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Stereotactic Radiosurgery for Pituitary Tumors

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Stereotactic Radiosurgery for Pituitary Tumors Neurosurg Focus 14 (5):Article 10, 2003, Click here to return to Table of Contents Stereotactic radiosurgery for pituitary tumors THOMAS C. WITT, M.D. Department of Neurosurgery, Indiana University Medical Center, Indianapolis, Indiana Pituitary adenomas frequently pose challenging clinical problems. Stereotactic radiosurgery (SRS) is one treatment option in selected patients. The purpose of this report is to identify the advantages and disadvantages of radiosurgery in cases of pituitary tumors to assess better its role in relation to other treatment. Methods for optimizing outcome are described. The author reviews several recent series to determine rates of growth control, endocrine response, and com- plications. In general, growth control is excellent, complications are very low, and reduction of excessive hormone secretion is fair. Depending on the clinical situation, SRS may be the treatment of choice in selected patients. KEY WORDS • stereotactic radiosurgery • pituitary tumor • acromegaly • prolactinoma • adrenocorticotropic hormone Although pituitary adenomas are histologically benign, structures must not occur. The purpose of this report is to the unrelated neurological and physiological conse- analyze the current status of SRS for pituitary tumors and quences can be devastating. Compression of the optic ap- to examine its utility compared with other treatment mo- paratus (nerves, chiasm, tracts) may lead to blindness or dalities. loss of peripheral vision. Interference with the function of nerves traversing the cavernous sinus can result in diplo- pia and/or facial paresthesias. Further expansion of the tu- RADIOSURGICAL TECHNIQUES mor laterally into the temporal lobes or posteriorly into the hypothalamus can lead to significant cognitive problems. Stereotactic radiosurgery is performed using three gen- eral types of devices: a cyclotron that generates heavy Excessive GH production associated with acromegaly 50,51 can lead to life-threatening cardiovascular and respiratory particles such as protons and helium ions; a linear ac- celerator that generates x-ray beams;19 and a GK that gen- conditions, diabetes mellitus, and possibly an increased 60 risk of colon cancer. Prolonged hypersecretion of ACTH erates gamma rays from 201 sources of Co. The majority in Cushing disease can lead to severe problems with hy- of studies published in the last 15 years regarding radio- pertension and osteoporosis. In patients with prolactino- surgery for pituitary adenomas are from centers at which mas, galactorrhea and infertility may occur.87 the GK is used. Because the author’s experience is also Successful management of these tumors can be chal- primarily with the GK, its use in cases of pituitary adeno- lenging. Treatment options include microresection, medi- ma radiosurgery will be described in more detail. cal therapy, fractionated radiotherapy, and SRS. Stereotac- Once the decision has been made to treat a patient with SRS, preparation may begin several months prior to the tic radiosurgery was defined in 1951 by Lars Leksell as 40 the “closed skull destruction of an intracranial target using procedure. In 2000, Landolt, et al., first reported a signi- ionizing radiation.”49 Applied to pituitary adenomas, de- ficantly lower incidence of GH and IGF-I normalization struction of the target means prevention of the tumor’s in patients with acromegaly in whom antisecretory med- growth and normalization of hormone production. De- ications were administered at the time of radiosurgery. In struction of surrounding neural, endocrine, and vascular a separate report on patients with prolactinomas, Landolt and Lomax42 found that patients not receiving dopamine agonists at the time of radiosurgery have a significantly better chance of experiencing endocrinological cure than Abbreviations used in this paper: ACTH = adrenocorticotropic those who continue to take these medicines. Since then, hormone; CA = carotid artery; CT = computerized tomography; GH = growth hormone; GK = gamma knife; GKS = GK radio- others have also documented a negative effect of antise- surgery; ICA = internal carotid artery; IGF-I = insulin-like growth cretory medications administered at the time of radiosur- 72 factor–I; LINAC = linear accelerator; MR = magnetic resonance; gery on the reduction of excessive hormone secretion. PRL = prolactin; SRS = stereotactic radiosurgery; UFC = urinary- Landolt, et al.,40 speculated that these drugs lower the met- free cortisol. abolic rate of the tumor and consequently make the tumor Neurosurg. Focus / Volume 14 / May, 2003 1 Unauthenticated | Downloaded 09/24/21 07:07 PM UTC T. C. Witt less susceptible to radiation. The optimal length of time For