We film orthogonal sets on Verification day then weekly during out port films. If the distance has changed between the two then the patient may need to take more or less of a breath. We match spine then compare sternum from port to DRR. The difference in sternum will indicated if they need to take a deeper breath or let air out. (tip-if you have a true beam you can take a MV/KV pair. Start with the MV and you can film ap and lat in one breath) We always recapture a new reference image after films and use that reference until the next set of films.