What ITV to PTV expansions do you use for free-breathing NSCLC SBRT using CT sim with 4DCT?  

5mm concentric PTV margins. Even with daily IGRT and fluoro, I find 3mm to be very tight and adding a little more margin does not change the OAR doses much unless PTV is touching. If 4DCT not available in sim, I do 7mm sup/inf and 5mm in other directions. I do not specifically add a CTV margin, but...

To be a stickler for terminology, PTV expansions should be based on uncertainty. My default is 5 mm although would consider 3 mm in certain (limited) circumstances. Discuss with your physicists based on your treatment machine, immobilization, and IGRT strategies. You can consider increasing your PTV...

My default expansion from IGTV to PTV is 7 mm concentrically. However, these margins can be customized to as little as 4 mm or as much as 10 mm in certain directions based on several considerations. These include: (1) proximity of the IGTV to chest wall or critical central structures (typically wil...