Radiosurgery Case Reviews
A 45 year-old-woman in excellent health noted a smallmass in her upper right eyelid. Workup by her ophthalmologistincluded a CT scan of the orbits (Figure1), which revealed a 1.8 cm homogeneous area ofenhancement in the region of the cribiform plate. Thiswas consistent with a meningioma of the olfactory groove.The patient was neurologically intact, including hersense of smell, and her eyelid mass was found to bebenign.
Having a small, incidentally discovered meningioma,the patient had several treatment options. These includedobservation and repeat CT scan, craniotomy andresection of the tumor, or radiosurgery. The patientchose radiosurgery, as it offered the possibility ofpreserved olfaction along with a high likelihood oftumor control.
As is routine for adults undergoing radiosurgery,the patient was treated as an outpatient under localanesthesia. An oral sedative was given and the stereotactichead ring applied. In this case, a CT scan, the mostspatially accurate imaging modality, was sufficientto image the lesion and important structures. Afterthe scan, the patient relaxed in the Radiosurgery Suitewhile treatment planning was carried out.
Structures contoured on the X-Knife system includedthe tumor, optic apparatus (eyes, nerves, and chiasm),and the brainstem (Figure2 ). The final plan involved the use of one isocenter,with five sagittally oriented arcs (Figure3 ). The volume dose image shows the 85% isodosevolume (Figure4 ), at which 2000 centiGray (cGy) was prescribed.Other algorithms used to confirm the efficacy and safetyof the prescribed dose included the surface dose summary(Figure5) and dose volume-histogram (Figure6 ).
To check the radiation dose prescription on the actualCT image, and not just the 3-dimensional rendering,a slice wash was generated (Figure7 ), confirming that the tumor will receive thedesired high dose, with minimal radiation to the eyes,optic nerves, and optic chiasm. The patient toleratedthe treatment easily, and was discharged home shortlyafterwards. An MRI scan will be done 6 months aftertreatment.
Meningiomas are neoplasms that arise fromarachnoidal cell rests in the dura mater. The greatmajority are benign and relatively slow-growing; theytend to occur in women of middle age or older. As withother brain tumors, symptoms depend on the locationand size of the lesion. Certain characteristics makepatients with meningiomas ideal candidates for radiosurgery:
they are usually well-demarcated, rarelyinvading the brain
they are seen easily on CT and/or MRI scans
the patients are often elderly, making craniotomymore hazardous
While surgical resection remains the treatmentof choice for most patients with newly diagnosed meningiomas,other factors may be considered. Even "completely resected"tumors have a risk of recurrence, especially those arisingfrom the base of the skull. Tumor involvement of cranialnerves or major vascular structures often limits thedegree of tumor resection. Had our patient undergonesurgical removal of her olfactory groove meningiomashe would have sustained permanent loss of her senseof smell. Radiosurgery at the prescribed dose shouldleave olfaction intact; published series suggest thatthe rate of tumor control is over 95%, similar to thatin the best results achieved with craniotomy. In addition,the patient avoided the risks (however low) of generalanesthesia, infection, CSF leak, and hemorrhage thatare associated with craniotomy. Fractionated radiationtherapy can be useful as an adjunct to incomplete surgicalremoval, of meningiomas, but is rarely, if ever, anappropriate choice as primary treatment.
Patients with meningiomas that are large (diameterover 4 cm) or too close to the optic chiasm (5 mm orless) are not candidates for radiosurgery. In thesecases, craniotomy should be considered first, as istrue for patients who can undergo resection withoutanticipated morbidity. For this reason, a multidisciplinary approachis required for patients presenting with meningiomas.
A 56-year-old man presented with progressive hearingloss and vertigo. The latter symptom was so severe thathe had to stop work as a contractor. Neurological examinationrevealed right-sided sensorineural hearing loss anddecreased ability to tandem walk. MRI scan (Figure1) showed an enhancing lesion in the right auditorycanal, consistent with an acoustic neuroma (also referredto as a vestibular schwannoma). Brainstem auditory evokedresponses and audiography confirmed sensorineural hearingloss in the right ear, with moderate loss (about 30%)of speech discrimination. The patient's medical historywas 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 conservativeapproach was taken at first. However, a followup MRI6 months after diagnosis showed increasing enhancementof the tumor (Figure2 ). The patient was referred for stereotactic radiosurgery(SRS) as definitive treatment of the acoustic neuroma.
