Primitive malignant melanoma arising in a spinal nerve root. A case report

We report the fourth case of primitive malignant melanoma arising in a spinal nerve root. A 39-year-old woman complained of one-year low back pain radiating to the right thigh and knee, and loss of 7kg. Clinical examination found moderate quadricipital amyotrophy and hypoesthesia of anterior side of the thigh. MRI study demonstrated an enlargement of right L3 root with scalloping of the L3/L4 foramen. The T1-weighted MRI images showed a tumor hyperintensity, the T2-weighted images showed tumor isointensity and mild contrast enhancement. Due to the scalloping of L3/L4 foramen with root enlargement and slow evolution (more than one year between the first symptom and surgery without clinical worsening), the initial preoperative diagnosis was L3 schwannoma. After laminoarthrectomy and dural opening, a firm black lesion, well encapsulated and involved in a dorsal spinal root, was totally removed. The tumor was composed of irregular melanocytoid cells with high proliferation index (20%). Immunohistochemistry showed melanin, HMB-45 and S100 positivy, but reticulin was negative (that eliminates malignant melanocytic schwannoma). An extensive clinical and paraclinical research of other melanotic localisation was negative. So, the final diagnosis was intradural primitive malignant melanoma. Radiotherapy was performed on the site of the tumor. Fatal pulmonary metastasis occurred 18 months after surgery. The most common tumor with root enlargement and bony scalloping is the benign schwannoma. Despite the above described radiological features, MRI characteristics (hyperintensity when images are T1-weighted) suggest a melanocytic tumor, a tumor with a high adipose component or an intratumoral bleeding. Specific MRI sequences can eliminate adipose tissue tumor, but diagnosis between melanin and methemoglobin is still difficult. According to the index of proliferation, a primitive central melanocytic lesion can be a meningeal melanocytoma (considered as benign) or a primitive malignant melanoma. These tumors show identical protein expressions in immunohistochemistry, and their prognosis is very variable (some long-term remissions are reported for malignant melanomas and fast disseminations are described for meningeal melanocytomas treated by subtotal surgery). The L3/L4 foramen scalloping is unusual for a malignant lesion with theoretic high-speed development. The other 3 patients (reported in the literature) survive more than 3 years. The histological features of malignant lesion with benign clinical features lead to interrogation upon the actual pathologic classification.