Neurosurg Focus 14 (5):Article 6, 2003, Click here to return to Table of Contents Stereotactic radiosurgery for meningiomas LAWRENCE S. CHIN, M.D., NICHOLAS J. SZERLIP, M.D., AND WILLIAM F. REGINE, M.D. Departments of Neurosurgery and Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland Meningiomas are benign tumors attached to the dura that typically have a slow growth rate. After gliomas, they are the most common primary tumor of the brain. They are ideal radiobiological targets because single-fraction radiation has a high biologically effective dose. Furthermore, a highly conformal radiation plan can provide effective treatment to the tumor while sparing the surrounding brain. Meningioma control rates range from 90 to 95%, and the risk of mor- bidity is low. Radiosurgery is an excellent treatment for asymptomatic, small- to moderate-sized meningiomas. It is also ideal for patients with incompletely resected meningiomas, recurrent meningiomas, and risk factors precluding conventional surgery. KEY WORDS • meningioma • stereotactic radiosurgery First described by Cushing5 in 1922, meningiomas are ces, with homogeneous contrast enhancement. Migration benign tumors that arise from arachnoid cap cells and of meningioma cells from the tumor margin along the dura commonly attach to the dural coverings of the brain and appears as a tail of enhancement that is characteristic but spinal cord. Except for relatively rare instances of malig- not pathognomonic for the diagnosis. Hyperostosis or nancy, these tumors maintain a separate plane from the frank invasion of adjacent bone structures, particularly in pia, thus resulting in displacement rather than infiltration the sphenoid wing, is also seen. The degree of edema can of normal brain.12 Large meningiomas, however, can par- vary widely: small meningiomas are often associated with asitize the pial blood supply and even superficially invade an inordinate amount of cerebral edema, whereas larger the pia.9 Its most common locations are convexity, falx/ tumors may present with little to no associated edema. parasagittal, cavernous sinus, sphenoid wing, anterior skull Causes of edema are likely multifactorial and due to brain base, petroclival, tentorial, and foramen magnum. De- invasion, venous congestion, and release of vasoactive pending on its location, the blood is usually supplied by a peptides.24 Cerebral angiography can demonstrate arterial branch of the external carotid artery with some pial supply supply to the tumor and can also be used to reduce vascu- 34 as well. Strategically located meningiomas can invade or larity by administration of small particle or glue emboli- completely occlude a dural venous sinus. zation.18 Angiography is also useful for determining if a Meningiomas are the second most common primary venous sinus is patent, although MR imaging may provide brain tumor, accounting for 27% of the total, with an inci- similar information.20 dence of four per 100,000.1 Unlike most brain tumors, there is a female predominance. Their most common clin- ical presentations include headache, seizures, and focal SURGICAL TREATMENT neurological deficits specific to the lesion’s location. Giv- Excision of meningiomas continues to be the standard en their indolent growth, neurological symptoms are typi- therapy. Because meningiomas are benign tumors fre- cally insidious and progress slowly. The classic appearance of a meningioma on MR imag- quently with a defined border, complete resection theoret- ically accomplishes a cure of the disease. Furthermore, ing is an isointense mass on T1-weighted sequences and an isointense to hyperintense signal on T -weighted sequen- resection can reduce focal neurological deficits and pro- 2 vide relief from mass effect. The Simpson grade was de- signed to correlate completeness of resection with risk 29 Abbreviations used in this paper: BED = biological equivalent of tumor recurrence (Table 1). Even complete resection dose; GKS = gamma knife surgery; ICA = internal carotid artery; with a margin of uninvolved dura is associated with tumor LINAC = linear accelerator; MR = magnetic resonance; SRS = recurrence.2 In addition, complete excision carries poten- stereotactic radiosurgery. tial risks that may be significant depending on the location Neurosurg. Focus / Volume 14 / May, 2003 1 Unauthenticated | Downloaded 10/01/21 06:54 PM UTC L. S. Chin, N. J. Szerlip, and W. F. Regine TABLE 1 tumor and prescription volumes with dose–volume his- Summary of Simpson grades and definititons tograms (Fig. 1). Following the treatment, the patient is observed overnight for immediate side effects such as Grade Definition of Corresponding Resection seizures. We do not routinely treat patients with corticos- Imacroscopically complete