Radiosurgery Case Reviews
A 45 year-old-woman in excellent health noted a small mass in her upper right eyelid. Workup by her ophthalmologist included a CT scan of the orbits which revealed a 1.8 cm homogeneous area of enhancement in the region of the cribiform plate. This was consistent with a meningioma of the olfactory groove. The patient was neurologically intact, including her sense of smell, and her eyelid mass was found to be benign.
Having a small, incidentally discovered meningioma, the patient had several treatment options. These included observation and repeat CT scan, craniotomy and resection of the tumor, or radiosurgery. The patient chose radiosurgery, as it offered the possibility of preserved olfaction along with a high likelihood of tumor control.
As is routine for adults undergoing radiosurgery, the patient was treated as an outpatient under local anesthesia. An oral sedative was given and the stereotactic head ring applied. In this case, a CT scan, the mostspatially accurate imaging modality, was sufficient to image the lesion and important structures. After the scan, the patient relaxed in the Radiosurgery Suite while treatment planning was carried out.
Structures contoured on the X-Knife system included the tumor, optic apparatus (eyes, nerves, and chiasm),and the brainstem. The final plan involved the use of one isocenter, with five sagittally oriented arcs. The volume dose image shows the 85% isodose volume, at which 2000 centiGray (cGy) was prescribed. Other algorithms used to confirm the efficacy and safety of the prescribed dose included the surface dose summary and dose volume-histogram.
To check the radiation dose prescription on the actual CT image, and not just the 3-dimensional rendering, a slice wash was generated, confirming that the tumor will receive the desired high dose, with minimal radiation to the eyes, optic nerves, and optic chiasm. The patient tolerated the treatment easily, and was discharged home shortly afterwards. An MRI scan will be done 6 months aftertreatment.
Discussion
Meningiomas are neoplasms that arise from arachnoidal cell rests in the dura mater. The great majority are benign and relatively slow-growing; they tend to occur in women of middle age or older. As with other brain tumors, symptoms depend on the location and size of the lesion. Certain characteristics make patients with meningiomas ideal candidates for radiosurgery:
• the patients are often elderly, making craniotomy more hazardous
While surgical resection remains the treatment of choice for most patients with newly diagnosed meningiomas, other factors may be considered. Even “completely resected” tumors have a risk of recurrence, especially those arising from the base of the skull. Tumor involvement of cranial nerves or major vascular structures often limits the degree of tumor resection. Had our patient undergone surgical removal of her olfactory groove meningiomas he would have sustained permanent loss of her sense of smell. Radiosurgery at the prescribed dose should leave olfaction intact; published series suggest that the rate of tumor control is over 95%, similar to that in the best results achieved with craniotomy. In addition, the patient avoided the risks (however low) of general anesthesia, infection, CSF leak, and hemorrhage that are associated with craniotomy. Fractionated radiation therapy can be useful as an adjunct to incomplete surgical removal, of meningiomas, but is rarely, if ever, an appropriate choice as primary treatment.
Patients with meningiomas that are large (diameter over 4 cm) or too close to the optic chiasm (5 mm or less) are not candidates for radiosurgery. In these cases, craniotomy should be considered first, as is true for patients who can undergo resection without anticipated morbidity. For this reason, a multidisciplinary approach is required for patients presenting with meningiomas.
Acoustic Neuroma
A 56-year-old man presented with progressive hearing loss and vertigo. The latter symptom was so severe that he had to stop work as a contractor. Neurological examination revealed right-sided sensorineural hearing loss and decreased ability to tandem walk. MRI scan showed an enhancing lesion in the right auditory canal, consistent with an acoustic neuroma (also referred to as a vestibular schwannoma). Brainstem auditory evoked responses and audiography confirmed sensorineural hearing loss in the right ear, with moderate loss (about 30%)of speech discrimination. The patient’s medical history was notable for significant cardiac and pulmonary disease, with an angioplasty having been done within the last12 months, and decreased respiratory reserve due toa past history of smoking. He was moderately overweight.
Due to the medical risks of surgery, a conservative approach was taken at first. However, a followup MRI6 months after diagnosis showed increasing enhancement of the tumor . The patient was referred for stereotactic radiosurgery(SRS) as definitive treatment of the acoustic neuroma.
