Can Epid-Based in Vivo Dosimetry Reliably Trigger Adaptive Replanning? A Comparative Analysis with Daily CBCT.
Abstract
Purpose
Anatomical variations during photon radiotherapy often cause discrepancies between planned and delivered doses. This study evaluates the clinical utility of combining EPID-based transmission in vivo dosimetry (IVD) with daily cone-beam CT (CBCT) review as a decision-support tool for adaptive radiotherapy (ART).
Methods
Adaptive plans from a 24-month period were retrospectively reviewed for IMRT/VMAT patients. Weekly 2D transit EPID measurements were analyzed using global gamma analysis (5%/3mm) relative to the predicted dose from the original treatment plan. Gamma pass-rate trends were correlated with CBCT-identified changes, such as external contour variation or target deformation. Cases with persistent gamma degradation were flagged for physician review to determine replanning necessity.
Results
The combination of IVD and CBCT provided a complementary workflow for identifying the need for adaptation, though sensitivities were treatment site dependent. Gamma pass-rate degradation was most sensitive in lung cases, correlating strongly with density changes. Conversely, IVD did not consistently detect anatomical variations in H&N treatments; nodal regression and lateral shrinkage were visible on CBCT without triggering significant gamma degradation given the clinic's selected gamma criteria. Consequently, CBCT review proved invaluable for identifying PTV deformation that IVD missed. No pelvic cases required ART based on IVD triggers.
Conclusion
Integrating EPID-based IVD with CBCT provides a robust framework for identifying plans requiring adaptation. While IVD is a powerful tool in combination with CBCT for all sites - particularly for lung treatments - it should supplement rather than replace daily CBCT image review, which remains essential for verifying volumetric target changes. Future protocols should consider site-specific gamma criteria and passing thresholds to improve trigger sensitivity.