301 Chapter 23
Tumors of the anterior visual pathway are almost without exception benign. However, this does not mean that they are radiation insensitive. This is true for pituitary adenomas as well as for meningiomas. For the latter type, radiation therapy is a valid alternative treatment when complete excision is impossible or would be complicated by high morbidity.
Radiotherapy for Tumors
of the Anterior Visual Pathway
G. Becker, R.-D. Kortmann, and H. Wilhelm
Side Effects of Radiation Therapy
Radiation therapy of the anterior visual pathway has long been used with a certain degree of reluctance, since the side effects are potentially disastrous. For many years, it was be- lieved that radiation therapy of optic nerve tumors was of little use, since the healthy portions of the nerve had practi- cally the same radiosensitivity as the tumor. The risk of bilateral radiation optic neuropathy and total blindness prevented even consideration of radiotherapy as a valid choice for treatment. In addition, if the globe lay within the field of treatment, the risk of a radiation retinopathy and/or cataract were additional possibilities. If the tumor was located close the pituitary gland, endocrinopathies were also possible. ■ Table 23.1 lists the critical radiation doses for the various structures in and around the anterior visual pathway.
Three-Dimensional Planning of Radiotherapy
and Conformational Irradiation
Three-dimensional planning of treatment has led to a sig- nificant reduction in the side effects of radiotherapy, as compared with the classic two-dimensional method. The advent of computed tomography and magnetic resonance imaging has also allowed improvements to be made in the use of radiation as a therapeutic medium. The basis for cal- culation of volumetric dosing of radiation has paralleled the development of stereotactic methods of neurosurgery.
The computational process is similar, but the tumor, rather than being imaged, is exposed to gamma or X-rays. The tumor is computationally located, and then the “virtual”
tumor is irradiated by an instrument that uses the same pattern. This process is referred to as conformational dos- ing, a method that spares surrounding normal tissues to a maximal extent; the contralateral optic nerve is now largely spared. Focused stereotactic targeting allows a highly precise delivery of radiation. For this method a mask is prepared, so that no significant inaccuracies can result from inadvertent movements or changes in body position (■ Fig. 23.1). The accuracy of the method has a precision of 1 to 2 mm.
Table 23.1. Critical dosage thresholds
Target tissue Dosage (Gy)
Optic nerve 54
Retina 45
Lens 10
Lacrimal glands 32
Pituitary gland 25–30
302
Chapter 23 G. Becker, R.-D. Kortmann, H. Wilhelm
Radiosurgery and Fractionated Irradiation There are two primary methods for the use of radiation therapy, radiosurgery, in which the total dose of radiation is administered in a single session, and fractionated irra- diation, in which the total dose is given over a period of several sessions. In the case of a meningioma, this may be as many as 30 separate sessions. Radiosurgery has the in- herent risk of a high single dose causing damage to healthy tissues, even though the total dose of radiation is within an acceptable range. Experience has taught us that for the pro- duction of radiation optic neuropathy, single dosing often plays an important role. If the total single dose remains less than 2 Gy, optic nerve damage is very unlikely.
• Pearl
As a general rule, when seeking to retain function, frac- tionated application is preferred.
This is radiobiologically more favorable, since the tumor cells can partially recover between the single sessions and begin to divide again, increasing their susceptibility to the radiation effect.
Indications for Radiation Therapy
Radiation therapy is indicated when surgical removal of a tumor is incomplete or when growth of the neoplasm recurs following surgical excision. This applies not just to pituitary adenomas, but also to meningiomas and gliomas (■ Fig. 23.2). Primary tumors of the optic nerve or its men- ingeal sheath cannot be surgically removed without also sacrificing visual function. Radiation therapy for menin- giomas can usually be expected to stop further growth, and will often improve visual function, even though the tumor shows no reduction in size on CT or MRI images. It is like- ly that the number of useful applications for radiation oncology will continue to expand in coming years.
Conclusion
Radiation therapy plays an increasingly important role in the treatment of benign tumors of the anterior afferent visual pathway, due primarily to the evolution of stereotac- tic methods that allow some degree of protection for healthy tissues while either shrinking the size or preventing further growth of neoplasm. It can be expected that the indications for this type of therapy will continue to expand.
Further Reading
Pitz S, Becker G, Schiefer U, Wilhelm H, Jeremic B, Bamberg M, Zrenner E (2002) Stereotactic irradiation of optic nerve sheath meningi- oma: a new treatment alternative. Br J Ophthalmol 86: 1265–1268 Fig. 23.1. Patient fitted with mask for application of stereotacti-
cally guided conformational irradiation
Fig. 23.2. Regression of optic disc edema in a patient with an optic nerve sheath meningioma after fractionated, stereotactically guided, conformational irradiation
