RT EPID 3D uses the same algorithm for both pre-treatment and in vivo verification, allowing you to compare in vivo dose with pre-treatment dose for each delivered fraction. This makes it much easier for you to detect patient-related errors, such as anatomical changes and/or setup issues, which are specific to each fraction, and to better estimate their dosimetric impact.
In the figure on the left, it can be seen how the difference between in vivo and pre-treatment (virtual) patient results becomes larger as the changes in lung density increase. After the result of the 3rd in vivo verification the radiation oncologist was consulted.
Source: Olaciregui-Ruiz et al, Phys Med. 2017 May;37:49-57, doi: 10.1016/j.ejmp.2017.04.016