Ini J Rodiorion Oncology Biol Phvs Vol. Printed in the U.S.A All rights reserved.

22. pp.

1093-1098

0360-3016192 $5.00 + .oO Copyright Cc 1992 Pergamon Press Ltd.

@ Technical Innovations and Notes PRECISION

RADIATION

THERAPY

FOR OPTIC NERVE

SHEATH

MENINGIOMAS

TONY Y. ENG, M.D.,* N. W. ALBRIGHT, PH.D.,* G. KUWAHARA, R.T.T.,* C. N. AKAZAWA, R.T.T.,* D. DEA, R.T.T.,* G. L. CHU, PH.D.,* W. F. HOYT, M.D.,+ W. M. WARA, M.D.* AND D. A. LARSON, M.D., PH.D.* Universityof California,Schoolof Medicine,San Francisco,CA A more precise radiation therapy technique to treat unilateral optic nerve sheath meningioma is presented.1 It uses an immobilization device to align the ipsilateral optic nerve with a vertical axis and employs three small half-beam blocked fields to deliver radiation to a small conformal volume, thereby reducing the dose to the optic chiasm and the contralateral optic nerve. Three patients were successfully treated with this technique, and a fourth,patient with optic nerve glioma was also treated in a similar fashion and was included in this study. The new tethnique irradiates a much smaller volume of tissue to high dose levels: 58 cm3 is irradiated to the 80% isodose level and only 18 cm3 to the 95% level. In contrast, the opposed lateral technique irradiates 171 and 73 cm3 to these levels, respectively. Thus, a considerable reduction in the volume of normal tissue irradiated was accomplished. 0oses to the pituitary and contralateral optic nerve were 4% of the treatment dose for the new technique, where& these doses were 40% and 100% for opposed laterals and 10% and 3% for wedged pair, respectively. The avera$e setup error for this technique was very small, 50% of the setups measured were less than 1 mm off, and 92.5% were less than 3 mm off. However, for the conventional setups without a mask, only 21% of the setups were less thqn 1 mm off and 55% less than 3 mm off. We recommend this technique for localized unilateral optic nerve sheath meningioma and other optic nerve lesions that may require radiation therapy. Optic nerve, Optic nerve sheath, Meningioma,

Radiation therapy, Technique, Dose-volume

histogram.

meningiomas (4, 15, 16, 26, 28). To our knowledge, the conventional techniques being used currently to treat optic nerve lesions usually involve two laterally opposed beam split portals similar to that described by Donaldson et al. for treatment of Graves’ ophthalmopathy (8) and orbital pseudotumor (9). This technique has been used in optic nerve sheath meningioma as well as optic nerve glioma (7, 10, 19, 2 1, 26, 29, 3 1). Depending gn the extent of disease involvement, other techniques uqd to treat orbital lesions include wedged pairs of fields, a single lateral field with or without an anterior field, three fields, multi-fields, and rotational arcs (2, 7, 9, 10, 14, 17, 18, 2 1, 29, 30), details of which, in most cases, are not supplied. Because of the tumor’s location and the critical structures surrounding it, we have developed and used a more precise technique to deliver radiation to a smaller and conformal volume. This technique has been used in three patients diagnosed with optic nerve sheath menipgioma and one patient with a juvenile pilocystic astrocfloma involving the left optic nerve and chiasm, who was treated in a similar fashion. This paper will present this technique and its advantages and disadvantages.

INTRODUCTION

Optic nerve sheath meningioma is a rare tumor, with an incidence varying between 2 to 6% of orbital tumors and a 67 to 84% female predominance (1, 6, 25). It is often slowly progressive, with decreased vision and transient visual obscurations being the most common initial symptoms (6, 25, 33). Occasionally it may be more aggressive in younger patients and in patients with neurofibromatosis (1, 5, 6). With the advent of CT and MRI, the diagnosis of optic nerve sheath meningioma can frequently be made without the complications from tissue biopsy (3, 12, 16, 23, 24). The management of optic nerve sheath meningiomas remains controversial. Conventional management has been observation and often, if indicated, surgical excision with relatively high surgical complication and/or recurrence rates (1, 4,6, 15, 16, 32, 33). Other treatment options, which must be individualized for each patient, include primary radiation therapy or surgical excision of the tumor followed by postoperative radiation therapy (15, 16, 26). Although meningiomas have been thought to be relatively radioresistant, many authors have reported the efficacy of radiation therapy in the management of

* Department of Radiation Oncology.

