Introduction
The use of stereotactic radiosurgery (SRS) to treat patients with benign tumors is still debatable. However, a consensus exists in favor of SRS for patients with high surgical and anesthesiological risk, patients of advanced age, and those who refuse open surgery for other reasons. Benign tumors in eloquent areas pose considerable surgical risks. Unrelated to a specific diagnosis, there are several important factors concerning patient assessment and SRS that should be taken into consideration before treatment. In all cases of radiosurgically treated tumors, the histological nature of the process should be evaluated. Variations of radiosensitivity of tumors and radiation tolerances of different structures within and outside the normal brain, mostly important for dose planning in different tumor locations, have to be taken into consideration. The volume of the tumor itself and the influence on the normal brain tissue around are important factors. The patient must be aware that the primary aim of treatment is to control the lesion and must be informed about possible risks of SRS. For example, SRS is strictly contraindicated in patients with compression of the optic apparatus (Fig. 1), or with signs of acute brainstem compression from cystic tumors.
All these factors should be borne in mind before using SRS to treat benign tumors, including meningiomas, acoustic or trigeminal schwanno-mas, benign tumors of the pituitary region, small, well-delineated, low-grade gliomas, hemangioblastomas, and glomus jugulare tumors.
Meningiomas
Patients with residual or recurrent meningiomas are often ideal candidates for radiosurgery especially those with skull base meningiomas in the para-sellar region. For instance, many neurosurgeons still advocate extensive dissection for aggressive removal of cavernous sinus meningiomas (CSM), yielding ”acceptable levels” of morbidity and mortality [1-5]. With open surgery, new neuropathies are reported in 18% to 43% of patients [4,6], and extensive resection of the cavernous sinus region carries a risk of permanent ocular palsy of about 20% [7]. The patient should be informed about these risks and the alternative of SRS.
Figure 1 A 64-year-old female with recurrent sellar meningioma after open surgery 11 years before. Acute visual field defects within 6 weeks will not allow radiosurgery. Note the compression of the chiasm.
Our own series of SRS for patients with CSM found no further cranial nerve deficit after treatment [8,9]; a report by Duma noted 6% transient deficits [10]. Although it is, of course, almost mandatory to have histological proof of the nature of a lesion to be treated by SRS, as computed tomography (CT) or magnetic resonance imaging (MR) may be misleading [11], such proof should not be obtained if this involves an unacceptable risk to the patient. This holds mainly for elderly or clinically disabled patients who are precluded from an open microsurgical approach. Figure 2A,B presents a case of a 69-year-old patient who refused open surgery despite VIth nerve palsy.
The radiosurgical prescription dose commonly used for meningiomas varies between 12 and 16 Gy. Decreasing the tumor dose to less than 12 Gy may defeat the therapeutical purpose of the radiosurgical procedure [12]. Therefore, patients in whom the optic nerves, chiasm, or tract are stretched, distorted, or displaced over the entire tumor surface should be excluded from SRS, as safe and effective doses cannot be delivered simultaneously by a single fraction dose plan under those circumstances [13]. Nevertheless, if surgery is required for visual pathway decompression or some other reason, the option of SRS means that the surgeon may be less aggressive when dissecting in the region of the optic apparatus or circle of Willis, leaving residual tumor for treatment with adjuvant SRS. Moreover there is evidence that SRS may control, even reduce, tumor size in meningiomas also with suboptimal doses less than 10 Gy, suggesting a further option in selected cases [14].
Figure 2 A, Gradual shrinkage of a parasellar meningioma of a 69-year-old female patient with VIth nerve palsy before treatment with 12 Gy at the 30% isodose volume. Shrinkage with radionecrosis occurred after slight enlargement 9 months after radiosurgery.
Figure 2 B, Magnetic resonance imaging control of patient in Figure 2A, 7 years after radiosurgery with a small cyst-like parasellar scarformation unchanged for 3 years.
As some of the basal meningiomas are residual or recurrent tumors of considerable size and the growth rate is rather low, even these large volumes can be managed by SRS as a staged procedure separated by 6 months. By this method, the complications that are expected in treatment volumes of more than 3 cm in diameter will be minimized, if present at all—an option that includes all other benign tumors as well.