A contrast-enhanced stereotactic MRI scan was donea week before radiosurgery. On the day of treatment,the Radionics head ring was applied, making sure thatthe skull base would be visualized. A stereotactic CTscan was then performed. All data was downloaded tothe treatment planning computer, and the MRI and CTscans were fused. A radiosurgical plan was generatedusing the fused image.
Treatment was carried out using a single isocenter;1800 centiGray were prescribed to the 80% isodose line(Figure3a , Figure3b , Figure4 ). After 6 months the patient is clinically stableand has suffered no further hearing loss; followup MRIshowed tumor shrinkage (Figure5 ).
Patients with acoustic neuromas are in some ways idealcandidates for radiosurgery. The radiobiological advantageof single-dose irradiation is greater, in general, forhistologically benign lesions such as these. SRS hasbeen used in acoustic management for nearly 30 years;thus, there is long-term followup available on relativelylarge groups of patients.
Data from series using linear accelerator or gammaknife SRS indicate that tumor control (i.e. lack ofgrowth or shrinkage) can be expected in nearly 95% ofcases. Earlier results included significant rates ofhearing loss and facial palsy (as high as 80% and 50%respectively, with 25% sustaining lasting facial nerveinjury). These complications were mostly sustained usingtreatment doses over 2000 cGy; more recent work withlower doses has greatly improved the rate of hearingpreservation, minimized the chance of facial palsy,and maintained excellent tumor control. Some clinicianshave recommended using fractionated SRS as a means offurther decreasing complications. However, this methodhas been associated with sudden hearing loss, and long-termdata on tumor control are not yet available.
At New Jersey Medical School, all patients with acousticneuromas are seen by a neurosurgeon and a neuro-otologist.Microsurgical resection is recommended for young patientsin good general health. Radiosurgery is offered to olderpatients, those with medical problems rendering surgerywith general anesthesia hazardous, and those who refusemicrosurgery but desire treatment. Single-dose SRS isrecommended for this group of patients with tumors 2.5cm in diameter or less; those with larger tumors undergofractionated SRS.
A 58-year-old right-handed woman had the slow onsetof gait difficulty and right upper extremity weakness.On examination she had a 4+/5 right hemiparesis withslight hyperreflexia on that side. MRI scan revealeda fairly well circumscribed, moderately enhancing massin the left hemisphere, deep to the motor cortex (Figure1). Stereotactic biopsy was performed under localanesthesia, and the diagnosis of oligoastrocytoma grade2 was made (Figure2).
As surgical resection of the tumor was not safelypossible, radiosurgery was performed as the next step.A stereotactic CT scan was done and the lesion contoured(Figure3). Treatment was carried out using five arcs; 1500centiGray (cGy) were prescribed to the 80% isodose lineat the tumor boundary (Figure4). The patient tolerated the radiosurgical treatmentwell. Two weeks later, a course of fractionated radiationtherapy 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 andher gait has improved. MRI showed the area of enhancementto be nearly gone (Figure5).
Another patient, a 30-year-old man with renal failuredue to Type I diabetes mellitus, sustained a generalizedseizure. He had a left homonymous hemianopsia on examination.CT scan showed a mass in the right parietooccipitalarea (Figure6), consistent with an oligoastrocytoma; this wasconfirmed on biopsy (grade 2). He too underwent stereotacticradiosurgery, receiving a presciption dose of 1750 cGyto the 80% isodose line (Figure7). Fractionated radiation therapy was given asin the previous patient. One year later his visual fieldsare improved and CT revealed the tumor to be smaller,hypodense, and partially calcified (Figure8).
Oligoastrocytomas are rare mixed gliomas that arefound most commonly in patients between 30 and 40 yearsold and are most often low-grade or "benign".Grading, on a scale of 1 to 4, is based on the predominantcell type (usually astrocytic), and the prognosis variesaccordingly. When possible, surgical resection is theinitial treatment of choice. Fractionated radiationtherapy (RT) is prescribed for patients in whom completetumor removal is impossible; patients with totally resectedlow-grade oligoastrocytomas may be observed with serialCT or MR scans before beginning RT. There is no provenrole for the use of chemotherapy in these patients.A 10-year survival rate of 40% or better can be expectedin patients with low-grade tumors.
