Recurrence of meningiomas after seemingly complete resection, as well as progression after subtotal resection, has been studied for many decades. In a seminal paper by Simpson [19] in 1957, the rate of recurrence was stratified according to the extent of tumor resection and removal or coagulation of the associated dura: grade I - complete surgical resection of the tumor and its dural attachment; grade II - complete surgical resection of the tumor with coagulation of its dural attachment; grade III -complete surgical resection of the tumor without coagulation of its dural attachment; grade IV - partial tumor removal, leaving the dura in situ; and grade V - simple decompression with or without biopsy. In Simpson's series of 265 meningiomas, 55 (21%) had recurrence. The rates of tumor recurrence according to the extent of resection were: grade I, 9%; grade II, 19%; grade III, 29%; grade IV, 44%. Other studies appearing subsequently, with at least 10 years' follow-up, found similar recurrence rates, which depended on the extent of resection. Using Simpson's classification, Melamed et al. [20] reported recurrence rates of: grade I, 8%; grade II, 15%; grade IV, 29%; and grade V, 33%. Similarly, Chan et al. [21] reported recurrence rates as: grade I, 11%; grade II, 22%; grade IV, 33%; and grade V, 100%. Both of these studies confirmed Simpson's initial finding that the extent of meningioma resection is the single most important prognostic factor for tumor recurrence in benign meningiomas [20,21].

In a study of 225 patients, Mirimanoff et al. [22] reported a recurrence-free rate after total resection of 93%, 80% and 68% at 5, 10 and 15 years, respectively. After subtotal resection, however, the progression-free rates were 63%, 45% and 9% at the same time intervals, respectively. They also evaluated tumor location as a factor for recurrence and found that 96% of their convexity meningiomas were totally resected, with a 3% recurrence rate at 5 years. Fifty-eight percent of the parasellar menin-giomas in their study were totally excised, with a 5-year recurrence rate of 19%, while only 28% of the sphenoid ridge meningiomas were totally excised, with a 5-year recurrence rate of 34% [22]. Kallio et al. [23] analyzed the surgical outcome of their series of 935 patients in terms of relative survival rate (RSR) following resection. RSR was defined as the ratio of the observed to the expected survival rate (SR). The expected SR was considered to be that for a population identical to the patient group, except for the meningioma. They found no difference between the observed and expected SR of their patients following resection at 3 months (91%) and at 1 year (89%). At 15 years' follow-up, they found a cumulative observed SR of 63%, which was 78% of the expected SR. The RSR was largely dependent on the degree of resection: the RSR at 15 years was 84% following a complete resection (Simpson grades I & II) and 50% following an incomplete resection (Simpson grades III-V) [23]. Overall, several studies have reached the similar conclusions that surgical outcome and recurrence of meningiomas are dependent on the degree of resection. The degree of resection is, in turn, dependent on the tumor location and, therefore, the accessibility of the tumor and the involved dura and bone [22].

Jaaskelainen observed that the following three factors were significantly associated with meningioma recurrence: (1) coagulation of the involved dura instead of removal, (2) attachment of the tumor to bone, and (3) the soft consistency of the tumor. If none of these criteria were present, the recurrence rate at 20 years was 11%, while if one or two of these criteria were present, the recurrence rates were 15-24% and 34-56%, respectively. Although the histological criteria for grading meningiomas have evolved in recent years, Jaaskelainen et al. [24] reported in 1986 that the 5-year recurrence rates following complete resection were 3% for benign, 38% for atypical, and 78% for anaplastic menin-giomas. Pathological features associated with a significantly higher rate of recurrence include histological anaplasia, (20 mitoses per 10 high-powered fields, nuclear atypia, and papillary or

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