Meningiomas make up about one-fifth of all primary intracranial tumors. Meningiomas are usually slow growing. Approximately a quarter of patients with meningioma present with epilepsy. In the remainder the effects of the pressure of the growing tumor (headache, vomiting, etc.) often become evident first.
While medical writings from as early as the 18th century make reference to tumors arising from the meninges, it was not until 1922 that the great American neurosurgeon Harvey Cushing first introduced the term "meningioma." Cushing documented his work on these tumors in his classic book Meningiomas, first published in 1938. In this book, Cushing describes the development (histogenesis) of these tumors:
The tumors, as there is every reason to believe, arise from the cell clusters principally associated with the arachnoid villi which in later life, as Luschka first showed (1852), develop into Pacchionian granulations. The precise nature of these cells, however, remains in dispute.
Cushing then describes how he decided upon the term "meningioma" as an apt description for these tumors:
Their cellular composition being in dispute, a "histogenetic name" was likely to be misleading; a simple "place-name," comparable to acoustic tumor, was inadequate since the tumors were widely distributed and took their origin from the leptomeninges almost anywhere; a "tissue-name" was therefore sought.........the simple and non-committal designation Meningioma as a catchword was thought to be suitable and all-embracing.
Now, in the early part of the 21st century, it is incredible to review the writings of pioneer neurosurgeons and anatomists to see how much of what they wrote about meningiomas remains relevant today.
What Are Meningiomas? How Are They Diagnosed?
Simply, meningiomas are tumors that arise from the leptomeninges (tissue that serves as the lining of the brain). They develop from cells of arachnoid granulations, and they can occur throughout the supratentorial and infratentorial compartment. They are generally benign tumors, and only rarely do they undergo malignant transformation. Meningiomas account for approximately 15% of all brain tumors. Consistent with the benign nature of these tumors, meningiomas grow very slowly, often over many years. Because these tumors grow so slowly, they can sometimes become extremely large before being detected. This was especially true in past eras, before advanced imaging modalities were available (i.e. CT scans and MRI scans). Back then, meningiomas could only be diagnosed after they became large enough to result in severe symptoms, or when they emerged from the skull as visible protrusions. Today, with our multitude of MRI and CT scan machines, meningiomas are most often detected when they are relatively small. MRI scanning is currently the best diagnostic tool for detecting and defining meningiomas. CT scans with "bone windows" or using "3D reconstruction" technologies are also helpful in some cases. Occasionally, formal angiograms, to define blood supply and venous drainage, are useful as well.
Meningiomas are often diagnosed when a physician orders an MRI scan based on a patient's symptoms - such as seizures, focal weakness, sensory loss, or cranial nerve (including visual) disturbance. Also, many meningiomas are today discovered as "incidental" findings - when a patient is scanned for an unrelated complaint. It is not uncommon, for example, for a patient to be diagnosed with a meningioma after a CT scan has been performed in the setting of trauma, such as following a motor vehicle accident or other injury.
The "ideal treatment" for meningiomas continues to evolve as new technologies give rise to new treatment options. Often, there is more than one "right" way to treat a tumor, and it is not uncommon for various experts in the field of neurosurgery to have different opinions on how to best treat a particular meningioma. Open surgery has long been regarded as the standard treatment for large or symptomatic meningiomas. Refinement in microsurgical techniques and the introduction of computer-based localization and mapping during microsurgical resection have greatly enhanced the surgeon's ability to successfully resect these tumors. Many small tumors have traditionally been "treated" with simple observation and follow-up with serial MRI or CT scans.
More recently, interest has surged in the use of stereotactic radiosurgery to treat meningiomas. The last decade has seen exponential growth in the number of meningiomas (and other brain tumors) treated with radiosurgery. However, radiosurgery itself is not a new treatment. In an effort to provide patients with a safer, non-invasive method of "brain surgery," Lars Leksell, a Swedish neurosurgeon, developed early techniques of radiosurgery more than 50 years ago. In the past decade, high-tech computer software coupled with detailed MRI imaging has made the use of stereotactic radiosurgery more feasible in a wider variety of cases than ever before.
