Since 2014, 219 lung VMAT SABR patients have been treated at our centre. However several patients could not have a plan created that met the ROSEL criteria, leading to delays in starting alternative treatment. The purpose of this study was to investigate at what lung density and GTV volume it is likely to be difficult to reach a clinically acceptable SABR plan. The density of lung tissue around a tumour and the gross tumour volume (GTV) diameter were varied independently within clinically representative ranges, using benchmark CT data from the CtE lung SABR study. Using Monaco, plans were produced by varying objectives and constraints to attempt to create acceptable plans. Low lung densities often correspond to patients with severe emphysema, indicating that those patients may not be suitable for SABR. This study identifies physical parameters that make a patient unsuitable for SABR treatments, allowing for alternatives to be discussed and making planning more efficient. Two recommendations were made as a result of this study: to seek advice before planning when the mean lung density around the tumour is less than 0.06g/cm3, or if the GTV volume is greater than 1.6% of the volume of the lungs, as an acceptable plan may not be possible. Knowing the limit of our planning system means we can optimise the time spent on plans without unnecessarily delaying our patients’ treatments. Clinicians can also manage the expectations of the patient at the start of the process so there are fewer delays and disappointments for them.