Stereotactic ablative radiotherapy for lung cancer

Stereotactic ablative radiotherapy (SABR), which is also referred to as stereotactic body radiotherapy (SBRT), is defined as “the precise irradiation of an image defined extra-cranial lesion associated with the use of high radiation dose in a small number of fractions”. SABR is a relatively recent advance in the treatment of small peripheral non-small cell lung tumours. The concept of SABR was originally proposed in the early 1990s by the Karolinska Institute in Stockholm, and the underlying principles are those of intracranial radiosurgery, applied in an extra-cranial site. Conventional radiotherapy for lung cancer usually involves 20 to 30 daily radiotherapy fractions, each delivering a dose of around 2 to 2.75Gy. Such a dose is designed to try to kill tumour cells without causing excessive damage to the surrounding normal cells of the lung and mediastinum. Unfortunately, because of the need to respect the surrounding normal tissues, the dose received by the tumour is insufficient to cure the vast majority of cases. In fact, a standard dose of 60Gy in 30 fractions is predicted to only control about 30% of lung cancers at three years. Delivering higher doses could improve tumour control but doing this using conventional treatment methods would result in excessive normal tissue toxicity. SABR employs a small number of high dose fraction treatments, such as 54Gy in three fractions. The higher dose per fraction means that the biological effect of such treatments is much greater than conventional radiotherapy, and is equivalent to approximately 96 to 126Gy in 2Gy fraction treatments. Doses around this level are predicted to result in three-year control levels in excess of 85%, and this is what is observed clinically, and is thus a vast improvement on what has been achieved using conventional doses and fractionations. The high dose per fraction delivered with SABR, however, means that there are specific requirements for SABR to be delivered safely. The consequences of inaccurate SABR delivery could result in significantly reduced tumour control as well as excessive normal tissue toxicity. The delivery of SABR doses is not possible using the same margins around a tumour as are used with conventional fractionation (ie 1.5cm to 2cm) as this would result in too much normal tissue irradiation. As such, smaller treatment margins are required and clearly where smaller margins are used, a robust method of image guidance is essential. The specific considerations for contouring, planning and delivering SABR are considered in greater detail below.