What do you do to keep patients within the 2mm tolerance? We use chest compression, and even with a 3mm tolerance, I would say most patients exceed this tolerance at some point with their breathing pattern. Once in a while we will have a patient with excursions upward of 5mm in their natural breathing pattern (even with compression). As long as their breathing is such that they are within that 3mm threshold for a large enough fraction of their breath cycle, we will continue the treatment. If they do not return to 0 or inside that 3mm threshold at some point in their breath cycle, we will re-CBCT. We use 4DCTsimulation, and use a 5mm expansion on the ITV for our PTV.