Reprint requests to: Tony Y. Eng, M.D.

+ Department

Accepted

of Neuro-ophthalmology. 1093

for publication

5 September

199 1.

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METHODS

Volume 22, Number 5, 1992

AND MATERIALS

In 1989, three patients with a diagnosis of localized unilateral optic nerve sheath meningioma and one patient with juvenile pilocystic astrocytoma of the optic nerve were treated in the Department of Radiation Oncology of the University of California at San Francisco. The radiation therapy technique reported here involves the patient in a supine position with the head rotated so that the affected optic nerve is approximately perpendicular to the horizontal plane. The appropriate lateral head rotation, which ranged from 15 to 27 degrees for the four patients, was estimated from diagnostic CT scans. The amount of neck flexion was estimated by an imagined straight line through the center of the orbit and a point just above the pituitary sella on a lateral film. Three small half-beam blocked fields were set up isocentrically with axes lying in the horizontal plane. The beam was split perpendicular to the vertical axis and the anterior half was blocked. The treatment table was rotated to produce the three treatment portals: a vertex field and two superiorlateral wedged fields. The patient’s head was immobilized using an immobilization system* that consists of a head support,+ a thermoplastic mask that is molded to the patient’s head, and a frame that locks the mask and head in a reproducible position. After the immobilization mask was made, a CT of the patient in the mask was obtained for treatment planning purposes. Patient setup is shown in Figure 1 and a typical CT scan is shown in Figure 2. The superior-lateral fields were chosen to avoid the contralateral eye and optic nerve. The blocked field size, which ranged from 4 X 3.5 cm to 4 X 4.5 cm for the three patients with optic nerve sheath meningioma, was based on the tumor volume shown on the treatment planning

Fig. 1. Photograph of patient setup. It shows the proper rotation and flexion of the head immobilized by the mask.

* Nuclear Associates, Victoreen Inc., Carle Place, NY. + Timo Industries, Pittsburgh, PA.

Fig. 2. Treatment planning CT scan. It shows the perpendicular orientation of the affected optic nerve. Three beads on the mask that were used for positioning are also seen.

CT scans. A 4 X 7 cm blocked field was used for the fourth patient who had chiasmal involvement. The collimator settings for these half-beam blocked fields were 4 X 7, 4 X 9, and 4 X 14 respectively. Each field was weighted differently to optimize the resulting isodose distribution for the radiotherapy machine.* A dose of 5400 cGy at 180 cGy per day, 5 days per week, was prescribed to the 95 or 100% isodose of the target volume, depending on the patient, which gave approximately a 1 cm margin about the tumor. Figure 3A shows the orientations of the beams and the beam splitters. Figure 3B shows an example of isodose curves from one of the patients. Since treatment portals were not obtainable, anterior (AP) and lateral verification films were taken during the course of treatment. These films were analyzed, and the distance from the field center to an identifiable anatomical structure, typically the inner rim of the bony orbit, was measured vertically and horizontally to determine reproducibility of the treatment position. The distances measured on the verification films were compared with those measured on the simulation films. A total of 40 distances were measured on 20 verification films of the four patients. The port films of seven patients treated without masks were analyzed in a similar manner to determine the reproducibility of the setup without a mask. These were patients with optic nerve sheath meningioma, optic nerve glioma, or acoustic neuroma. All were treated using opposed lateral or wedged-field techniques with a three-point setup. A total of 62 distances were measured on 5 1 verification films and compared with 11 simulation films of the seven patients. To check the accuracy of our measurements, the new setup technique was repeated using a structurally equivalent phantom9 with a metallic bead in the optic nerve region. The immobilization system was disassembled be-

* Varian Clinac 6 (6MV photon). p Rando Phantom, Alderson Research

Laboratories,

Inc.

