Although generally benign tumors, meningiomas can cause serious neurological injury and, at times, vexatious management difficulties. Currently, the accepted management of these tumors is attempted total surgical excision when technically possible and associated with an acceptable risk. However, even with innovations in instrumentation and refinements in surgical technique, the goal of total resection may not be achievable. For these patients, and for those with recurrent tumors, options for treatment include reoperation, radiation therapy, and chemotherapy. Recent developments in surgical technique and instrumentation, radiosurgery, and brachytherapy have increased the treatment options, while clinical trials with tamoxifen and mifepristone (RU486) are adding information on the effectiveness of these drugs as chemotherapeutic agents. While the search continues for a uniformly successful management plan, physicians must be aware of the available options and try to help the patient decide which treatment is appropriate, based on current medical knowledge.
Histologically, meningioma is a typically benign neoplasm that would not present a therapeutic challenge if located in any less vital or less anatomically complex region of the body than the intracranial or intraspinal spaces. When located inside the cranium, and especially along the skull base, this usually benign tumor must be viewed as a locally aggressive soft-tissue tumor that will recur if incompletely treated. The following case presentation highlights just such a problem.
In 1965, a 31-year-old woman had a left frontotemporal craniotomy and a subtotal resection of a sphenoid ridge meningioma. Postoperatively, a wound infection led to the removal of the craniotomy bone flap. In 1968, progressive left-sided proptosis due to tumor regrowth developed, and the patient underwent a left orbital exenteration with subtotal removal of the meningioma. Progressive enlargement of the residual meningioma led to two subsequent cranial operations in 1974. Following these surgeries, external beam radiation to a dose of 50 Gy was delivered. The patient remained stable for 17 years.
In 1991, an increased frequency of seizures, somnolence, and memory loss led to reinvestigation. Massive tumor recurrence was found. The initially left-sided sphenoid ridge meningioma had now spread to the right middle fossa and into the posterior fossa. The large right middle fossa component of the tumor was resected. In early 1994, the patient, now 59 years old, developed an ataxic gait and right-sided hearing loss. Enlargement of the tumor was noted. The patient was enrolled in an experimental trial using the antiprogesterone agent mifepristone (RU486). Progressive tumor enlargement with the onset of obstructive hydrocephalus led to the discontinuation of the medication and the insertion of a ventriculoperitoneal shunt. The patient's current magnetic resonance (MR) images are shown in Figure 1. Surgical removal of the large petroclival component of the meningioma has been recommended.
Reported incidence rates of meningioma vary from less than 1 to more than 6 per 100,000 population [1,2]. If the results of three large recent studies of intracranial neoplasms are combined, the overall incidence of meningioma is 2.6 per 100,000 population [1,3,4]. The ratio of male to female cases ranges from 1:1.4 to 1:2.8. The incidence of intracranial meningioma increases with age [1,3,4]. The rates peak in the sixth decade for males and in the seventh for females. These peak rates are 6.0 and 9.5 per 100,000, respectively . Autopsy data suggest that the declining incidence rates beyond the seventh decade may be the result of a less aggressive investigative posture in the elderly .
The World Health Organization (WHO) classification of meningiomas is shown in Table 1. Numerous histologic variants of meningioma exist, none of which has any prognostic significance; all are considered histopathologically benign tumors. The most common of these variants are the meningothelial and fibrous meningiomas (Figure 2).
Certain histopathologic features, which can be detected by light microscopy, portend an increased tumor aggressiveness and increased likelihood for recurrence (Table 2) [6,7]. It is these features that define the atypical meningioma. The diagnosis of malignant meningioma generally requires histologic evidence of brain invasion or distant metastasis, which, in most cases, is accompanied by further evidence of aggressivity such as is seen with the atypical meningiomas. An exception to this requirement is the finding of a papillary pattern . This pattern is associated with a predictable aggressive behavior, with late distant metastases occurring with significant frequency . When dissemination occurs, the more common sites of implantation and growth are the lungs and/or pleura, bones, abdominal viscera (especially the liver), and lymph nodes .
