Geometrical and dosimetrical uncertainties in hypofractionated radiotherapy of the lung: A review.

The application of high precision hypofractionated regimes (a.k.a. stereotactic body radiotherapy (SBRT)) to the treatment of lung cancer is a 'success story' of radiotherapy. From the technical perspective, lung SBRT is a challenging technique as all aspects of the treatment workflow, from imaging to dose calculation to treatment delivery, should be carefully handled in order to ensure consistency between planned and delivered dose. In this review such technical aspects are presented and discussed, looking at what has been developed over the years. The use of imaging techniques such as slow-CT, breath-hold CT, four-dimensional CT and mid-ventilation is reviewed, presenting the main characteristics of each approach but not necessarily to single out 'the best' solution. Concerning dose calculation, a number of studies clearly separate dose algorithms that should be considered inadequate for lung SBRT (e.g. simple pencil beam algorithms) from approaches such as convolution algorithms, Monte Carlo, and solution of the transport equation, that are much better at handling the combination of small fields and heterogenenous geometries that make dose calculation not trivial. Patient positioning and management of intrafraction motion have been two areas of significant developments, to the point where it is difficult to identify which solution represents the best compromise between technical complexity and clinical effectiveness. The review analyses several of these methods, outlining the residual uncertainties associated with each of them. Last but not least, two subjects are discussed, adaptive therapy and particle therapy, that may represent in the near future additional tools for the technical improvement of lung SBRT.