Precision radiation 0 T. Y. ENG CI a/.

beam

t

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1095

CT slice thickness (0.5 cm). The contour areas were calculated by the treatment planning computer. Dose to the pituitary (and thus the optic chiasm), the contra-lateral optic nerve, the contra-lateral eye (near the lens and lacrimal gland), and the ipsilateral thyroid gland was measured with thermo-luminescent dosimeters (TLD’s) in Rando phantom. The phantom was set up using either the new technique, wedged pair, or opposed lateral fields, and two separate measurements were made for each setup. To measure small doses accurately, 900 cGy was given to the 100% isodose for the new technique or to midplane for the opposed laterals. Other TLD’s were irradiated in a calibration phantom with doses of 20, 50, 100,200, 300, 500, and 1000 cGy to determine the doseresponse curve of the TLD’s. RESULTS Figure 4 shows the dose-volume histograms for the new technique, wedged pair and opposed lateral techniques. The new technique and wedged pair both irradiate a larger volume to low dose levels than does the opposed lateral technique, that is, they distribute their entrance and exit doses over a larger volume. However, the opposed laterals irradiate a much larger volume to high dose levels. At the 80% isodose level, the opposed laterals irradiate 17 1 cm3, but the new and wedged pair techniques irradiate only 58 and 54 cm3, respectively. At the 95% level the new and wedged pair techniques irradiate a very small volume, only 18 and 2 1 cm’, respectively, but the opposed laterals irradiate much more tissue to this dose, 73 cm3. Table 1 shows the results of the TLD measurement. The opposed laterals gave a full dose to the contralateral optic nerve. The value of 103- 105% is relative to 100% at midplane for a treatment plan without heterogeneity corrections. The pituitary was on the edge of the opposed

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tween each set of films simulating daily patient treatment. Ten sets of films were taken. These films were analyzed in the same manner as were the verification films of the patient setup. A dose-volume histogram was generated from one of the patients for the new technique, the opposed lateral and the wedged pair techniques. The blocked field size was 4 X 4.5 cm. The volume inside an isodose surface was obtained by adding the areas inside the corresponding isodose contours for each CT slice and multiplying by the

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Fig. 4. Dose-volume histograms for opposed lateral, wedged pair, and the new techniques. The graphs show the volume of tissue treated at a given isodose level.

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Table 1. Dose to various sites (W) Site Contralateral Contralateral Pituitary Thyroid

eye optic nerve

New technique

Opposed laterals

Wedged pair

1.6-1.7 3.2-3.5 4.1-4.4 1.1

4.2-4.9 103-105 38-42 .02

0.7 2.6-2.9 8.5-l 1.3 0.7

lateral fields, and consequently it received a dose of 38 to 42%. However, the new technique gave only about 4% dose to the pituitary for these setups, which is about 10 times less than for opposed laterals. The dose to the cornea of the contralateral eye was 4-5% for the opposed lateral technique, which corresponds to a dose of 2 16-270 cGy over a complete course of treatment (5400 cGy). The equator of the lens is approximately 9 mm deep and would receive a somewhat higher dose than this, which raises concern for the possibility of radiation-induced cataract. On the other hand, the dose to the contralateral cornea from the new technique was only about 1.7%, less than half as much as for opposed laterals. The lens and the lacrimal gland should also receive less dose from the new technique than from opposed laterals. Consequently, the new technique is less likely to induce a cataract or cornea1 damage secondary to decreased tear production. The dose to thyroid is slightly higher for the new technique than opposed laterals, but the difference is insignificant and the overall dose is small, 1% versus 0.2%. The wedged pair gave similar doses as the new technique except for a higher pituitary dose, about 10% versus 4%. Table 2 shows the differences in distances from the field center measured on the treatment verification films compared to those measured on the simulation film for the four patients treated with a mask. These were single measurements and &ere corrected for film magnification. The table shows that the overall setup accuracy was very high. Of the 40 distances measured, 50% were less than 1 mm off, 62.5% less than 2 mm off, and 92.5% were less than 3 mm off. In only one case was a large setup error observed, 6.9 mm. Table 2. Distance differences between simulation and treatment verification films (mm)