Procedure
A contrast-enhanced stereotactic MRI scan was done a week before radiosurgery. On the day of treatment, the Radionics head ring was applied, making sure that the skull base would be visualized. A stereotactic CTscan was then performed. All data was downloaded to the treatment planning computer, and the MRI and CT scans were fused. A radiosurgical plan was generate dusing the fused image.
Treatment was carried out using a single isocenter;1800 centiGray were prescribed to the 80% isodose line. After 6 months the patient is clinically stable and has suffered no further hearing loss; followup MRI showed tumor shrinkage.
Discussion
Patients with acoustic neuromas are in some ways ideal candidates for radiosurgery. The radiobiological advantage of single-dose irradiation is greater, in general, for histologically benign lesions such as these. SRS has been used in acoustic management for nearly 30 years; thus, there is long-term follow up available on relatively large groups of patients.
Data from series using linear accelerator or gamma knife SRS indicate that tumor control (i.e. lack of growth or shrinkage) can be expected in nearly 95% of cases. Earlier results included significant rates of hearing loss and facial palsy (as high as 80% and 50%respectively, with 25% sustaining lasting facial nerve injury). These complications were mostly sustained using treatment doses over 2000 cGy; more recent work with lower doses has greatly improved the rate of hearing preservation, minimized the chance of facial palsy, and maintained excellent tumor control. Some clinicians have recommended using fractionated SRS as a means of further decreasing complications. However, this method has been associated with sudden hearing loss, and long-term data on tumor control are not yet available.
At New Jersey Medical School, all patients with acoustic neuromas are seen by a neurosurgeon and a neuro-otologist. Microsurgical resection is recommended for young patients in good general health. Radiosurgery is offered to older patients, those with medical problems rendering surgery with general anesthesia hazardous, and those who refuse microsurgery but desire treatment. Single-dose SRS is recommended for this group of patients with tumors 2.5cm in diameter or less; those with larger tumors undergo fractionated SRS.
Oligoastrocytoma
A 58-year-old right-handed woman had the slow onset of gait difficulty and right upper extremity weakness. On examination she had a 4+/5 right hemiparesis with slight hyperreflexia on that side. MRI scan revealed a fairly well circumscribed, moderately enhancing mass in the left hemisphere, deep to the motor cortex. Stereotactic biopsy was performed under local anesthesia, and the diagnosis of oligoastrocytoma grade2 was made.
As surgical resection of the tumor was not safely possible, radiosurgery was performed as the next step. A stereotactic CT scan was done and the lesion contoured. Treatment was carried out using five arcs; 1500centiGray (cGy) were prescribed to the 80% isodose line at the tumor boundary. The patient tolerated the radiosurgical treatment well. Two weeks later, a course of fractionated radiation therapy was begun; 5400 cGy were administered in 30fractions to the tumor volume plus a margin of 3 cm. Six months later, the patient has normal strength and her gait has improved. MRI showed the area of enhancement to be nearly gone.
Another patient, a 30-year-old man with renal failure due to Type I diabetes mellitus, sustained a generalized seizure. He had a left homonymous hemianopsia on examination. CT scan showed a mass in the right parietooccipital area, consistent with an oligoastrocytoma; this was confirmed on biopsy (grade 2). He too underwent stereotactic radiosurgery, receiving a prescription dose of 1750 cGyto the 80% isodose line. Fractionated radiation therapy was given asin the previous patient. One year later his visual fields are improved and CT revealed the tumor to be smaller, hypodense, and partially calcified.
Discussion
Oligoastrocytomas are rare mixed gliomas that are found most commonly in patients between 30 and 40 years old and are most often low-grade or “benign”. Grading, on a scale of 1 to 4, is based on the predominant cell type (usually astrocytic), and the prognosis variesaccordingly. When possible, surgical resection is the initial treatment of choice. Fractionated radiation therapy (RT) is prescribed for patients in whom complete tumor removal is impossible; patients with totally resected low-grade oligoastrocytomas may be observed with serial CT or MR scans before beginning RT. There is no proven role for the use of chemotherapy in these patients .A 10-year survival rate of 40% or better can be expected in patients with low-grade tumors.
