Stereotactic Body Radiation Therapy As a Derivative of Stereotactic Radiosurgery: Clinically Independent But With Enduring Common Themes

Cranial stereotactic radiosurgery (SRS) and extracranial stereotactic body radiation therapy (SBRT), despite their modern high-tech incarnations, trace their historical roots to a technique initially explored more than a century ago in the first decade after the discovery of x-rays. In 1902, the Austrian radiotherapy pioneers Holzknecht and Kienbock described the first clinically applied radiation dosimeter and promoted an approach known as “expeditive radiotherapy,“ whereby an entire course of treatment would be given in a single large-dose session, as opposed to multiple small exposures given daily over a period of numerous weeks. The indications for radiotherapy in those days included a quaint assortment of non–life-threatening maladies such as alopecia from fungal infection. However, one impetus for expeditive radiotherapy arose from a serious consideration, namely the significant risk of radiation-induced illness among health care providers administering treatment using unshielded sources with minimal safety features. Awareness of the danger of excess exposure to ionizing radiation emerged quickly. The Law of Bergonie and Tribondeau, published in 1906, is the classic initial observation that mitotically active cells are more sensitive to radiotherapy than quiescent cells; less well remembered is that these authors also stated in their article that “the practice of delivering small and repeated doses, in contradistinction to the technique of few and heavy doses, is more apt to produce [in health care workers] . . . nondestructive irritations with resulting monster cells [ie, carcinogenesis]. Therefore, one should prefer the method of massive doses.” Holzknecht himself succumbed to a series of radiation-induced ailments, and it is important to remember the martyrdom of many early radiation scientists. Fortunately, radiation treatment delivery technology quickly evolved to eliminate the exposure threat to medical personnel. After a variety of sporadic forays into the realm of high-dose-per-treatment external-beam radiation therapy throughout the 1900s, by the dawn of the 21st century, advances in technology and clinical knowledge had converged to translate the lessons of SRS into the rapidly burgeoning field of SBRT. In recent years the American Society for Radiation Oncology (ASTRO) has issued guidelines on SBRT and a white paper on quality and safety in SRS and SBRT. Apart from providing specifics about personnel and training requirements and quality assurance methodologies, these documents also confirm that multiple radiation delivery systems could be effectively used for SRS and SBRT. While each has somewhat different means of achieving the same goals, the common end point is to deliver an intense radiation dose conforming to the intended target, with a steep dose gradient away from the target. Typically this goal is achieved by employing multiple nonoverlapping static or dynamically arcing beams that converge on the target so that the normal tissues in the entrance and exit parts of the beam path are relatively spared while the tumor receives a heavy concentration of ionizing energy.