CT scan, the lesion shows heterogeneous density. Its margins are moderately defined from surrounding tissues. After enhancement, areas of marked enhancement within the tumor are intermixed with low-density necrotic or cystic changes.
3. MRI shows T1-weighted signals that are relatively isointense/slightly hypointense, with some high signal areas on T2-weighted images. A high signal in the center indicates possible cystic or necrotic changes. Post-contrast scans reveal pronounced ring-like or nodular enhancement, with a lack of central enhancement.
4. The tumor diameter exceeds 50 mm, has a substantial volume, and local muscle spaces are involved. The adjacent sciatic nerve may be compressed or encased. No clear bone destruction or significant osseous erosion is currently observed.
5. Given the patient’s history of prostate cancer, a metastatic lymph node was initially suspected. However, due to the lesion’s shape and MRI characteristics—which are not typical—subsequent biopsy findings indicate a peripheral nerve origin.
1. Malignant Peripheral Nerve Sheath Tumor (MPNST)
Reasoning: The lesion is spindle-shaped and located along the sciatic nerve. Imaging suggests rich vascular supply, central necrosis or cystic changes, and although the tumor boundary is distinguishable, the internal signal is uneven. MPNST has considerable malignant potential, aligning with this patient’s imaging features.
2. Metastatic Tumor
Reasoning: The patient has a history of prostate cancer, which can theoretically metastasize to the pelvic region or lymph nodes. However, this lesion’s spindle shape and tight association with the nerve sheath appears more consistent with a nerve-origin tumor, and its internal MRI features are not typical of a prostate cancer metastasis.
3. Schwannoma (benign, e.g., Schwann cell tumor)
Reasoning: Schwannomas also originate from the nerve sheath and can occur in the sciatic nerve. However, benign schwannomas usually grow slowly and remain small, with minimal necrosis or cystic changes; they generally appear more homogenous. A tumor larger than 5 cm with significant necrosis or irregular enhancement raises strong suspicion of malignancy.
Because of the large tumor size (>50 mm), noticeable internal necrosis, and the lack of a history of neurofibromatosis (NF1) or other known benign nerve tumors, a malignant process is more likely.
Based on the patient’s advanced age (74 years), prior history of prostate cancer, the size of the pelvic soft tissue lesion (>50 mm), imaging evidence of a spindle-shaped mass with marked necrosis and heterogeneous enhancement, and corroborating biopsy pathology results, the most likely diagnosis is: Malignant Peripheral Nerve Sheath Tumor (MPNST).
If uncertainty remains, further immunohistochemistry and genetic testing are recommended to determine whether neurofibromatosis (type I) or a radiation-induced etiology might be present.
1. Surgical Treatment: Radical resection of the tumor is the first choice, aiming for an adequate surgical margin to reduce local recurrence. Intraoperative frozen section pathology can be utilized to ensure negative margins.
2. Chemoradiotherapy: Malignant peripheral nerve sheath tumors, after extensive local resection, often warrant adjuvant radiotherapy to lower the rate of recurrence. For larger tumors or in cases with a high risk of lung metastases, adjuvant chemotherapy may be considered, although large-scale clinical data on its efficacy are limited.
3. Key Rehabilitation Points (FITT-VP Principle):
4. Individual Considerations: Given the patient’s older age and history of prostate cancer treatment, close monitoring of bone health and cardiopulmonary endurance is essential. If there is a tendency toward osteoporosis or adverse effects from hormonal therapy, adjust exercise intensity accordingly and take precautions to prevent falls.
Disclaimer: The above report and analysis are for clinical reference only and do not replace an in-person diagnosis or professional medical advice. Specific treatment plans should be determined based on the patient’s actual condition, pathological findings, and multidisciplinary consultation.
Low-grade malignant sheath tumour of sciatic nerve.