Because the identification of histologic features of aggressiveness is occasionally imprecise, and their presence does not necessarily correlate with future regrowth or recurrence of the tumor, researchers have attempted quantitative measurements of various parameters. Using flow cytometry, May et al  have shown that recurring meningiomas have a significantly higher proliferative index (% S-phase + % G(2)/M-phase) than do nonrecurrent meningiomas. They indicated that a proliferative index of more than 20%, irrespective of the histopathologic appearance, strongly suggested that the tumor would recur.
The determination of the bromodeoxyuridine (BUdR) labeling index or of the number of argyrophilic nucleolar organizer regions (AgNOR) has been used to identify intracranial meningiomas with a higher propensity to recur. Hoshino et al  found that a BUdR labeling index of 1% or higher was indicative of meningiomas with a faster than typical growth rate (higher proliferative potential) and that meningiomas with a BUdR labeling index of 5% or higher had a 100% recurrence rate. The recurrence rate dropped to 55.6% for meningiomas with a BUdR labeling index between 3% and 5% and to 30.6% for those with a BUdR labeling index between 1% and 3% [12,13]. Chin and Hinton  reported that the mean AgNOR counts were statistically different among benign (245 ± 156), atypical (497 ± 135), and malignant meningiomas (921 ± 59). They also noted a statistically different AgNOR count for recurrent meningiomas (544 ± 76) when compared with nonrecurrent meningiomas (329 ± 183).
Finally, positron emission tomography (PET) studies have shown glucose utilization to be lower (1.9 mg/dL/min ± 1.0) in nonrecurring tumors than in recurrent tumors (4.5 mg/dL/min ± 1.9) .
Contrast-enhanced MR imaging provides the best means of detecting meningiomas . Most meningiomas enhance intensely and homogeneously with intravenous paramagnetic contrast material, and in approximately 10% of cases, small additional meningiomas are encountered that are missed on unenhanced MR images. Likewise, contrast enhancement of the dura extending away from the margins of the mass is typical of meningioma, although it can be seen with other dural-based lesions. This "dural tail" can indicate tumor extension, and its resection is important to reduce the risk of recurrence. Postoperative enhanced MR imaging has also been found to be more sensitive and specific in the detection of residual or recurrent meningioma. Thick and nodular enhancement has a high correlation with recurrent or residual neoplasm .
The MR characteristics of meningiomas are relatively consistent. On noncontrasted T(1)-weighted images, 60% to 90% of meningiomas are isointense, whereas 10% to 30% are mildly hypointense when compared with gray matter. T(2)-weighted imaging reveals that 30% to 45% of meningiomas have increased signal intensity, whereas approximately 50% are isointense to gray matter [16,18-20].
Vascular distortion or encasement and tumor vascularity are better assessed by MR imaging than by computed tomography (CT) scanning. Flow-voids produced by flowing blood identify the vasculature local to the tumor (Figure 1).
There is increasing interest in using MR characteristics to tissue-subtype meningiomas preoperatively. The results of these studies have been varied, with some reporting 75% to 96% accuracy, and others finding no correlation [18-21]. The MR characteristics that allowed accurate preoperative identification of meningioma subtypes were confined to findings on T2-weighted studies. Specifically, meningothelial meningiomas were found to have a consistently higher signal intensity on T2-weighted sequences than did fibroblastic or transitional meningiomas, which demonstrated a higher relative signal intensity on intermediate images. High signal intensity on T2-weighted images has also been correlated with microscopic hypervascularity and soft tumor consistency .
1. Kurland LT, Schoenberg BS, Annegers JF, et al: The incidence of primary intracranial neoplasms in Rochester, Minnesota. Ann NY Acad Sci 381:6-16, 1982.
