Meningiomas are the most common lesions located in the foramen magnum region, and they account for 60–75% of tumors in this location.[2
] Most lesions are in the anterior or anterolateral intradural space, but they may arise posterolaterally as well. An en plaque variety with extradural extension also exists.[7
] These are slow growing lesions that commonly cause significant distortion of the brainstem or spinal cord and often encase key vascular structures and cranial nerves by the time of presentation. For this reason, the majority of patients have multiple symptoms and/or deficits on presentation. The classic foramen magnum syndrome describes motor and sensory deficits developing first in one arm with progression to the ipsilateral leg prior to involvement of the contralateral leg before completing the progression in the contralateral arm. Although this clockwise symptomatology is well associated with these lesions, common presentations simply involve occipital headaches and neck pain with a predominance of cerebellar and spinal cord dysfunction.[1
] In our series, headache or headache and cervical pain, dizziness or imbalance, and alterations in sensation were the most common presenting symptoms.
As a result of the complex nature and difficult location of these lesions, many surgical approaches have been used to resect these lesions, including anterior–lateral transcervical, transoral, posterolateral (retrocondylar), and far lateral (transcondylar).[1
] The first studies concerning surgical treatment of foramen magnum meningiomas often showed disappointing results with mortality rates of 19%[30
] and 45%.[31
] With the advent of microsurgery, mortality rates decreased, but mortality rates of 10–30% were not uncommon.[9
] With refinements in surgical techniques, patient selection, more conservative operative plans and approaches, and adjuvant radiosurgery, surgical series have reported surgical mortality rates of 0–10%.[2
] Nevertheless, these patients often experience a significant morbidity due to cranial nerve injury and damage to critical vasculature, brain stem, and spinal cord. The resulting deficits can be severe and complications may include hypertension, respiratory depression, aspiration, pneumonia, and mediastinitis among others.[1
Similarly, reported complete resection rates of tumors of foramen magnum (Simpson grade 1–2) range from 40% to 96% and are dependent on many characteristics of the tumor. In one series, intradural-based lesions could be resected in 94% of the cases, whereas tumors with extradural components could be resected in only 50% of the cases. Recurrence rates range from 12% to 91% and depend on a number of variables, including tumor characteristics, treatment paradigms, and follow-up.[1
The high recurrence rates of lesions in this location foretell a less than favorable prognosis. In the series by Stein et al
., 20% of the patients died because of problems related to tumor recurrences.[33
] In the series by Meyer et al
., 5% of the patients died subsequent to tumor recurrence within 3 years of surgery.[27
] In the majority of reports, re-operations for recurrent tumors could only be subtotal and were related to a significant postoperative neurologic compromise.[2
] Tumors with adherence to the brainstem, encasement of the vertebral arteries and/or cranial nerves, en plaque growth pattern, high tumor grade, high mitotic activity/Ki-67 labeling, and loss of 1p36.1–p34 have been associated with an increased rate of incomplete resection and/or tumor recurrence.[34
In a significant number of cases, complete surgical resection is not possible,[34
] and the best follow-up treatment remains unclear. Previous authors have recommended radical resection in recurrent meningiomas of the foramen magnum to prolong life in neurologically stable patients.[6
] Others argue that radiosurgery may be used as an adjunctive treatment after subtotal tumor resection or to arrest the progression of recurrent tumors.[14
] In these complex lesions, the treatment plan is often to achieve the most aggressive surgical resection with the goal of preserving full function using radiosurgery for residual tumors.[14
Although there have been more than 40 published articles concerning the operative outcomes of meningiomas of the foramen magnum, there is a paucity of literature concerning radiosurgery of lesions extending into the foramen magnum. Additionally, the majority of series note the use of radiosurgery of these lesions without having evidence or sources to define the outcomes and complications of radiosurgery of meningiomas of the foramen magnum.
Nicolato et al
. reported the use of radiosurgery in the treatment of 62 meningiomas of the posterior fossa.[40
] Only 1 lesion involved the foramen magnum. During a follow-up of 6–64.3 months (median, 28.7 months), neuroimaging evaluation documented the disappearance or reduction of the meningioma mass in 34/62 (55%) cases, a stable imaging in 25/62 (40%), and progression in 3/62 (5%). Two patients died from tumor progression, and 6.5% of the patients experienced transient complications due to post-radiosurgical edema.
We believe that radiosurgery is an alternative treatment in patients with advanced age, high operative risks, and in those who refuse surgery or have residual or recurrent tumors. In a select group of patients with small lesions in which imaging is diagnostic of meningiomas, radiation may be used as a primary treatment option. In our series of 5 patients, 4 had a decrease or no increase in the size of their lesions (follow-up range, 4–13 years). The fifth patient achieved had imaging confirming tumor progression. The patient achieved stabilization aftera second treatment with GKRS. One patient experienced intermittent seizures during the following microsurgery and GKRS that were medically controlled and was without seizures in the last 6 years of follow-up. No other patient experienced symptoms or complications related to radiosurgery.
To our knowledge, there is only one other similar report of foramen magnum meningiomas treated with radiosurgery.[41
] Muthukumar et al
. described the treatment of 3 patients with recurrent tumor progression after surgery (n = 3), and 2 patients who refused surgical treatment or were poor surgical candidates. During the follow-up interval of 1-5 years (median 3 years), 1 patient died of a concurrent illness and all others were stable without deterioration of their clinical condition. One patient exhibited a decrease in lesion volume and 4 patients had no change on follow-up imaging.
We know of no reported radiation-induced toxicity to the brainstem or spinal cord reported in the literature, but radiosurgery does have inherent risks. In 1990, Engenhart et al
. reported the results of linear accelerator-based radiosurgery in a series of 17 meningiomas.[42
] The tumors ranged from 10 to 54 mm in diameter and received single-fraction doses from 10 to 50 Gy. One patient died from brain herniation after radiosurgery (attributed to a shunt malfunction) and another patient died from radiation necrosis and herniation after receiving 35 Gy to a large treatment volume. Five patients developed a large volume of brain edema and brain necrosis was suspected in 3 of them. These complications may be attributed to the use of high dose radiosurgery as the primary modality of treatment of large lesions. We limit our radiosurgical treatment to lesions with a diameter of less than 3.5 cm and use multiple isocenters and three-dimensional image planning.
Limitations of the use of GKRS for foramen magnum meningiomas include difficulties with targeting below the first cervical vertebra. However, this is largely obviated with the current Perfexion version of the Gamma Knife. Additional refinements are necessary to address the cervical spine mobility and target planning in the treatment of lesions below this level. Further limitations of treatment include the necessity of long-term follow-up as the exact rates of recurrence and natural disease progression are not clearly defined.