resection w/ excision of dural teroid agents after radiosurgery, unless they are already re- attachment & abnormal bone ceiving this medication. Follow-up examination involves II macroscopically complete resection w/ coagulation of MR imaging studies obtained at 3 and 9 months after dural attachment treatment and then annually thereafter. Because of their III macroscopically complete resection w/o resection or indolent nature, meningioma regression is not expected in coagulation of its attachment IV subtotal resection the first few years of observation. V simple decompression of the tumor RADIOBIOLOGY AND DOSIMETRY A critical concept in understanding the radiobiology of of the tumor. Although complete resection of cavernous ␣ ␤ sinus meningiomas has been proposed, the debilitating SRS is the / ratio. Each type of tissue has a measurable ␣/␤ ratio that reflects the survival of cells in relation to the and disfiguring cranial neuropathies, as well as potential ␣ ␤ ischemic injury, make this option untenable.6,7,22 Foramen dose of radiation. This / ratio is the key to determining magnum and petroclival meningiomas are associated with the BED, which allows different radiation fractionation schemes to be compared. The following formula is used: risk of lower cranial nerve dysfunction and cerebrospinal ␣ ␤ fluid leakage; the tumors that invade or occlude the poste- BED = nd(1 + d/ / ), where n equals the number of frac- tions and d equals the dose per fraction. rior sagittal sinus and torcula carry unacceptable risks of ␣ ␤ venous infarction. The development of modern radio- Meningiomas have a low / that is characteristic of late-responding tissues such as benign tumors and normal surgical techniques has provided an adjunct, or in some ␣ ␤ cases, an alternative therapy to the conventional surgery. brain. A low / translates into a higher BED for any given single fraction of radiation.15 This means that be- nign tumors such as meningiomas can receive a lower dose of radiation, yet have the same biological effect as a STEREOTACTIC RADIOSURGERY higher single fraction targeting a malignant tumor. Frac- Stereotactic radiosurgery is defined as focused-beam tionation schemes are less important in meningiomas radiation delivered in one session to an intracranial tar- because both the brain and tumor have similar late-re- get and involving the use of spatially defined localization sponding properties, and therefore there is no biological and a rigid head fixation device. A modification of this difference in ␣/␤ to exploit. Additionally, the highly con- technique involves fractionated, smaller-dose radiation formal nature of radiosurgery in conjunction with the dis- delivered stereotactically and is called stereotactic ra- tinct tumor–brain border allows the high BED of a single diotherapy. Typically, stereotactic radiotherapy involves dose to be restricted to the tumor. relocatable, nonrigid skull fixation for its stereotactic lo- The ␣/␤ of different tissue types determines the optimal calization. As will be discussed in the proceeding section, dose range for each tumor type. Other important factors in the theoretical benefit of fractionation over SRS for treat- ment of meningiomas is minimal. Stereotactic radiosurgery is predominantly delivered using one of two types of devices: the gamma knife or modified LINAC. In GKS, 201, 60 Co sources are focused on the target by interchangeable helmets with 4-, 8-, 14-, or 18-mm collimators. The treatment is created by the additive effect of multiple isocenters of radiation that re- sults in a high dose of radiation delivered in a highly con- formal manner. Similarly, LINAC can produce the same effect by moving a single beam of radiation in arcs around a patient’s head. The size of the beam can also be colli- mated, thus improving conformity of the plan. In GKS, a Leksell G stereotactic coordinate frame is placed after administration of a local anesthetic agent. Given the spatial constraints of the Leksell frame and the gamma knife helmet, it is critical to center the tumor in the frame; for parasagittal tumors, the base of the frame will need to be at the level of the orbits. A contrast-enhanced MR imaging study is then performed, or computerized tomography scanning in patients in whom MR imaging is Fig. 1. Treatment plan for a sphenoid wing meningioma. The contraindicated due to the existence of a device such as a contrast-enhanced axial MR image demonstrates a sphenoid wing cardiac pacemaker. Treatment planning is performed us- meningioma (red outline) with superimposed 30 and 50% isodose ing the interactive computer program that also provides lines. The tumor has infiltrated into the overlying bone flap, which important information such as isodose curves, as well as has