2. Schoenberg GS, Christine BW, Whisnant JP: The descriptive epidemiology of primary intracranial neoplasms: The Connecticut experience. Am J Epidemiol 104:499-510, 1976.
3. Preston-Martin S, Henderson BE, Peters JM: Descriptive epidemiology of central nervous system neoplasms in Los Angeles County. Ann NY Acad Sci 381:202-208, 1982.
4. Sutherland GR, Florell, R, Louw D, et al: Epidemiology of primary intracranial neoplasms in Manitoba, Canada. Can J Neurol Sci 14:586-592, 1987.
5. Rohinger M, Sutherland GR, Louw DF, et al: Incidence and clinicopathological features of meningioma. J Neurosurg 71:665-672, 1989.
6. Burger PC, Scheithauer BW, Vogel FS: Surgical Pathology of the Nervous System and its Coverings, 3rd Ed, p 83. New York, Churchill Livingstone, 1991.
7. De la Monte SM, Flickinger J, Linggood RM: Histopathologic features predicting recurrence of meningiomas following subtotal resection. Am J Surg Pathol 10:836-843, 1986.
8. Ludwin SK, Rubenstein U, Russell DS: Papillary meningiomas: A malignant variant of meningioma. Cancer 36:1363-1373, 1975.
9. Pasquier B, Gasnier F, Pasquier D, et al: Papillary meningioma: Clinicopathologic study of seven cases and review of the literature. Cancer 58:299-305, 1986.
10. Younis GA, Sawaya R, DeMonte F, et al: Aggressive meningeal tumors: Review of a series. J Neurosurg (in press).
11. May PL, Broome JC, Lawry J, et al: The prediction of recurrence in meningiomas: A flow cytometric study of paraffin-embedded archival material. J Neurosurg 71:347-351, 1989.
12. Hoshino T, Nagashima T, Murovic JA, et al: Proliferative potential of human meningiomas of the brain: A cell kinetics study with bromodeoxyuridine. Cancer 58:1466-1472, 1986.
13. Shibuya M, Hoshino T, Ito S, et al: Meningiomas: Clinical implications of a high proliferative potential determined by bromodeoxyuridine labeling. Neurosurgery 30:494-498, 1992.
14. Chin LS, Hinton DR: The standardized assessment of argyrophilic nucleolar organizer regions in meningeal tumors. J Neurosurg 74:590-596, 1991.
15. DiChiro G, Hatazawa J, Katz DA, et al: Glucose utilization by intracranial meningiomas as an index of tumor aggressiveness and probability of recurrence. Radiology 164:521-526, 1987.
16. Zimmerman RD: MRI of intracranial meningiomas, in Al-Mefty O (ed): Meningiomas, pp 209-223. New York, Raven Press, 1991.
17. Weingarten K, Ernst RJ, Jahre C, et al: Detection of residual or recurrent meningioma after surgery: Value of enhanced vs unenhanced MR imaging. Am J Radiol 158:645-650, 1992.
18. Demaerel P, Wilms G, Lammens M, et al: Intracranial meningiomas: Correlation between MR imaging and histology in fifty patients. J Comput Assist Tomogr 15:45-51, 1991.
19. Elster AD, Challa VR, Gilbert TH, et al: Meningiomas: MR and histopathologic features. Radiology 170:857-862, 1989.
20. Spagnoli MV, Goldberg HI, Grossman RI, et al: Intracranial meningiomas: High-field MR imaging. Radiology 161:369-375, 1986.
21. Kaplan RD, Coons S, Drayer BP, et al: MR characteristics of meningioma subtypes at 1.5 Tesla. J Comput Assist Tomogr 16:366-371, 1992.
22. Chen TC, Zee CS, Miller CA, et al: Magnetic resonance imaging and pathological correlates of meningiomas. Neurosurgery 31:1015-1022, 1992.
23. Simpson D: The recurrence of intracranial meningiomas after surgical treatment. J Neurol Neurosurg Psychiatry 20:22-39, 1957.
