35‐year‐old Woman with Progressive Bilateral Leg Weakness

CLINICAL HISTORY & IMAGING

A 35‐year‐old woman presented with one month’s history of progressive bilateral leg weakness and altered sensation. There had been no pain. She had noted urinary frequency and constipation in the previous two weeks. On examination, the patient had diffuse lower extremity weakness (2–3/5), with a T6 sensory level to pain and temperature sensation. Proprioception was preserved. Post‐void residuals exhibited urinary retention. There was sacral hypesthesia and decreased rectal tone. She was mildly hyperreflexic (3/4) at the knees and ankles without clonus; both great toes were upgoing. A T1‐weighted MRI demonstrated a T4‐5 intradural mass ventral to the spinal cord (Figure 1A), with an enhancing dural tail (Figure 1B), consistent with meningioma. The lesion was dark on T2‐weighted images (Figure 1C).

Figure 1.

GROSS AND MICROSCOPIC DESCRIPTION

At surgery via a posterior, transpedicular approach, an intradural, extramedullary, firm, black neoplasm was encountered, which invaded the ventral dura and elevated and distorted the spinal cord. The mass was removed, leaving only microscopic invasion of the ventral dura (Figure 1D, a sagittal MR view, post‐contrast, after resection). There was no bone invasion. The tumor fragments measured 2.0 × 1.5 × 1.0 cm in aggregate. Serial sections revealed a homogeneous black tumor without necrosis. Hematoxylin and eosin stained sections showed an occasionally fascicular tumor of melanocytes and small round blue tumor spindle cells with melanin pigmentation and 1–2 mitotic figures per 10 high‐powered fields (Figure 2A,B). The nuclei are generally oval‐shaped and elongated, with prominent nucleoli. Necrosis, hemorrhage, and nuclear and cellular pleomorphism are not present and mitotic figures are rare. MIB‐1 labeling (Figure 3A,B) averaged 1–2% in multiple fields. Immunohistochemical staining was positive for S‐100 and HMB‐45 (Figure 3C) and S‐100 (Figure 3D).

Figure 2.

Figure 3.

DIAGNOSIS AND DISCUSSION

Diagnosis: Intermediate Grade Melanocytoma

Primary meningeal melanocytic tumors (PMMTs) are rare; fewer than 100 cases have been described. PMMTs of the CNS consist of a spectrum of tumors ranging from well‐differentiated melanocytoma to its overtly malignant counterpart, melanoma 1, 3, 4. Intermediate grade melanocytomas (IMGs) are the least common variant, comprising about 10% of PMMTs reported 1, 3. Similar to the less and more aggressive variants of PMMTs, IGMS occur in the spinal leptomeninges and intracranially in approximately equal proportions 1, 4, 6, 7. IGMs are more cellular than the well‐differentiated variant, with 1–3 mitotic figures per 10 HPFs and MIB‐1 labeling of <6%(1). By contrast, melanomas contain more mitotic figures (3–15 per 10 HPF) and MIB‐1 labeling rates up to 15%(1). Once metastasis, including drop metastasis from pigmented medulloblastomas, have been excluded, the differential includes pigmented meningiomas and schwannomas (solitary or as part of Carney complex), as well as other, pigmented CNS tumors such as ependymoma and pineoblastoma and systemic diseases such as lymphoma 1, 2, 3, 4, 6, 8. For lesions that mimic meningioma, a decision‐tree has been worked out, which in this instance used the presence of S‐100 and HMB‐45 immunostaining to confirm the origin of the tumor in melanocytes, as demonstrated by the widespread presence of melanin (reviewed in reference 8). The low level of proliferation, as demonstrated by rare mitotic figures and MIB‐1 levels of 1–2% suggested an intermediate grade melanocytic tumor 1, 3, 8.

For primary CNS melanocytic neoplasms, complete tumor resection is preferred, as it leads to cure of well‐differentiated and intermediate‐grade melanocytomas and most melanomas 1, 3, 4. Radiotherapy is recommended for incomplete resection of IMGs and melanomas; the recurrence potential of low‐grade melanocytomas is less clear and watchful waiting may be employed, since recurrent tumors may be treated surgically prior to radiation 1, 4

In this instance, the patient noted immediate improvement in strength and sensation after surgery, and at the time of discharge 4 days post‐operatively, the patient was able to ambulate with a cane, with 4–5 to 4+/5 strength diffusely in the lower extremities. Bowel and urinary function was normal. Two months after surgery, the patient had normal sensation and strength. She was given focused radiotherapy to the region of the ventral thecal sac to 40 cGy. At 20 months following surgery, the patient’s neurological examination is normal and she remains free of residual disease by MR examination.