For patients with hemispheric meningiomas, the indication for SRS should be limited, as they have a greater risk of edema after treatment compared with those with basal meningiomas [15-17]. In these patients, the minimal risk of open surgery should be taken into consideration as well.
Acoustic Schwannomas
The typical imaging appearance of these tumors, along with the history of signs and symptoms, allows for their reliable diagnosis without biopsy. In dose planning for patients with these tumors, one must bear in mind the proximity of the facial and trigeminal nerves to the tumor surface, mandating a steep dose gradient of the marginal dose. Therefore, multiple small shots should be applied. The marginal dose itself should be 15 Gy in very small, 12 Gy in medium-sized, and 10 Gy in large tumors, especially if hearing can be preserved [18]. The growth of acoustic schwannomas can be controlled by SRS in 95% of patients with tumors with diameters up to 3 cm [18,19]. Figure 3 shows a 68-year-old woman with a medium-sized acoustic schwannoma in whom no visible tumor could be detected 2.5 years after SRS. Useful hearing was preserved.
The incidence of transitory facial and trigeminal nerve dysfunction after SRS is currently less than 2%, which is superior to the results of microsurgery [20]. After microsurgery, permanent trigeminal symptoms may occur in 11% of patients, and the incidence of persistent facial nerve paresis varies between 0 and 35% correlating with tumor size [21,22]. Transitory hemifacial spasm, subsiding spontaneously, might occur as a side effect after SRS [18].
Figure 3 A, A 68-year-old woman with a 1.5 X 1.0-cm large acoustic schwannoma. Stereotactic radiosurgery with 12 Gy peripheral dose to the 50% isodose volume was applied. B, No visible tumor 2.5 years after radiosurgery. Useful hearing as before radiosurgery was preserved with no further neurological deficit.
Rates of useful hearing preservation in the immediate postoperative period (defined as Gardner and Robertson classes I and II) are reported to be 100%, but decline to 62% to 70% after at least 2 years of follow-up. Similar rates of hearing preservation are reported only for selected small volume schwannomas after microsurgery [19,23,24].
Patients with neurofibromatosis type 2 (NF2) who still have useful hearing on both or either sides may be offered SRS as the optimal alternative to open surgery. Long-term follow-up of these patients has demonstrated that hearing loss may not be inevitable [24]. This is illustrated by our case of a 16-year-old female with large NF2 tumors. Because on the left side useful hearing was still preserved, SRS was applied on this side, followed by microsurgery with only partial resection to exclude any risk on the right side. Useful hearing could be preserved along with intact facial nerves on both sides (Fig. 4). In general, risk factors involving radiosurgical injury to cranial nerves increase with the irradiated length of the nerve, high total peripheral dose, and decreased conformality of the prescription isodose shell. This is especially the case in patients with intracanalicular tumors, in whom the use of multiple shots and relatively low marginal dose radiosurgical planning (on the order of 12 Gy) is mandatory [25,26].
Figure 4 A, A 16-year-old female patient with NF2 tumors. Radiosurgical dose plan on the left neurofibroma: 10 Gy peripheral dose to 50% iso-dose volume. B, One year after stereotactic radiosurgery and microsurgical partial resection of the right neurofibroma showing central radi-onecrosis on the left side.
Hydrocephalus induced by tumor compression may necessitate placement of a shunt before radiosurgery. Patients with large intracranial acoustic schwannomas who are not candidates for primary SRS because of brainstem compression should undergo tumor resection before SRS. In cases of cystic schwannomas, one should apply radiosurgery with caution, as the chance of increasing the cyst volume after radiosurgery is high. This can result in acute symptoms of brainstem compression, and open surgery for decompression might be needed as an emergency [27]. Cyst formation may also develop after SRS of larger acoustic schwannomas [28]. It may be appropriate to be cautious in advising radiosurgery for any intracranial tumor with a major cystic component [29].
Overall, however, the inevitable risks of open surgery—cerebrospinal fluid (CSF) leak, intracranial infection, and intraoperative and postoperative hemorrhage—can be avoided by SRS. Radiosurgery is also useful for treatment of recurrent tumors after microsurgery. The problems in performing microsurgery after SRS are sometimes discussed [22-30], although this has not been our observation.