Radiosurgery refers to the use of a single fraction of high dose radiation to precisely treat a target within the brain. Treatment plans are generated using powerful computer software. No surgical incision is required, and therefore "normal" surgical risks such as hemorrhage, infection, CSF leak, etc., are avoided. When the target is a meningioma, for example, the radiation induces changes within the tumor's cells such that cellular reproduction is interfered with. Additionally, radiosurgery produces changes within the walls of blood vessels supplying the tumor, and therefore the tumor's blood supply may be decreased over time.
Most centers treat patients with either a modified linear accelerator system (LINAC) or with the Gamma Knife. The Gamma Knife is a unit with 201 fixed cobalt60 sources that precisely emits gamma rays onto the target. The precision of delivery with the Gamma Knife is less than 0.5 mm. A few centers also offer stereotactic charged-particle irradiation.
Indications for treatment of meningiomas with radiosurgery continue to evolve. Most experts agree that smaller tumors (less than 3.0 to 3.5 cm average diameters) respond best to treatment. Potential side effects of radiosurgery, such as edema or neurologic dysfunction, are also minimized with smaller treatment volumes. At The Ruth C. Heede Gamma Knife Center at Good Samaritan Medical Center in West Palm Beach , FL, our patients with meningiomas are considered for radiosurgery if their meningioma is small and not located too closely to radiosensitive structures, such as the optic apparatus. We feel that radiosurgery is the treatment of choice for small tumors involving the skull base (including those within the cavernous sinus) and those closely associated with the venous sinuses. Larger tumors and those that can be resected easily, completely and with low morbidity (such as convexity meningiomas) are generally considered for microsurgical resection. Of course, patient age, medical status and patient preference are important factors, which we consider in every case. Additionally, we continue to advocate "cautious observation" in elderly patients with asymptomatic tumors. At our Gamma Knife center, we generally treat meningiomas with a margin dose ranging from 11 to 16 Gray (Gy), at the 50% isodose line. In order to minimize the potential for radiation-induced optic neuritis, we try to keep the dose of radiation to the optic nerves and chiasm below 8 Gy.
Outcomes and Patient Satisfaction
Radiosurgery has proved to be a highly successful treatment for meningiomas. Thousands of patients have been treated worldwide. A recent published report by Kondziolka et. al. found that the clinical tumor control rate (no resection required) for meningiomas treated with radiosurgery was 93%. Interestingly, success rates appear to be lower for certain meningiomas (e.g. parasaggital) which have recurred or are residual after prior microsurgical resection. Patient satisfaction following radiosurgery is also extremely high. Patient satisfaction surveys used at our center and others reflect that the vast majority of patients view their Gamma Knife treatment positively. Patients appreciate the fact that radiosurgery is performed either on an outpatient basis, or as a 24-hour admission. Patients resume their normal activities within one to two days of treatment. One of our patients even changed a flat tire on his car on the way home from the hospital after his Gamma Knife treatment!
Follow-up care at our center consists of an office visit one week after the procedure. At four to six months following radiosurgery, the first follow-up MRI is performed. Generally, this reveals no significant change in the meningioma, although transient enlargement is occasionally seen. Following this, yearly MRI scans are performed for a minimum of five years, to assure tumor stability.
Meningiomas are generally benign, slowly growing tumors that carry an excellent prognosis for many patients. Both radiosurgery and microsurgery represent acceptable treatment options for patients with meningiomas who require intervention. In the past, microsurgery, with the goal of total tumor removal, had been the technique favored by most neurosurgeons. This continues to be true for many convexity meningiomas that are easily accessible and can be removed completely, with minimal morbidity. However, with the increasing availability of radiosurgery throughout the U.S. and the world, surgeons have had to rethink radical surgical removal of difficult meningiomas. Radiosurgery now has a proven track record for providing long term tumor control with low morbidity. Therefore, the question arises: with radiosurgery widely available as a treatment option, is aggressive open surgery for difficult meningiomas (such as skull base tumors) worth the risk? The explosive growth in the number of meningioma patients selecting radiosurgery in recent years attests to the fact that the goal of controlling meningioma growth while preserving neurologic function is very attractive to patients. The use of radiosurgery as the primary treatment of smaller meningiomas, and it's use in combination with surgical decompression or subtotal removal (with avoidance of neurologic deficits that could arise from more radical removal) in the treatment of larger tumors, is emerging as the standard of care at many centers.