Feasibility of prostate offline adaptive radiation therapy using iterative reconstructed CBCT to optimize the ART workflow

Background Adaptive Radiotherapy (ART) updates treatment plans based on newly acquired anatomical information to maintain dosimetric accuracy. Image-Guided Radiotherapy (IGRT) is essential for detecting anatomical changes and guiding replanning decisions.Purpose This study evaluates the feasibility of using iterative Cone Beam CT (iCBCT) acquired on a TrueBeam system as a reference image dataset for dose calculation and offline ART in prostate radiotherapy. The approach avoids additional simulation CTs and reduces uncertainties from deformable image registration (DIR).Materials and methods A CIRS electron-density phantom was embedded in solid-water slabs and surrounded by bolus to simulate varying patient-scanned volumes. HU-density calibration curves were generated for a standard prostate patient volume (sHU-density) and volume-specific conditions (vHU-density) and imported into Eclipse planning system along with the planning (pCT) calibration curve (pHU-density). Twenty prostate VMAT plans were retrospectively recalculated on registered iCBCT images using these curves. Dosimetric parameters (Dmean, D98%, D2%) and 3D gamma analysis (1%/1 mm, 10% threshold) assessed dose differences. An Alderson Rando phantom study was used to compared pCT-based (pHU-density) and iCBCT-based (sHU-density) calculations.Results HU-density curves were stable across scanned volumes, particularly for soft tissues. Dosimetric differences between sHU-density and vHU-density curves were  99%. Using the pHU-density curve on iCBCT introduced 1% systematic dose deviations. Phantom measurements confirmed strong agreement between pCT- and iCBCT-based calculations using an iCBCT-specific calibration curve.Conclusion iCBCT with a dedicated HU-density calibration curve provides dose calculation accuracy comparable to planning CT, supporting reliable offline ART in prostate radiotherapy.