Purpose: Deformable image registration (DIR) based dose accumulation is essential for adaptive radiation therapy (ART) and can be subjective to errors. We accumulate daily doses based on DIR and estimate their uncertainties during RT for head and neck cancer (HNC).
Methods: A commercial ART tool (PreciseART, Accuray, Inc.) was used to reconstruct daily doses and accumulate them via DIR for ten HNC patients treated with helical tomotherapy. The accuracy of dose accumulation was assessed through a consistency check that comprises: i) warping the structures defined in the planning kVCT to the daily MVCT, ii) mapping the dose calculated in the MVCT to the kVCT, iii) calculating and overlaying DVHs from both images. At a given volume level, the absolute difference between the doses computed in the DVH overlays was considered the leading uncertainty component of the assessed metric. By relating the evaluated dose/volume level in DVHs of both single fractions and cumulative distributions, daily uncertainties were propagated to estimate confidence intervals in the accumulated distribution.
Results: Accumulated doses differed from planned values, e.g., the changes in PTV-D95%, larynx-D33%, esophagus-D50%, and ipsilateral parotid gland-D50% ranged from 97 to 101%, 90-111%, 63-106%, and 76-165%, respectively. For 87% of the regions-of-interest and metrics studied, uncertainties were less than 2% of the planned value, including all points of the PTV (D95%) and larynx (D33% and mean dose). For D50% in the esophagus and parotids, 24% of the points presented uncertainties from 3.5% to 5.5% of the accumulated dose; a maximum uncertainty of 20% was observed for the esophagus.
Conclusion: The DIR-related dose reconstruction errors were estimated for HNC patients. Relative uncertainties can be greater than 5% for structures undergoing significant deformations during radiotherapy (around 20% volume change), e.g., the esophagus and parotids. In ART, both daily dose accumulation and its uncertainty should be considered.