24. Mirimanoff RO, Dosoretz DE, Linggood RM, et al: Meningioma: Analysis of recurrence and progression following neurosurgical resection. J Neurosurg 62:18-24, 1985.
25. Chan RC, Thompson GB: Morbidity, mortality, and quality of life following surgery for intracranial meningiomas. J Neurosurg 60:52-60, 1984.
26. Jääskeläinen J: Seemingly complete removal of histologically benign intracranial meningioma: Late recurrence rate and factors predicting recurrence in 657 patients. Surg Neurol 25:461-469, 1986.
27. Jääskeläinen J, Haltia M, Servo A: Atypical and anaplastic meningiomas: Radiology, surgery, radiotherapy and outcome. Surg Neurol 25:233-242, 1986.
28. Kallio M, Sankila R, Hakulinen T, et al: Factors affecting operative and excess long-term mortality in 935 patients with intracranial meningioma. Neurosurgery 31:2-12, 1992.
29. Sawaya R, Rämö OJ: Systemic and thromboembolic effects of meningiomas, in Al-Mefty O (ed): Meningiomas, p 137. New York, Raven Press, 1991.
30. Derome PJ, Visot A: Bony reaction and invasion in meningiomas, in Al-Mefty O (ed): Meningiomas, p 169. New York, Raven Press, 1991.
31. DeMonte F, Al-Mefty O: Anterior clinoidal meningiomas, in Rengachary SS, Wilkins RH (eds): Neurosurgical Operative Atlas, vol 3, pp 49-61. Baltimore, Williams & Wilkins, 1993.
32. Al-Mefty O, Fox JL, Smith RR: Petrosal approach for petroclival meningiomas. Neurosurgery 22:510-517, 1988.
33. Sen CN, Sekhar LN: An extreme lateral approach to intradural lesions of the cervical spine and foramen magnum. Neurosurgery 27:197-204, 1990.
34. DeMonte F, Smith HK, Al-Mefty O: Outcome of aggressive removal of cavernous sinus meningiomas. J Neurosurg 81:245-251, 1994.
35. Jane JA, Park TS, Pobereskin LH, et al: The supraorbital approach: Technical note. Neurosurgery 11:537-542, 1982.
36. Spetzler RF, Daspit CP, Pappas CTE: The combined supra- and infratentorial approach for lesions of the petrous and clival regions, experience with 46 cases. J Neurosurg 76:588-599, 1992.
37. Brackmann DE: Translabyrinthine/transcochlear approaches, in Sekhar LN, Janecka IP (eds): Surgery of Cranial Base Tumors, pp 351-365. New York, Raven Press, 1993.
38. DeMonte F, Warf P, Al-Mefty O: Intraoperative monitoring of the lower cranial nerves during surgery of the jugular foramen and lower clivus, in Loftus C, Traynelis V (eds): Intraoperative Monitoring Techniques in Neurosurgery, pp 205-212. New York, McGraw-Hill, 1993.
39. Goldsmith BJ, Wara WM, Wilson CB, et al: Postoperative irradiation for subtotally resected meningiomas. J Neurosurg 80:195-201, 1994.
40. Miralbell R, Linggood RM, de la Monte S, et al: The role of radiotherapy in the treatment of subtotally resected benign meningiomas. J Neurooncol 13:157-164, 1992.
41. Taylor BW, Marcus RB, Friedman WA, et al: The meningioma controversy: Postoperative radiation therapy. Int J Radiat Oncol Biol Phys 15:299-304, 1988.
42. Bloom HJG: Intracranial tumors: Response and resistance to therapeutic endeavors, 1970-1980. Int J Radiat Oncol Biol Phys 8:1083-1113, 1982.
43. Carella RJ, Ransohoff J, Newall J: Role of radiation therapy in the management of meningioma. Neurosurgery 10:332-339, 1982.
