FINAL DIAGOSIS -- ATYPICAL MENINGIOMA
Meningiomas are neoplasms of the middle decade of life and account for 13-19% of all primary intracranial tumors (1). They can occur anywhere along the neuraxis, including intracranial, spinal, intraventricular, and extracranial sites. The majority are benign, slow growing, and well-circumscribed. Meningiomas in children, in contrast, are uncommon; they represent less than 5% of intracranial tumors in the pediatric age group and show a tendency toward aggressive behavior.
In the revised edition of the WHO classification of tumors of the central nervous system (2), three categories of meningiomas have been delineated: common, (WHO grade 1), atypical (WHO grade 2), and anaplastic (WHO grade 3). Atypical meningiomas represent an intermediate category of tumor that has a higher relative risk for relapse (3). In a recent study of 160 cases, recurrence occurred in 9% of common meningiomas, 29% of atypical meningiomas, and 50% of anaplastic meningiomas (4). While patients with atypical meningiomas have improved long-term survival rates compared to patients with anaplastic meningiomas (79% 10 year survival versus 34.5% 10 year survival), 26% of recurring atypical meningiomas will assume a malignant phenotype, thus underscoring the importance of identification of this tumor subtype (5).
Histologic features of atypical meningiomas include: 1.) increased mitotic activity, 2.) increased cellularity, 3.) small cells with a high nuclear-cytoplasmic ratio and prominent nucleoli, and 4.)patternless, uninterrupted or sheet-like (syncytial) growth and areas of micronecrosis unrelated to vascular occlusion. Invasion of the dura, bone, or soft tissue is not a feature of atypia, nor is the presence of pleomorphic or atypical nuclei unassociated with mitotic activity or necrosis (6).
While a moderate correlation exists between histologic grade and recurrence (4), meningiomas in general, and especially those which belong to the atypical type, may vary in clinical presentation and morphologic features, making classification and prediction of recurrence difficult. The use of ploidy analysis and proliferation markers may assist in the identification of recurrence-prone meningiomas (4, 6), as demonstrated by this case. In addition, cytogenetic analysis may be useful (4,6,7). Chromosomal changes other than monosomy 22 and, especially, deletion of the short arm of chromosome 1 (1p-), have been correlated with more aggressive tumor behavior.
Contributed by Linda M. Dallasta, MD, PhD and Ronald L. Hamilton, MD