Volume 22, Number 5, 1992

The setup errors for the seven patients treated without masks were much larger than the patients treated with masks. Of the 62 distances that were measurable for patient setups without masks, only 21% were less than 1 mm off, 40% were less than 2 mm off, and 55% were less than 3 mm off. The most dramatic difference between setups with and without a mask concerned the number of setups with a large error. Approximately 21% of the setups without a mask were more than 5 mm o@,however, only one setup out of 40 with masks was off by more than 5 mm. Cumulative histograms of the setup errors with and without masks are shown in Figure 5. These graphs show the percent of setups whose errors, in magnitude, were less than or equal to a given value. For example, 50% of the setups with a mask had errors less than or equal to 1 mm. The new technique using mask immobilization and the conventional opposed lateral or three-field techniques without masks all use three-point setups. However, the graphs show that the accuracy of these setups is much better when a mask is used. The precision with which the setup errors were measured is shown by the corresponding differences in distances observed using the phantom. In that case 65.6% of the setups were less than 0.5 mm off, and 90.6% were less than 1.0 mm off. The standard deviation of the differences in distances for the phantom setup was 0.6 mm. Since the phantom measurements include both measurement errors and setup variations, the error in measurement alone must be less than about 0.6 mm. We concluded that the patient setup variations were accurately measured.

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Precision radiation 0 T. Y. ENGet al.

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DISCUSSION

The conventional technique for radiation treatment of the optic nerve is usually laterally opposed fields or wedged pairs (7, 10, 19, 21, 26, 29, 31) which may contribute needless irradiation of the contralateral optic nerve. Unlike other immobilization systems used in treating orbital lesions, which include a simple head holder, bite block, or custom head shell (8, 13, 22), the technique presented here immobilizes the patient with a mask and aligns the tumor with the vertical axis about which the treatment table rotates. This enables one to produce a smaller conformal treatment, reduce the dose to the opposite orbit, and distribute the entrance and exit dose over a larger volume, thereby redu:+g the possibility of injury to the surrounding normal tissues. The important aspect of this technique is the proper rotation of the patient’s head and flexion/extension of his neck so that the affected optic nerve is vertical and aligned with the treatment table axis. The beam is split perpendicular to the vertical axis and the anterior half is blocked, which also reduces the dose to the eyes (Figure 3A). The resulting treatment volume has a cross section that is a square with rounded corners. The treatment volume is coaxial with the affected optic nerve and has one sharp nondiverging field edge anteriorly. The dose-volume histogram shows that the new technique and the wedged pair method irradiate only a very small volume at the 95 to 100% isodose level. This is achieved at the expense of irradiating a larger volume at low isodose levels. In contrast with this, the opposed lateral fields irradiate a much larger volume to high isodose levels, but irradiate a smaller volume than the new technique at low isodose levels. In some ways, the new technique is similar to the wedged pair since both use wedges and have small high-dose volumes. The advantage of the new technique over that of the wedged pair lies in the shape of high isodose surfaces and the way they encompass the optic nerve. As mentioned above, the high-isodose volume of the new technique has a more rounded cross section whose axis is aligned with the optic nerve. In contrast, the high-isodose volume of the wedged pair technique has a more diamond-shaped cross section, and the optic nerve is not coaxial with the treatment volume. Instead, it runs almost diagonally across the volume. Consequently, a larger volume may be needed to cover the tumor adequately with a wedged pair than with the new technique. In addition, separation of the tumor from the other critical normal structures is better accomplished with the new technique because of the vertical orientation of the optic nerve. See Figure 6. The doses to pituitary, optic chiasm, and contralateral optic nerve are much lower with the new technique than with the opposed lateral technique. In addition, the doses to contralateral cornea and lens are also significantly lower with the new technique. While doses to the contralateral

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eye and optic nerve and thyroid were similar, with wedged pair the pituitary received about 6% more than that with the new technique. Therefore, the new technique should be used for unilateral localized optic nerve lesions, so that unnecessary doses to the opposed orbit, optic nerve, and chiasm are avoided. The setup accuracy of the new technique is very high: 50% of the cases measured were less than 1 mm off and 92.5% were less than 3 mm off. This is acceptable since the dose is prescribed to a target volume that has a 1 cm margin about the tumor. A large setup error of 6.9 mm was noted in only one instance, and even this is within the prescribed margin. On the other hand, the setup errors for the conventional opposed lateral technique without mask immobilization were typically much higher. The median setup error without a mask was more than twice the median error with a mask, and when no mask was used, 2 1% of the setups had errors in exce$s of 5 mm. The effect of immobilization has definitely improved setup accuracy as demonstrated by other investigators (11, 20, 27). The main disadvantage of this technique is the additional time required for making the individual patient’s mask, which is an additional pretreatment step. The other pretreatment steps are unchanged: performing a CT scan of the patient in the treatment setup, generating a computer treatment plan, and verifying the treatment plan. Also, the actual treatment time on the machine is not significantly prolonged compared to the alternative techniques, which is about 15-20 minutes. In summary, the technique presented here is superior to the alternatives such as opposed lateral$ or wedged pairs because it is more accurate and irradiates less normal tissue to a high dose volume. Although the dose-volume histogram does not show superiority of the new technique because same field size was used, we believe the wedged pair would require a larger field size for an identical tumor. As shown in Figure 6, to encompass the tumor with same margin, wedged pair requires slightly larger field area than the new technique does (Fig. 6). In addition, a little tighter margin may be used for the new technique because of its better immobilization and better separation of the tumor from the chiasm and the eye. Therefore, this technique is

