Purpose: To assess uncertainty in deformable image registration (DIR) based dose accumulation, we estimate the inverse consistency error (ICE) based on a DVH overlay method as applied to daily dose accumulation for prostate cancer cases.
Methods: Dose accumulations of five fractions for ten prostate cancer cases treated with daily MVCT IGRT were performed by applying two commonly used DIR algorithms: a normalized correlation coefficient technique (NCC-DIR) and a free-form intensity-based registration (IB-DIR). Regions of interest (ROIs) included the PTV (prostate or prostate bed), bladder, and rectum. ICE was estimated using a DVH overlay technique, where a DIR applied between the plan CT and daily MVCT was used to forward-transfer the plan ROIs to the MVCT and inverse-transfer the daily dose to the plan CT. For the five summed daily doses, ICE was calculated as the weighted sum of the separation between the overlaid DVHs from the plan CT and daily MVCT for each fraction.
Results: ICE for the PTV V(100%Rx) is similar for both algorithms, 1.6±1.6% for IB-DIR and 1.2±0.6 for NCC-DIR. For the bladder and rectum V(70%Rx), ICE is on average greater for IB-DIR (3.6% and 3.3%, respectively) than for NCC-DIR (1.9% for both). Among all cases, the maximum value of ICE for PTV V(100%Rx) is 5.6% for IB-DIR and 2.1% for NCC-DIR; the maximum ICE for bladder V(70%Rx) was 6.3% for IB-DIR and 2.6% for NCC-DIR; and for rectum V(70%Rx) the maximum ICE was 6.3% for IB-DIR and 3.9% for NCC-DIR.
Conclusion: We demonstrated that the inverse consistency error, a contributor to the overall uncertainty in DIR-based dose accumulation, can be estimated using the DVH overlay technique. The data obtained presently for prostate cancer cases indicate that caution should be exercised in using the dose accumulation for clinical decision making if ICE exceeds pre-defined thresholds.
Dose Volume Histograms, Registration, Prostate Therapy