Stereotactic body radiation therapy: Rationale and clinical experience

INTRODUCTION

Stereotactic body radiation therapy (SBRT) is a technique that utilizes precisely targeted radiation to a tumor while minimizing radiation to adjacent normal tissue. This targeting allows treatment of small- or moderate-sized tumors in either a single or limited number of dose fractions.

Stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT) have been successfully used for intracranial, orbital, and base of skull tumors, as well as benign conditions that can use the skull as a reference system. The success of such an approach for intracranial indications has led to the development of new techniques to extend this approach to extracranial targets. Extension of these approaches to extracranial sites has required significant technical advances in the use of tumor imaging to guide radiation administration, patient immobilization, and conformal radiation delivery techniques.

The term stereotactic body radiotherapy has been adopted by the Centers of Medicare and Medicaid Services. Although the term stereotactic implies the use of an external frame of reference, many current approaches no longer utilize an external stereotactic localization method. Other names that have been applied to this approach include extracranial radiosurgery and extracranial radiotherapy.

The rationale for SBRT, its techniques, and early results in selected tumors will be reviewed here.

RATIONALE

Radiation therapy (RT) requires a balance between cytotoxicity to a tumor and the adjacent normal tissue. Conventional external beam RT encompasses the tumor as well as a significant margin of normal tissue to avoid missing any part of the tumor and to maximize the likelihood of a favorable therapeutic outcome. Technical factors that necessitate irradiation of a margin of normal tissue include limited accuracy in delineating the tumor target, organ movement due to respiration, and variation in patient positioning from one treatment to the next.