Incorporating uncertainty bounds in daily deformable dose accumulation for adaptive radiation therapy of head‐and‐neck cancer
Authors: García‐Alvarez Juan A. , Zhong Hualiang , Schultz Christopher J. , Li X. Allen , Kainz Kristofer 
Published: November 2022

Background

The widespread use of deformable dose accumulation (DDA) in adaptive radiotherapy (ART) has been limited due to the lack of clinically compatible methods to consider its related uncertainties.

Purpose

We estimate dose reconstruction uncertainties in daily DDA during CT‐guided radiotherapy of head‐and‐neck cancer (HNC). We project confidence intervals of cumulative dose–volume parameters to the parotids and determine threshold values to guide clinical decision‐making in ART.

Methods

Doses from daily images (megavoltage CTs [MVCTs]) of 20 HNC patients treated with tomotherapy were reconstructed and accumulated in the planning CT (PCT) utilizing a commercial DDA algorithm (PreciseART, Accuray, Inc.). For each mapped fraction, we warped the planning contours to the MVCT. Dose–volume histograms (DVHs) calculated in the MVCT (with warped contour and native dose) and the PCT (with native contour and mapped dose) were compared; the observed inconsistencies were associated with dose reconstruction errors. We derived uncertainty bounds for the transferred dose to voxels within the structure of interest in the PCT. The confidence intervals of cumulative dose–volume parameters were mid‐treatment projected and evaluated as predictors of the end of treatment cumulative metrics. The need for plan adaptation was tested by comparing the projected uncertainty bounds with the treatment constraint points.

Results

Among all cases, the uncertainty in mean values of daily dose distributions mapped to the reference parotid's contours averaged between 2.8% and 3.8% of typical single fraction planning values and less than 1% for the planning target volume (PTV) D95%. These daily inconsistencies were higher in the ipsilateral compared to the contralateral parotid and increased toward the end of treatment. The magnitude of the uncertainty bounds for the cumulative treatment mean dose, D50%, and V20 Gy to the parotids, and PTV D95% were on average 3.5%, 6.6%, 4.6%, and 0.4% of the planned or prescribed values, with confidence intervals of 97.1%–107.0%, 98.2%–110.4%, 95.6%–111.1%, and 98.2%–100.2% respectively. The uncertainty intervals projected at mid‐treatment intersected with the end of treatment bounds in 82% of the parotid's metrics; half of them presented an overlapping percentage greater than 60%. In five patients, the cumulative mean doses were projected at mid‐treatment to exceed the total treatment constraint point by at least 3%; this threshold was exceeded at the end of treatment in the five cases. Underdosing was projected in only one case; the cumulative PTV D95% at the end of treatment was below the clinical threshold.

Conclusion

Uncertainty bounds were incorporated into the results of a commercial DDA tool. The cohort's statistics showed that the parotids’ cumulative DVH metrics frequently exceeded the planning values if confidence intervals were included. Most of the uncertainty bounds of the PTV metrics were kept within the clinical thresholds. We verified that mid‐treatment violation projections led to exceeding the constraint point at the end of the treatment. Based on a 3% threshold, approximately one fourth of the patients are expected to be replanned at mid‐treatment for parotids sparing during HNC radiotherapy.