The Role of Radiosurgery in the Treatment of Pituitary Tumors

Antonio A.F. De Salles, M.D.,Ph.D. and Yvette Ficekova, M.D.
Division of Neurosurgery, University of California, Los Angeles

Radiosurgery

Stereotactic radiosurgery is a technique used for the treatment of intracranial tumors that was developed to avoid the need for an open surgical removal. Radiosurgery is a combination of radiation therapy and stereotactic surgery. These two techniques, when performed together, provide a high concentration of radiation to the tumor, in such intensity, that the tumor is rendered sterile. When the tumor cells go on to divide, they die. The tumor decreases in size progressively, depending on the speed of reproduction of its cells. The process of tumor death and reabsorption may last from months to years. Scar tissue in the area of the tumor may be seen forever on MRI or CT.

Stereotactic surgery is a surgical technique designed to localize lesions inside of the skull with mathematical accuracy. This technique uses geometrical principles to direct probes, to guide sites of surgical incisions, and in the case of radiosurgery, to direct radiation beams to the intracranial lesion. Several forms of irradiation have been directed by stereotactic techniques. The particulate irradiation and the electromagnetic radiation are the most commonly used. Particulate radiation is made of nuclei of atoms accelerated by cyclotrons that were built in the forties for the development of the atomic bomb. Particulate energy has a special property called the Bragg peak. This allows calculations to make the beam stop at the tumor, thereby avoiding radiation of normal brain beyond the tumor. Recently, similar cyclotrons have been built for medical purposes. Few centers in the world have these cyclotrons because of the high cost of their construction and maintenance.

The electromagnetic energy, also called photon beam, is generated from the decay of the Cobalt-60 isotope, or artificially by collision of electrons accelerated to high speed to a plate of heavy metal. The collision releases photons which are generated and directed on high voltage linear accelerators. The energy generated by decay of Cobalt-60 or linear accelerators is the same. It has the same biological effect on the tumor. It also has the same ability to spare normal issue. This ability depends on the geometric strategies to avoid repeat beams crossing the same normal tissue. The two most popular forms of radiosurgery are based on either Cobalt-60 decay or linear accelerator beam. When Cobalt-60 is used, the radiosurgery instrument receives the commercial name of Gamma Knife. When the linear accelerator beam is used, the radiosurgery receives the popular name of LINAC radiosurgery. Again, both forms of radiosurgery have the same effect on the tumor.

Indications

Over the years, the use of radiosurgery for the treatment of pituitary tumors has been controversial. Extensive experience on the pituitary gland and tumor use of radiosurgery was developed with the Harvard cyclotron and the Berkeley syncyclotron. Destruction of the pituitary gland affords remarkable improvement of pain secondary to metastatic breast cancer. Thousands of patients underwent destruction of the pituitary gland in those two cyclotrons during the 50's and 60's. The data collected during those years are used today for radiosurgery treatment of pituitary tumors. Destruction of the pituitary for the treatment of pain is no longer performed because of better more modern means of pain control.

Hormone secreting pituitary tumors such as prolactinomas, ACTH secreting tumors and GH secreting tumors respond well to radiosurgery, however, there is a long period after radiosurgery before the hormone level starts to fall to normal. Therefore, the tumor-related symptoms persist long after radiosurgery. Surgical resection, on the other hand, provides immediate control of the hormone hypersecretion. Because of this immediate control of the hormonal levels, microsurgery, usually transphenoidal, is the technique of choice for treatment of pituitary secreting tumors. Residual tumor, the portion of tumor infiltrating the cavernous sinus, or non-secreting pituitary tumor that is not compressing the optic pathways can be treated by radiosurgery. When there is compression of the optic apparatus, or the tumor is very close to the visual pathways, surgical resection is recommended. Recently, stereotactic radiotherapy has been developed to treat tumors in contact with optic apparatus. This technique affords better ability to spare normal brain than the conventional fractionated radiotherapy. Because of the exquisite visualization of the pituitary gland in relation to the tumor, radiosurgery also provides a less risk of affecting the production of other hormones than conventional radiation therapy.

Radiosurgery is a complement to microsurgery resection and it must be offered to the patient soon after surgery, usually immediately after demonstration of re-growth of the tumor. Radiosurgery has a high control rate of hormonal levels and less side effects to the normal brain than conventional fractionated radiation therapy.

In summary, radiosurgery is a non-invasive technique for treatment of residual pituitary adenomas or treatment of tumors in patients that are not fit for a microsurgery resection. Radiosurgery must be kept in mind during the follow up after surgical removal. When there is reoccurrence of the tumor, radiosurgery is an excellent option. Radiosurgery and stereotactic radiotherapy must replace conventional radiation therapy for the majority of the patients with pituitary adenomas. These two techniques differ substantially from conventional radiation therapy because they spare normal brain and the extent that other tissues in and around the skull are irradiated. This potentially allows for more effectiveness and less side effects.