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less likely to cause radiation injury to the surrounding normal tissues. The sole disadvantage is the effort required to construct a mask to immobilize the patient. This tech-

Volume 22, Number 5, 1992

nique is useful for unilateral optic nerve sheath meningiomas and, in addition, for other optic nerve lesions that may require radiation therapy.

REFERENCES 1. Alper, M. G. Management

of primary optic nerve menin-

giomas: current status-therapy in controversy. J. Clin. Neuroophthalmol. 1: 10 1- 117; 198 1. 2. Austin-Seymour, M. M.; Donaldson, S. S.; Egbert, P. R.; McDougall, I. R.; Kriss, J. P. Radiotherapy of lymphoid diseases of the orbit. Int. J. Radiat. Oncol. Biol. Phys. 11: 371-379; 1985. 3. Azar-Kia, B.; Naheedy, M. H.; Elias, D. A.; Mafee, M. F.; Fine, M. Optic nerve tumors: role of magnetic resonance imaging and computed tomography. Radio]. Clin. North Am. 25(3): 561-81; 1987. 4. Barbaro, N. M.; Gutin, P. H.; Wilson, C. B.; Sheline, G. E.; Boldrey, E. B.; Wara, W. M. Radiation therapy in the treatment of partially resected meningiomas. J. Neurosurg. 20: 525; 1987. 5. Cibis, G. W.; Whittaker, C. K.; Wood, W. E. Intraocular extension of optic nerve meningiomas in a case of neurofibromatosis. Arch. Ophthalmol. 103(3): 404-406; 1985. 6. Clark, W. C.; Theotilos, C. S.; Fleming, J. C. Primary optic nerve sheath meningiomas-report of nine cases. J. Neurosurg. 70: 37-40; 1989. 7. Danoff, B. F.; Kramer, S.; Thompson, N. The radiotherapeutic management of optic nerve gliomas in children. Int. J. Radiat. Oncol. Biol. Phys. 6: 45-50; 1980. 8. Donaldson, S. S.; Bagshaw, M. A.; Kriss, J. P. Supervoltage orbital radiotherapy for Graves’ ophthalmopathy. J. Clin. Endo. Met. 37(2): 276-285; 1973. 9. Donaldson, S. S.; McDougall, I. R.; Egbert, P. R.; Enzmann, D. R.; Kriss, J. P. Treatment of orbital pseudotumor (idiopathic orbital inflammation) by radiation therapy. Int. J. Radiat. Oncol. Biol. Phys. 6: 79-86; 1980. 10. Flickinger, J. C.; Torres, C.; Deutsch, M. Management of low-grade gliomas of the optic nerve and chiasm. Cancer 61: 635-642; 1988. 11. Gerber, R. L.; Marks, J. E.; Purdy, J. A. The use of thermal plastics for immobilization of patients during radiotherapy. Int. J. Radiat. Biol. Phys. 8: 1461-1462; 1982. 12. Haik, B. G.; Zimmerman, R.; Louis, L. S. GadoliniumDTPA enhancement of an optic nerve and chiasmal meningioma. J. Clin. Neuro-ophthalmol. 19(2): 122-25; 1989. 13. Harnett, A. N.; Doughty, D.; Hirst, A.; Plowman, P. N. Radiotherapy in benign orbital disease. II: ophthalmic Graves’ disease and orbital histiocytosis X. Br. J. Ophthalmol. 72: 289-292; 1988. 14. Horwich, A.; Bloom, H. J. G. Optic gliomas: radiation therapy and prognosis. Int. J. Radiat. Oncol. Biol. Phys. 11: 1067-1079; 1985. 15. Ito, M.; Ishizawa, A.; Miyaoka, M.; Sato, K.; Ishii, S. Intraorbital meningiomas: surgical management and role of radiation therapy. Surg. Neurol. 29: 448-53; 1988. 16. Kennerdell, J. S.; Maroon, J. C.; Malton, M.; Warren, F. A. The management of optic nerve sheath meningiomas. Am. J. Ophthalmol. 106: 450-457; 1988.

