First clinical investigation of CBCT and deformable registration for adaptive proton therapy of lung cancer
Résumé
Purpose: An adaptive proton therapy workflow using cone-beam computed tomography (CBCT) is
proposed. It consists of an online evaluation of a fast range-corrected dose distribution based on a
virtual CT (vCT). This may be followed by more accurate offline dose recalculation on the vCT
which may trigger a rescan CT (rCT) for replanning.
Methods and Materials: The workflow was tested retrospectively on twenty consecutive lung cancer
patients. A diffeomorphic Morphons algorithm was used to generate the lung vCT, by deforming the
average planning CT (pCT) onto the CBCT. An additional correction step was applied to account for
anatomical modifications that cannot be modeled by deformation alone. A set of clinical indicators for
replanning were generated based on water equivalent thickness (WET) and dose statistics, and
compared to those obtained on a rCT. The fast dose approximation consisted of warping the initial
planned dose onto the vCT based on changes in WET. Potential under/over-ranges were assessed as
variation in WET at the target’s distal surface.
Results: The range-corrected dose from the vCT reproduced similar clinical indicators as the rCT.
The workflow performed well under different clinical scenarios: atelectasis, lung reinflation and
different types of tumor response. Between vCT and rCT, we found a difference in the measured 95%
percentile of the over-ranges distribution of 3.4±2.7mm. The limitations of the technique consisted of
inherent uncertainties of deformable registration and drawbacks of CBCT imaging. The correction
step was adequate when gross errors occurred but could not recover subtle anatomical or density
changes in tumors with complex topology.
Conclusions: A proton therapy workflow based on CBCT provided similar clinical indicators as rCT
on lung patients with considerable anatomical changes.
Domaines
Physique Médicale [physics.med-ph]
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