SU‐D‐WAB‐05: Image‐Based Proton Range Verification Using Intensity‐Corrected CBCT

Purpose: Using CBCT to detect variations in proton beam path prior to treatment is an important step in the quality assurance of proton therapy. However, the Hounsfield Units (HUs) from a CBCT differs slightly from a simulation CT, thereby introducing additional uncertainties in the conversion from HUs to proton stopping power. The aim of this work is to demonstrate the feasibility of using deformable image registration to map simulation CT HUs onto a treatment CBCT for range verification during proton therapy. Methods: A simulation CT and two simulated proton CBCT of a head and neck patient were used in this study. We used a diffeomorphic deformable image registration algorithm with symmetric normalization in conjunction with cross correlation similarity metrics (ANTs: Advanced Normalization Tools) to deform the simulation CT onto the CBCTs. Polar plots of proton beam water equivalent thickness (WET) were generated using a ray tracing algorithm at different gantry angles in the unmodified CBCT, corrected CBCT and simulation CT for analysis. Results: With no anatomical variation, corrected CBCT and simulation CT image exhibited excellent agreement in WETs (mean difference of 1.26 mm, interquartile range 0.42∼1.79mm). In the unmodified CBCT image, larger difference in WETs (mean value 3.38 mm, interquartile range 1.90∼5.03mm) attributed to errors in HU to stopping power conversion were observed. With tumor shrinkage the differences in the WET polar plots between planning CT and corrected CBCT correlated with the location of anatomical change in the nasal/oral cavity, indicating potential deviations in proton dose deposition. Conclusion: It is feasible to use deformable image registration to correct CBCT HUs for image based proton range verification. Polar plots of deviations in WET between CBCT and simulation CT is potentially an efficient and practical quality assurance tool in the use of CBCT for proton therapy applications.