Investigating deformable image registration and scatter correction for CBCT-based dose calculation in adaptive IMPT

Area of Science:

• Medical Physics
• Radiotherapy Oncology
• Image-Guided Therapy

Background:

• Accurate dose calculation is crucial for adaptive intensity modulated proton therapy (IMPT).
• Cone-beam x-ray computed tomography (CBCT) is used for image guidance in IMPT.
• Intensity correction and scatter reduction in CBCT are essential for reliable dose calculations.

Purpose of the Study:

• To investigate intensity-corrected CBCT images for accurate dose calculation in adaptive IMPT for prostate and head and neck (H&N) cancer.
• To compare a deformable image registration (DIR)-based method with a scatter correction approach using DIR-derived data as prior.
• To evaluate these methods on a clinical patient cohort for the first time.

Main Methods:

• Utilized planning CT (pCT) and daily CBCT data from H&N and prostate cancer patients.
• Employed a validated Morphons algorithm for DIR to create a virtual CT (vCT).
• Applied DIR-warped pCT as prior for scatter correction of CBCT projections and performed dose calculations on vCT and scatter-corrected CBCT (CBCT_cor).

Main Results:

• High agreement between vCT and CBCT_cor in proton range and gamma-index analysis (96-100% pass rate for 2%,2mm).
• For H&N cases, vCT and CBCT_cor showed agreement comparable to reference replanning CT (rpCT).
• Prostate cases revealed insufficient vCT contour accuracy from DIR, while CBCT_cor contours closely matched raw CBCT delineations.

Conclusions:

• For H&N cancer, vCT and CBCT_cor showed no significant differences, suitable for adaptive IMPT.
• For prostate cancer, DIR-derived vCT contours were inaccurate due to anatomical changes and low soft-tissue contrast in CBCT.
• Scatter correction of CBCT projections, using vCT as prior, overcomes contour inaccuracies, enabling precise delineation and dose calculation for adaptive IMPT.