Stereotactic cranial radiosurgery and radiotherapy

INTRODUCTION

The potential utility of ionizing radiation to treat cancer was recognized shortly after the discovery of x-rays. The ability of radiation to kill tumor cells is thought to be derived from the induction of extensive DNA damage.

Prior to the development of stereotactic techniques, radiation was delivered to the cancer and surrounding normal tissues. Therapeutic efficacy was based upon the increased DNA-repair capacity after radiation exposure in normal cells compared to tumor cells. This fractionated treatment is known as radiation therapy (RT) (figure 1).

Major advances in stereotactic localization, noninvasive neuroimaging, and radiation physics made it possible to selectively irradiate a sharply defined target, largely sparing the surrounding normal tissue. This is achieved by converging multiple, non-parallel radiation beams (figure 1). This approach is called stereotactic radiosurgery (SRS).

The biologic differences between fractionated RT and SRS and the technology of administering SRS are reviewed here. The application of SRS in various clinical settings (both malignant and nonmalignant) is discussed elsewhere in the topics on specific lesions, and the complications of cranial SRS and the application of SRS to extracranial sites are presented separately. (See "Complications of cranial stereotactic radiosurgery" and "Stereotactic body radiation therapy: Rationale and clinical experience".)

DEFINITIONS

Fractionated (conventional) RT — Fractionated or conventional RT refers to the repeated administration of small doses of radiation in a relatively large target, as in whole brain RT or involved-field RT. This fractionation of the total dose minimizes damage to normal tissues and maximizes the killing of tumor cells. Conventional dose fractionation schemes for intracranial lesions typically consist of 1.8 to 2.0 Gy in daily sessions with cumulative doses of 30 to 60 Gy [1].