44. Forbes AR, Goldberg ID: Radiation therapy in the treatment of meningioma: The joint center for radiation therapy experience, 1970-1982. J Clin Oncol 2:1139-1143, 1984.
45. Salazar OM: Ensuring local control in meningiomas. Int J Radiat Oncol Biol Phys 15:501-504, 1988.
46. Busse PM: Radiation therapy for meningiomas, in Schmidek HH (ed): Meningiomas and Their Surgical Management, p 506. Philadelphia, WB Saunders, 1991.
47. Al-Mefty O, Kersh JE, Routh A, et al: The long-term side effects of radiation therapy for benign tumors in adults. J Neurosurg 73:502-512, 1990.
48. Harris JR, Levene MB: Visual complications following irradiation for pituitary adenomas and craniopharyngiomas. Radiology 120:167-171, 1976.
49. Spiegelmann R, Friedman W: Radiosurgical treatment of meningiomas, in Schmidek HH (ed): Meningiomas and Their Surgical Management, pp 508. Philadelphia, WB Saunders, 1991.
50. Steiner L, Lindquist C, Steiner M: Meningiomas and gamma knife radiosurgery, in Al-Mefty O (ed): Meningiomas, pp 263-272. New York, Raven Press, 1991.
51. Luxton G, Petrovich Z, Jozsef G, et al: Stereotactic radiosurgery: Principles and comparison of treatment methods. Neurosurgery 32:241-259, 1993.
52. Lunsford LD: Contemporary management of meningiomas: Radiation therapy as an adjuvant and radiosurgery as an alternative to surgical removal? J Neurosurg 80:187-190, 1994.
53. Engenhart R, Kimmig BN, Hover KH, et al: Stereotactic single dose radiation therapy of benign intracranial meningiomas. Int J Radiat Oncol Biol Phys 19:1021-1026, 1990.
54. Souhami L, Olivier A, Podgorsak EB, et al: Fractionated stereotactic radiation therapy for intracranial tumors. Cancer 68:2101-2108, 1991.
55. Kondziolka D, Lunsford LD, Coffey RJ, et al: Stereotactic radiosurgery of meningiomas. J Neurosurg 74:552-559, 1991.
56. Gutin P, Leibel SA, Hosobuchi Y, et al: Brachytherapy of recurrent tumors of the skull base and spine with iodine-125 sources. Neurosurgery 20:938-945, 1987.
57. Kumar PP, Patil AA, Leibrock LG, et al: Brachytherapy: A viable alternative in the management of basal meningiomas. Neurosurgery 29:676-680, 1991.
58. Al-Mefty O: Comment on Kumar PP, Patil AA, Leibrock LG, et al: Brachytherapy: A viable alternative in the management of basal meningiomas. Neurosurgery 29:680, 1991.
59. Goodwin JW, Crowley J, Stafford B, et al: A phase II evaluation of tamoxifen in unresectable or refractory meningiomas: A Southwest Oncology Group study. J Neurooncol 15:75-77, 1993.
60. Grunberg SM, Weiss MH, Spitz IM, et al: Treatment of unresectable meningiomas with the antiprogesterone agent mifepristone. J Neurosurg 74:861-866, 1991.
61. Lamberts SWJ, Tanghe HLJ, Avezaat CJJ, et al: Mifepristone (RU 486) treatment of meningiomas. J Neurol Neurosurg Psychiatry 55:486-490, 1992.
62. Schrell UMH, Fahlbusch R, Adams EF, et al: Growth of cultured human cerebral meningiomas is inhibited by dopaminergic agents. Presence of high affinity dopamine-D1 receptors. J Clin Endocrinol Metab 71:1669-1671, 1990.
63. Todo T, Adams EF, Fahlbusch R: Inhibitory effect of trapidil on human meningioma cell proliferation via interruption of autocrine growth stimulation. J Neurosurg 78:463-469, 1993.