How do you approach liver SBRT in patients with hepatocellular carcinoma who aren't candidates for surgery or other interventional procedures when the overall liver volume is small making it difficult to achieve liver constraints?

These are tough situations. I have used proton and photons for HCC. In the situation you described, I would generally favor IMPT due to improved low dose distribution and keeping the parenchyma not treated to a minimum while still giving an ablative dose more comfortably. Although the mean liver dose and volume liver spared constraints are useful, our understanding of the ideal metrics is still evolving. For example, how much parenchyma needs to be spared is likely dependent on the underlying liver function and also the patient's size. As a field, we generally have not used volume metrics standardized to body surface area in these decisions. If the lesion is small (<3cm) and accessible, exploring ablation approaches is often reasonable when one is concerned of limited normal parenchyma due to heavy pretreatment, prior resections, etc. 

Whether to treat at all would also depend, of course, on the liver function, performance status, and overall goal (transplant eligible?).