How do you do immobilization for liver SBRT?

There are 2 components of internal organ motion to be concerned with: Inter-fraction motion and intra-fraction motion. Most people have  good solutions for respiratory motion control and external immobilization (intra-fraction motion). It's difficult to compare between them because people only know their solution well, and do not know other solutions as well. So that's why everyone thinks their solution is the best and they are probably right because it works well for them. The bottom line is that the most important thing is to have a solution and be consistent with it so your team is very good at it. In general, abdominal compression can reduce respiratory motion to about 5 mm, deep inspiration breath hold is more like 2 mm or less, and end expiratory gating is similar to deep inspiration breath hold. We use all of these, but use DIBH as our preferred solution, and end expiratory gating as our backup for those that can't do it. We use abdominal compression only for liver and lung cases for the following important reason.

The use of abdominal compression in the upper abdomen is problematic if you are considering the treatment of pancreatic cancer for instance because it compresses the organs into  the tumor target. We give 100 Gray BED in 75Gy in 25 fractions with millimeters separation between the duodenum and stomach, and do not want these organs to be compressed into the treatment volume. For those that give low-dose palliative SBRT (33Gy in 5 fx, BED 53Gy), compression is probably okay because that dose is too low to cause harm. It is also too low to help patients much. In contrast, abdominal compression does not compress visceral organs into the liver as much, except for maybe the left lobe, so it can be an easy solution for those cases.

The inter-fraction solutions for organ motion are related to image guidance and adaptive planning to account for day to day visceral organ motion. We use deep inspiration breath-hold cone beam CT and will be using MR linear accelerator for our IGRT, and rely on increased numbers of fractions up to 25 to randomize the motion of the stomach, jejunum, colon, and duodenum. When the motion is not random we do an adaptive plan to ensure the safe delivery of 100Gy BED (10% of cases).