Stereotactic radiosurgery (SRS) has been found to achieveat least a partial response to treatment in patientswith low-grade astrocytomas. Radiobiological considerationsfavor the use of SRS for relatively benign lesions,in particular. In our patients, the critical tumor locationin eloquent brain and, in patient 2, concurrent medicalproblems ruled out the possibility of surgical resection.RT alone was a treatment option, but we chose to employradiosurgery as a substitute for resection; at the sametime, SRS alone would not be expected to yield adequatetumor control without the use of RT as well. An excellentclinical and CT/MR response has been demonstrated.
In selected patients with primary brain tumors, SRS can be used in lieu of open surgery with equivalentresults.
Sphenoid Wing Meningioma
A 60-year-old woman sustained a generalized seizure.Upon awakening, she was neurologically intact. Her pastmedical history was notable for mild hyperthyroidism;she had a slight proptosis of the right eye, deemedto be caused by Grave's disease. An MRI scan revealedan enhancing lesion, 4 cm in its maximum diameter, involvingthe anterior right temporal fossa and cavernous sinus(Figure1), consistent with a sphenoid wing meningioma.The patient was referred for stereotactic radiosurgery.
The large size of the tumor ruled out radiosurgeryas a primary option. Fractionated radiation therapy,with or without stereotactic localization, would notbe generally accepted treatment in this patient witha benign tumor. Observation with followup imaging wasa possibility, as the patient had no obvious symptomsfrom the tumor other than a seizure, which could betreated medically. Given her relatively young age andgood general medical condition, surgery was offeredas the main modality. However, due to involvement ofthe cavernous sinus, a curative resection would notbe possible without injury to cranial nerves and a resultingophthalmoplegia (injury to the internal carotid arterywould also be a significant risk). Therefore, a planwas made for surgical removal of the bulk of the tumorfollowed by radiosurgery to the small remnant in thecavernous sinus.
A right pterional craniotomy was performed. Usingmicrosurgical technique, the tumor was dissected offthe right middle cerebral artery, and the large massoutside of the cavernous sinus was removed (Figure2a & 2b).
Neuropathological examination confirmed that this wasa benign meningioma. Postoperative MRI scan showed,as expected, the residual mass in the cavernous sinus,along with expected postoperative changes (Figure3). The patient remained neurologically intact,and was discharged several days after surgery.
Eight weeks later, the patient underwent stereotacticradiosurgery as an outpatient. Imaging was done usingboth MRI and CT to optimize localization of the tumorand the optic chiasm (Figure4 and Figure5). The tumor border was no less than 5 mm fromthe optic chiasm. Treatment was performed using a singleisocenter, with 1500 centiGray (cGy) prescribed to the85% isodose line (Figure6). A small portion of the right optic chiasm received700 cGy. An MRI scan 6 months later showed decreasedsize of the tumor in the cavernous sinus (Figure7). The patient remains neurologically intact.
Over the last 15 years, advances in skull-basesurgery have encouraged neurosurgeons to be more aggressivein the removal of tumors involving the cavernous sinus.However, radical surgery in this area usually resultsin new cranial neuropathy, including diplopia . As meningiomasof the cavernous sinus have been demonstrated to infiltratethe cranial nerves themselves, a surgical cure oftenmeans the creation of a new and potentially disablingneurological deficit.
Stereotactic radiosurgery theoretically has themost benefit in the treatment of histologically benignlesions, and various published series have demonstrateda 5-year control rate of over 95% for meningiomas. However,this treatment is generally limited to patients withtumors 3 cm or less in maximum diameter, especiallyif the tumors lie close to the optic apparatus. Thisis simply a specific application of the principle thatthe complications of radiosurgery are a function oflesion 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 wouldnot have been possible had the patient undergone radiosurgerywithout prior surgical removal of the majority of thetumor. Primary radiosurgical treatment would have mandatedeither an ineffective low prescription dose, or a higherone with attendant risks to the optic chiasm and brain.This case history illustrates the benefits of the multidisciplinarytreatment of patients with skull-base tumors, and theattendant excellent clinical result.