Abstract
BackgroundDuring therapeutic beam irradiation, an unvisualized three-dimensional (3D) target position should be estimated using an external surrogate with an estimation model. Training periods for the developed model with no additional imaging during beam irradiation were optimized using clinical data.MethodsDual-source 4D-CBCT projection data for 20 lung cancer patients were used for validation. Each patient underwent one to three scans. The actual target positions of each scan were equally divided into two equal parts: one for the modeling and the other for the validating session. A quadratic target position estimation equation was constructed during the modeling session. Various training periods for the session—i.e., modeling periods (TM)—were employed: TM ∈ {5,10,15,25,35} [s]. First, the equation was used to estimate target positions in the validating session of the same scan (intra-scan estimations). Second, the equation was then used to estimate target positions in the validating session of another temporally different scan (inter-scan estimations). The baseline drift of the surrogate and target between scans was corrected. Various training periods for the baseline drift correction—i.e., correction periods (TCs)—were employed: TC ∈ {5,10,15; TC ≤ TM} [s]. Evaluations were conducted with and without the correction. The difference between the actual and estimated target positions was evaluated by the root-mean-square error (RMSE).ResultsThe range of mean respiratory period and 3D motion amplitude of the target was 2.4–13.0 s and 2.8–34.2 mm, respectively. On intra-scan estimation, the median 3D RMSE was within 1.5–2.1 mm, supported by previous studies. On inter-scan estimation, median elapsed time between scans was 10.1 min. All TMs exhibited 75th percentile 3D RMSEs of 5.0–6.4 mm due to baseline drift of the surrogate and the target. After the correction, those for each TMs fell by 1.4–2.3 mm. The median 3D RMSE for both the 10-s TM and the TC period was 2.4 mm, which plateaued when the two training periods exceeded 10 s.ConclusionsA widely-applicable estimation model for the 3D target positions during beam irradiation was developed. The optimal TM and TC for the model were both 10 s, to allow for more than one respiratory cycle.Trial registrationUMIN000014825. Registered: 11 August 2014.
Highlights
During therapeutic beam irradiation, an unvisualized three-dimensional (3D) target position should be estimated using an external surrogate with an estimation model
The root-mean-square error (RMSE) in the SI direction were associated with larger errors than those of the other directions
The median RMSEs in the LR and AP directions were < 1.0 mm
Summary
An unvisualized three-dimensional (3D) target position should be estimated using an external surrogate with an estimation model. Optimal detection of time-resolved 3D position involves the triangulation of two simultaneous two-dimensional (2D) projections using pre-defined camera parameters [7]. Detection of 3D target positions using a monoscopic view is possible with techniques such as kV intra-fraction monitoring [8, 9] and marker-less tumor detection [10,11,12]. These methods are inapplicable if the target or tumor cannot be detected via kV projections at certain gantry angles
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