The concept of stereotactic ablative radiotherapy (SABR), known in older reports as stereotactic body radiation therapy (SBRT), for the treatment of lung cancer traces its roots back to the use of stereotactic radiosurgery in the treatment of central nervous system (CNS) malignancies from the 1940s through the 1960s. Although noninvasive, radiosurgery defined the use of single, high-dose fractions of radiation in the treatment of intracranial processes. Drs. Leksell and Larsson of Sweden used the concept that limited high doses of ionizing radiation could ablate neoplastic activity and still limit normal tissue side effects if the treatment was targeted with high precision.1 Early in radiosurgery, protons and gamma rays from a radioactive cobalt-60 source were used to irradiate lesions. To promote precise targeting of treatment, patients’ skulls were immobilized with a frame, and fiducial markers regulating a coordinate system were implemented. Thus, a large ablative dose could be delivered with high levels of precision, allowing a safe and effective therapy.
Ultimately, multiple linear accelerator and nonlinear accelerator systems have been used to deliver high doses of radiation in limited treatments. For extracranial treatment, originally extracranial stereotactic radioablation, later coined stereotactic body radiation therapy, described the complex process of high-dose precision treatment of neoplastic events.2,3 The more recent term stereotactic ablative radiotherapy has been gaining traction, because the involvement of the term ablative may more accurately describe the radiobiologic and pathologic consequences of high dose-per-fraction treatment on cell division and tissue function.4 The capacity for SABR to deliver high doses to tumor while limiting adjacent normal tissue toxicity is predicated on the use of all available imaging modalities, before and during treatment, to ensure optimal disease delineation, precise targeting, and delivery of radiation dose. A term has been created, image-guided radiation therapy (IGRT), to describe the use of imaging for ensuring precision in radiation delivery to tumor, especially for treatments involving high doses per treatment, such as SABR. Both the American Society of Therapeutic Radiology and Oncology and the American College of Radiology have defined SABR to include all radiation therapy requiring very large doses per fraction.5 Therefore, each governing body has also created guidelines to ensure quality control in the setting of SABR, which requires adequate technology for target delineation, experience of radiation oncologists in understanding and implementing treatment with high doses, and support from radiation physicists, dosimetrists, and therapists.
Because of the inherent concern over normal tissue toxicity in the setting of fractionated radiotherapy, let alone SABR, anatomic considerations are paramount in deciding the practicality of using SABR for malignancy. As a regenerative and generally tolerable organ with known areas of high-dose radiation susceptibility, the liver has become a more applicable target for SABR. From the earliest days of radiosurgery, treatment of CNS malignancies has been standard. It has become apparent, however, that a leap in treatment paradigms has been evolving with the use of SABR for early-stage non–small cell lung cancer (NSCLC) in both the medically inoperable and borderline operable settings. This article provides a discussion of the indications, rationale, and methods of treating NSCLCs with SABR, and presents information that can be appreciated by the general medical community.
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