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. (1-4, 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.
Contributed by Kyle J. Mangels, MD, Mahlon D. Johnson, MD, PhD and Robert J. Weil, MD