FINAL DIAGNOSIS: Myxoid meningioma
The metaplastic variants of meningioma are an interesting subgroup that highlight the broad histological repertoire of these neoplasms (8). Of these variants, myxoid meningioma poses a particularly difficult diagnostic challenge. These lesions do not display the usual histological features of more typical meningiomas and have abundant mucoid stroma that mimics other myxoid tumors. Currently, the diagnosis of myxoid meningioma is contingent on: 1) recognition that the tumor arises from the leptomeninges, 2) discovery of obscure meningioma histology, 3) extensive Alcian blue staining, 4) identification of tight junctions and basal lamina, and 5) exclusion of other myxoid tumors (2-5, 7). Notable among the histologic features of myxoid meningiomas is that they demonstrate little or no immunoreactivity to recommended dilutions of EMA. In addition, immunostaining for other cytokeratins, smooth muscle actin, muscle-specific actin, and S-100 protein are all generally negative.
The myxoid variant of meningioma must be differentiated from both other dural-based lesions that radiographically mimic meningiomas and other myxoid spindle cell neoplasms including schwannomas, fibromyxomas or fibroxanthomas of the dura (6,8). For instance, if located near cranial nerves, the circumscription, combined spindle cell and myxoid histologic patterns, variable S-100 immunoreactivity, lack of EMA immunoreactivity, and ultrastructural evidence of basal lamina might suggest a diagnosis of schwannoma. Myxoid meningiomas must also be differentiated from myxoid solitary fibrous tumors (SFT) of the falx or cerebellopontine angle, which present in the same age group as meningiomas (mean age = 57 years) and occur more commonly in females [5:2, F:M ratio] (6). In theory, nerve sheath myxomas (neurothekomas) might be considered in the differential diagnosis but don't usually occur at this site and rarely develop in this age group (9). Similarly, myxoid liposarcomas may rarely metastasize to the dura but display notably different histologic features (1). Thus, recognition of myxoid variants requires use of all of the histologic tools available to the pathologist.
Our experience suggests that these tumors grow like other metaplastic variants, with similar recurrence rates. Nonetheless, due to the paucity of reported cases and long-term follow up, the behavior of this meningioma variant has yet to be established.
Contributed by Mahlon D. Johnson, M.D., Ph.D., Charles B. Stevenson, M.D., Reid C. Thompson, M.D., James Atkinson, M.D., Ph.D., and Phillip Boyer, M.D., Ph.D.