Room: AAPM ePoster Library
Purpose: Whole-heart dose metrics may not be as strongly linked to late cardiac morbidities as radiation doses to individual cardiac substructures. However, effective sparing of substructures may be complicated by the presence of respiratory motion. This work quantifies substructure excursion during respiration to identify dominant axes of motion.
Methods: Eleven patients with thoracic cancer treatments underwent 4-phase (n=8) or 10-phase (n=3) non-contrast 4-dimensional CTs. Cardiac gated T2-weighted MRIs were also acquired at end-exhalation (EE). The EE phase of the 4DCT (50% phase) was rigidly registered with the MRI and the registration was refined with an assisted alignment surrounding the heart. Thirteen cardiac substructures (e.g. chambers, great vessels, coronary arteries, etc.) were contoured by a radiation oncologist on the 4DCT using hybrid MRI/CT information. Contours were deformed to the other phases via an intensity-based deformable registration for radiation oncologist verification. Centroid locations, volumes, and maximum excursions between phases were evaluated.
Results: Paired t-tests revealed no statistical differences in volume for any substructure between the 0% and 50% phase (p>0.05). Overall, shifts were largest in the superior-inferior axis with the inferior vena cava and the right coronary artery having maximum displacements >6mm. No maximum displacements for any substructure in the left-right or anterior-posterior plane exceeded 5mm, when averaged over substructures. For one patient, left-right shifts exceeded 5mm for every substructure with displacements up to 13mm. Maximum shifts of >4mm occurred 16%, 5%, and 52% of the time in the right-left, anterior-posterior and superior-inferior axes, respectively. Excursion analysis of 4-phase 4DCTs (n=8) revealed that 52% and 28% of maximum excursion occurred in the 0-50% and 0-75% trajectories, respectively.
Conclusion: This work suggests non-isotropic planning organ at risk volumes may be necessary for certain substructures when treated under free-breathing conditions. Future work to determine the dosimetric effect of sensitive cardiac substructure motion is warranted.
Funding Support, Disclosures, and Conflict of Interest: The submitting institution holds research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA204189. The PI is on the Philips Healthcare Advisory Board.