Lung cancer: a model for implementing stereotactic body radiation therapy into practice

Primary and metastatic tumors to the lung have been principle targets for the noninvasive high-doseper- fraction treatment programs now officially called stereotactic body radiation therapy (SBRT). Highly focused treatment delivery to moving lung targets requires accurate assessment of tumor position throughout the respiratory cycle. Measures to account for this motion, either by tracking (chasing), gating, or inhibition (breath hold and abdominal compression) must be employed in order to avoid large margins of error that would expose uninvolved normal tissues. The treatments use image guidance and related treatment delivery technology for the purpose of escalating the radiation dose to the tumor itself with as little radiation dose to the surrounding normal tissues as possible. Clinical trials have demonstrated superior local control with SBRT as compared with conventionally fractionated radiotherapy. While late toxicity requires further careful assessment, acute and subacute toxicity are remarkably infrequent. Radiographic and local tissue effects consistent with bronchial damage and downstream collapse with fibrosis are common, especially with adequate doses capable of ablating tumor targets. As such, great care must be taken when employing SBRT near the serially functioning central chest structures including the esophagus and major airways. While mechanisms of this injury remain elusive, ongoing prospective trials offer the hope of finding the ideal application for SBRT in treating pulmonary targets.