17. Kim, R. Y.; Roth, R. E. Radiotherapy of orbital pseudotumor. Radiology 127: 507-509; 1978. 18. Knowles II, D. M.; Jakobiec, F. A.; Potter, G. D.; Jones, I. S. Ophthalmic striated muscle neoplasms. Surv. Ophthalmol. 21(3): 219-61; 1976. 19. Markoe, A. M.; Brady, L. W.; Grant, G. D.; Shields, J. A.; Augsburger, J. J. Radiation therapy of ocular disease. In: Perez, C. A., Brady, L. W. Principles and practice of radiation oncology. Philadelphia, PA: J. B. Lippincott Co; 1987: 453-472. 20. Marks, J. E.; Haus, A. G. The effect of immobilization on localisation error in the radiotherapy of head and neck cancer. Clin. Radiol. 27: 175-177; 1976. 2 1. Montgomery, A. B.; Griffin, T.; Parker, R. G.; Gerdes, A. J. Optic nerve glioma: the role of radiation therapy. Cancer 40: 2079-2080; 1977. 22. Olivotto, I. A.; Ludgate, C. M.; Allen, L. H.; Rootman, J. Supervoltage radiotherapy for Graves’ Ophthalmopathy: CCABC technique and results. Int. J. Radiat. Biol. Phys. 11: 2085-2090; 1985. 23. Peyster, R. G.; Hoover, E. D.; Hershey, B. L.; Haskin, M. E. High resolution CT of lesions of the optic nerve. Am. J. Roentgenol. 140: 869-874; 1983. 24. Sarkies, N. J. C. Optic nerve sheath meningioma: diagnostic features and therapeutic alternatives. Eye 1: 597-602; 1987. 25. Sibony, P. A.; Krauss, H. R.; Kennerdell, J. S.; Maroon, J. C.; Slamovits, T. L. Optic nerve sheath meningiomas: clinical manifestations. Ophthalmol. 9 l( I 1): 13 13-26; 1984. 26. Smith, L.; Vuksanovic, M. M.; Yates, B. M.; Bienfang, D. C. Radiation therapy for primary optic nerve meningiomas. J. Clin. Neuro-ophthalmol. 1: 85-99; 1981. 27. Soffen, E. M., Hanks, G. E., Hwang, C. C., Chu, J. C. Conformal static field therapy for low volume low grade prostate cancer with rigid immobilization. Int. J. Radiat. Oncol. Biol. Phys. 20: 141-146; 1991. 28. Wara, W. M.; Sheline, G. E.; Newman, H.; Townsend, J. J.; Boldrey, E. B. Radiation therapy of meningiomas. Am. J. Roentgenol. 123(3): 453-458; 1975. 29. Wechsler-Jentzsch, K.; Witt, J. H.; Fitz, C. R.; McCullough, D. C.; Harisiadis, L. Unresectable gliomas in children: tumor-volume response to radiation therapy. Radiology 169: 237-242; 1988. 30. Weiss, L.; Sagerman, R. H.; King, G. A.; Chung, C. T.; Dubowy, R. L. Controversy in the management of optic nerve glioma. Cancer 59: 1000-1004; 1987. 3 1. Wang, J. Y. C.; Uhl, V.; Wara, W. M.; Sheline, G. E. Optic Gliomas: a reanalysis of the University of California, San Francisco experience. Cancer 60: 1847-l 855; 1987. 32. Wright, J. E.; Call, N. B.; Liaricos, S. Primary optic nerve meningioma. Br. J. Ophthalmol. 64: 553-558: 1980. 33. Wright, J. E. Primary optic nerve meningiomas: clinical presentation and management. Trans. Am. Acad. Ophthalmol. Otolaryngol. 83: 6 17-625; 1977.

Precision radiation therapy for optic nerve sheath meningiomas.

A more precise radiation therapy technique to treat unilateral optic nerve sheath meningioma is presented. It uses an immobilization device to align t...
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