Tumors of the Pituitary Region
Using SRS to treat patients with pituitary tumors is a challenging task. There is a risk of radiation damage to optic nerves, optic chiasm, or the hypo-thalamus, especially with larger tumors. The possibility of control of tumor growth and pituitary endocrinopathy without producing pituitary failure is a relatively new but promising aspect of radiosurgery [31].
Although SRS is not the preferred primary treatment for patients with hormonally active tumors, it has a role to play as an adjunct to treatment after failed microsurgery. Tumors may invade the cavernous sinus relatively far away from the optic apparatus. This makes a surgically awkward location a reasonable and safe indication for SRS. The carotid artery and nerves in the wall of the cavernous sinus tolerate those radiosurgical doses that are effective for controlling tumor growth. Residual tumors in this region can be controlled very reasonably by SRS [32-34]. Figure 5 presents a case of a 37-year-old man with a large recurrent prolactinoma after transsphenoidal surgery. After 3 years, no evidence of tumor was apparent, and his prolactin level was normal.
There is now evidence that a marginal dose of at least 25 Gy may normalize elevated hormone levels relatively soon after SRS [35-37]. Nevertheless, special heed must be taken of sparing critical structures from excessive radiation. Therefore, the radiosurgical dose plan has to be created with exact visualization of tumor, in relation to the normal pituitary gland, optic pathways, the cavernous sinus, and of the isodose lines and corresponding doses tangent to these critical structures. Doses up to 10 Gy to the optic apparatus are reported to be acceptable [32], but to avoid optic neuropathy, we recommend limiting this dose to no more than 8 Gy. If the chiasm is stretched over the tumor edge and subject to compression, primary microneurosurgery has to be performed for decompression; this can be followed by adjuvant radiosurgery. Patients should be followed up after SRS for at least 5 years to assess the effects on the endocrinopathy, to exclude pituitary failure, and to check visual function.
Figure 5 A, Large tumor mass of a prolactinoma with 45.2 cc volume. Dose plan with 10 Gy. B, Magnetic resonance imaging follow-up 3 years after stereotactic radiosurgery. No further evidence of vital tumor structures in the sellar region in accordance with the endocrinological situation.
The tumor control rate for patients undergoing SRS for inactive and hormone-active pituitary adenomas (with peripheral doses ranging from 10 to 22 Gy) is as high as 98.3% [35,38-41]. The incidence of pituitary dysfunction ranges from 15% to 55% [36,42-44]. In 11% of 73 cases, improvement of pituitary function is reported [38]. Moreover, with sophisticated doses that deliver less than 9 Gy to the optic apparatus, patients will avoid radiation-induced visual damage [38]. The endocrinological cure rate in patients with hormonally active adenomas may be up to 57% [36,38].
Craniopharyngiomas
For selected patients, in whom microsurgery may not be appropriate, SRS may be a viable option [45,46]. In those with cystic craniopharyngiomas, intracystic bleomycin, initially described by Takahashi in 1985 [47], has proved effective in preparing these tumors for radiosurgery after shrinkage. With SRS, the radiation field can be closely tailored to the tumor volume, keeping the radiation dose to the surrounding hypothalamic region and optical structures to a minimum. After bleomycin instillation, radiosurgery may achieve volume reductions of the residual tumors in 74% of patients [48,49]. The prescription dose should be kept within 12 to 18 Gy. In these reports, SRS resulted in no mortality and no significant morbidity. Figure 6 shows a cystic craniopharyngeoma, which had been stereotactically punctured and treated by instillation of bleomycin into the evacuated cyst. Shrinkage of the craniopharyngioma could be observed 3 months later and SRS was then applied. Four years later, only a small area of tumor tissue in front of the chiasm could be noted, which is stable in size up to now. Vision has remained normal since before evacuation.
Glomus Jugulare Tumors
With rare exceptions, glomus jugulare tumors are histologically benign, non-secreting paragangliomas. They have a well-known predilection for local invasion of the surrounding structures, such as the middle ear, jugular vein, clivus, internal carotid artery, cavernous sinus and cranial nerves [50]. De- spite advances in neuroimaging and microsurgical techniques, some of these lesions defy radical resection because of their critical location. If complete excision is inadvisable owing to concerns of postoperative morbidity, the residual glomus tumor should be treated by SRS. The optimal dose for these tumors has not yet been established. Nevertheless, we recommend using a prescription dose above 18 Gy to achieve tumor control. This dose should be delivered while minimizing the amount of radiation received by the brain-stem [51,52]. With this strategy in mind, no complications are reported to occur after SRS of glomus juglare tumors [52].
Figure 6 A, Craniopharyngioma cyst of 3.5-cm diameter, bulging into the third ventricle. Calcified nodule at the base of the tumor cyst. B, Dose planning for stereotactic radiosurgery of the remaining small volume of the craniopharyngioma after bleomycin treatment to the evacuated cyst with 9 Gy to the 50% isodose volume. C, Control magnetic resonance imaging 4 years after stereotactic radiosurgery with residual calcified small tumor nodule near the chiasm.
Miscellaneous Lesions
Stereotactic radiosurgery has become a well-accepted adjuvant or even primary treatment option to reduce risk and to improve outcome for patients with midline lesions located within the thalamus, hypothalamus, pineal region, and even brainstem. In midline tumors, especially in the brainstem, the marginal dose should not exceed 14 Gy [53]. Low-dose radiosurgery to critically located hamartomas of the hypothalamus may be effective for tumor arrest and suppression of epileptic activity [54]. In general, SRS may be used in lesional epilepsy by incorporating epileptogenic areas outside the tumor into the dose plan [55,56].