identification of the tumor, MR imaging is far supe- for these medications to be withheld is not clear. For pa- rior to CT scanning. The latter may be used in cases in tients with prolactinomas, Landolt and Lomax42 recom- which a non–MR imaging–compatible metallic implant mended that dopamine agonists be withheld for 2 months exists or if the patient is too large to fit into a closed MR prior to radiosurgery. For patients with acromegaly receiv- imaging unit (the latter scenario is not unusual in patients ing the long-acting form of octreotide, the last injection with Cushing disease). When MR imaging is used, a post- should occur 4 months before the GK. Six weeks later, the Gd thin-slice (1-mm) volume-acquisition sequence is rec- medication should be switched to the immediate-release ommended. Depending on the patient’s history of treat- form, which should be maintained until 2 weeks before ra- ment and findings on preradiosurgery images, acquisition diosurgery. In these last 2 weeks before the procedure, no of precontrast studies or use of fat-suppression techniques antisecretory medications should be taken.40 Because the may help in identification of the tumor. If there is signifi- tumor may enlarge once these medications are stopped cant flow artifact across the sella from the CAs, the phase/ (Fig. 1), the potential benefits of these medication adjust- frequency-encoding direction can be flipped so that this ments must be weighed against the potential risks. A larg- artifact extends in an anteroposterior direction and does er tumor may mean a lower prescription dose and a high- not obscure the sella. er risk for surrounding structures. One relatively common problem in patients with Cush- On the day of the treatment, the stereotactic head frame ing disease is the inability to detect the tumor on the imag- is applied in the standard fashion. Mild intravenous seda- ing study,79 which may occur if the tumor is extremely tion is administered for adults, and general anesthesia is small or because neoplastic cells invaded the dura or have induced for children. The scalp is prepared with alcohol, diffusely infiltrated the gland.6,61,79 As long as pre-GKS and the areas of the pin placements are infiltrated with a tests strongly suggest that the tumor does reside in the long-lasting local anesthetic. It may be helpful to angle the sella, radiosurgery may still be successfully performed. frame parallel to the axis of the optic apparatus.26 In the Semple, et al.,79 Sheehan, et al.,81 and Shimon, et al.,84 author’s experience, this angle approximates a line joining have reported successful outcomes in microsurgical series the lateral canthus and the top of the pinna. One purpose when the tumor cannot be identified on a preoperative MR of this maneuver is to make identification of the optic images. Inferior petrosal sinus sampling can successfully nerves, chiasm, and tracts easier by having an MR or CT predict lateralization of the tumor 81% of the time.79 With- image that demonstrates the entire optic apparatus in a sin- out the aid of direct visualization and histological analysis gle slice. at the time of surgery, however, the predictive value of in- ferior petrosal sinus sampling may not be accurate enough to allow the radiosurgeon to target just one half of the gland. It is probably more reasonable to select the entire sella as the radiosurgical target in cases in which the tumor cannot be visualized to minimize chances of failure to nor- malize excessive hormone production. After stereotactic images have been acquired and trans- ferred into a computer workstation, multiple isocenter dose planning is performed to enclose the borders of the tumor within the prescription isodose line.16 The 50% iso- dose is the most common prescription isodose line in GKS series because this line is where the slope of the radiation falloff is the steepest.99 Beam blocking plug patterns are often used to distort the peripheral isodose curves away from the optic apparatus because of the radiosensitivity of this structure17 (Fig. 2). The need to use plug patterns can be reduced by adjusting the gamma angle so that the an- teroposterior axis of the peripheral isodose curves is par- allel to the optic apparatus in the sagittal plane. If the frame is placed parallel to the course of the optic appar- atus, then a gamma angle of 90° may be used. This ma- neuver takes advantage of the extremely steep falloff of radiation dorsal to the isocenter. Selection of the prescrip- tion dose is based partially on the integrated logistic for- mula13,15 as well as specific strategies for protecting the optic apparatus, controlling tumor growth, and establish- ing and maintaining normal endocrinological function. Fig. 1. Images obtained in a patient with a recurrent prolactin- EXPECTATIONS IN RADIOSURGERY oma. Upper: Coronal MR
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