Stereotactic radiosurgery (SRS) has been found to achieve at least a partial response to treatment in patients with low-grade astrocytomas. Radiobiological considerations favor the use of SRS for relatively benign lesions, in particular. In our patients, the critical tumor location in eloquent brain and, in patient 2, concurrent medical problems ruled out the possibility of surgical resection. RT alone was a treatment option, but we chose to employ radiosurgery as a substitute for resection; at the same time, SRS alone would not be expected to yield adequate tumor control without the use of RT as well. An excellent clinical and CT/MR response has been demonstrated.
In selected patients with primary brain tumors, SRS can be used in lieu of open surgery with equivalent results.
Sphenoid Wing Meningioma
A 60-year-old woman sustained a generalized seizure. Upon awakening, she was neurologically intact. Her past medical history was notable for mild hyperthyroidism; she had a slight proptosis of the right eye, deemed to be caused by Grave’s disease. An MRI scan revealed an enhancing lesion, 4 cm in its maximum diameter, involving the anterior right temporal fossa and cavernous sinus, consistent with a sphenoid wing meningioma. The patient was referred for stereotactic radiosurgery.
The large size of the tumor ruled out radio surgery as a primary option. Fractionated radiation therapy, with or without stereotactic localization, would not be generally accepted treatment in this patient with a benign tumor. Observation with followup imaging was a possibility, as the patient had no obvious symptoms from the tumor other than a seizure, which could be treated medically. Given her relatively young age and good general medical condition, surgery was offered as the main modality. However, due to involvement of the cavernous sinus, a curative resection would not be possible without injury to cranial nerves and a resulting ophthalmoplegia (injury to the internal carotid artery would also be a significant risk). Therefore, a plan was made for surgical removal of the bulk of the tumor followed by radiosurgery to the small remnant in the cavernous sinus.
A right pterional craniotomy was performed. Using microsurgical technique, the tumor was dissected off the right middle cerebral artery, and the large mass outside of the cavernous sinus was removed.
Neuropathological examination confirmed that this was a benign meningioma. Postoperative MRI scan showed, as expected, the residual mass in the cavernous sinus, along with expected postoperative changes. The patient remained neurologically intact, and was discharged several days after surgery.
Eight weeks later, the patient underwent stereo tactic radiosurgery as an outpatient. Imaging was done using both MRI and CT to optimize localization of the tumor and the optic chiasm. The tumor border was no less than 5 mm from the optic chiasm. Treatment was performed using a single isocenter, with 1500 centiGray (cGy) prescribed to the85% isodose line. A small portion of the right optic chiasm received700 cGy. An MRI scan 6 months later showed decreased size of the tumor in the cavernous sinus. The patient remains neurologically intact.
Discussion
Over the last 15 years, advances in skull-base surgery have encouraged neurosurgeons to be more aggressive in the removal of tumors involving the cavernous sinus. However, radical surgery in this area usually results in new cranial neuropathy, including diplopia . As meningiomas of the cavernous sinus have been demonstrated to infiltrate the cranial nerves themselves, a surgical cure often means the creation of a new and potentially disabling neurological deficit.
Stereotactic radiosurgery theoretically has the most benefit in the treatment of histologically benign lesions, and various published series have demonstrated a 5-year control rate of over 95% for meningiomas. However, this treatment is generally limited to patients with tumors 3 cm or less in maximum diameter, especially if the tumors lie close to the optic apparatus. This is simply a specific application of the principle that the complications of radiosurgery are a function of lesion size, location, and the prescribed dose.
To avoid radiosurgical injury to the optic chiasm, a lesion should be no closer than 5 mm to that structure, nor should the chiasmal dose exceed 750 cGy; this would not have been possible had the patient undergone radiosurgery without prior surgical removal of the majority of the tumor. Primary radiosurgical treatment would have mandated either an ineffective low prescription dose, or a higher one with attendant risks to the optic chiasm and brain. This case history illustrates the benefits of the multidisciplinary treatment of patients with skull-base tumors, and the attendant excellent clinical result.