BACKGROUND: With the ultimate aim of reducing time to start radiotherapy treatment in patients with rectal cancer, this study explores the feasibility of omitting a planning CT scan (pCT), by utilizing the diagnostic CT scan (dCT) for treatment planning, with or without plan adaption using online adaptive radiotherapy. METHODS: Fifteen rectal cancer patients, with both dCT and pCT available, were included. Target volumes and organs at risk (OARs) were delineated on both scans, followed by treatment planning based on the dCT contours. Plans were recalculated on the pCT to assess dosimetric differences for target volumes and OARs. Additionally, five patients with HyperSight CBCT scans underwent a similar planning process. An online adaptive treatment workflow was simulated using the Ethos system, where the dCT and its plan served as the reference, and the HyperSight CBCT was used for adaptation. RESULTS: dCT-based plans showed adequate target volume coverage. However, when recalculated on the pCT, median coverage decreased for both CTV and PTV, and OAR doses increased. None of the 15 plans met prescribed constraints without online adaptive radiotherapy. In contrast, for all five patients in the adaptive workflow, the treatment plans met target volume coverage and OAR constraints. CONCLUSION: Using dCT-based treatment planning is feasible for rectal cancer patients but requires at least one online adaptive session. A prospective trial (MEC XXXX-XXXX) is ongoing in patients with rectal cancer, aiming to reduce time to start treatment, by omitting the pCT and using online adaptive radiotherapy workflow.