Gamma Knife Radiosurgery for Patients with Foramen Magnum Meningiomas: Cohort Study

Background: Meningiomas are the most common tumors of the foramen magnum, but represent only 2 to 3% of all intracranial meningiomas. Standard management strategies include observation, resection, or stereotactic radiosurgery (SRS), with consideration for each depending on factors including size, anatomic configuration, patient age, medical comorbidities, and individual preferences. Surgical resection is typical challenging due to the intimate relationship between these tumors and surrounding neurovascular structures; correspondingly, SRS presents an attractive, safe, and effective strategy for appropriately selected patients including those without brainstem compression, or with recurrent/progressive tumors following resection. Given the relative rarity of these lesions, long-term clinical outcomes data after SRS are limited. We report our experience with a single-center cohort study of patients with foramen magnum meningiomas treated with primary SRS.

Methods: Cohort study of all foramen magnum meningiomas treated with SRS at our institution during the study period, 1990 to 2018. Histology- and imaging-based diagnoses were included, provided that they were the basis for treatment. Patients without 6 months of post-SRS imaging follow-up were excluded. Data abstraction included demographics, clinical presentation, treatment history, and dosimetry. The primary outcome was treatment failure (defined as radiographic progression after SRS based on post-SRS MRI), the secondary outcome was radiation-induced complications, and statistics were reported descriptively as counts with frequencies for categorical data, or medians with ranges for continuous data.

Results: Twenty-two patients were included, 16 of whom were female, with an overall median age at SRS of 59 years (range = 39–81). Incidental diagnoses accounted for 6 patients (27%), while the remaining 16 presented with cranial neuropathy (75%), imbalance (50%), craniocervical pain (43%), paresthesias (25%), or hydrocephalus (12%). Fourteen tumors were ventral (64%). A total of 12 patients underwent primary SRS (55%), while 10 were treated after subtotal resection (45%). Median pre-SRS volume was 6.3 cm³ (range = 0.8–19.9). Median SRS margin dose was 14.8 Gy (13–18); median maximal dose was 29.8 Gy (26–36). One individual was treated with volume-staged SRS for a larger tumor (16 Gy to 9 cm³ followed by 16 Gy to 10.4 cm³) in the setting of postoperative deficits. Median radiologic follow-up was 80 months (range = 6–178 months); median clinical follow-up was 80.7 months (range = 6–128 months). As of last follow-up, 7 tumors had regressed (32%), 14 were stable (64%), and 1demonstrated evidence of progression after SRS (4%)—an event that occurred in the only patient with a known grade II meningioma and which was treated with two courses of repeat SRS. RIC were limited to one instance of extremity numbness and hemifacial pain (4%) associated with medullary T2 changes on MRI; the patient improved clinically and radiographically in follow-up.

Conclusion: Foramen magnum meningiomas are rare, typically benign posterior fossa tumors. The current cohort study is small but extensively followed, and demonstrates that these lesions respond favorably to SRS in both the primary and adjuvant/salvage settings. Modest margin doses (13–18 Gy) appear to confer excellent tumor control, while also effectively minimizing the toxicity profile. Atypical lesions are predisposed to treatment failure and may require aggressive, multimodality treatment for achieve durable growth arrest.