(Saturday, 4/4/2020) 10:30 AM - 12:30 PM [Mountain Time (GMT-6)]
Purpose: Radiation-induced heart disease is linked to radiation dose to cardiac substructures that are not visible on treatment planning CTs. This work sought to evaluate the potential for leveraging the soft-tissue contrast from MR-guidance and the integration of daily MR-guided adaptive radiation therapy (MRgART) to further improve cardiac substructure sparing.
Methods: Four thoracic stereotactic body radiation therapy patients treated on a 0.35T MR-Linac were retrospectively evaluated. A hybrid MR/CT cardiac substructure segmentation atlas propagated 13 substructures to 0.35T MRI datasets. Manual contour modifications were performed as needed by radiation oncologists. Clinical step-and-shoot IMRT MRI-primary plans were first re-optimized to reduce substructure doses while maintaining target coverage and sparing organs at risk (OARs). Using a research workstation, MRgART workflow simulations were conducted for each fraction (n=15) after performing MR-based tumor alignment. Initial plan dose was predicted on the daily MRI. MRgART was performed if target coverage (=10%) or OAR sparing could be increased. Maximum and mean doses were evaluated for the left anterior descending arteries (LADAs), great vessels, atria, ventricles, coronary arteries, and chambers, as well as the left ventricular volume receiving 5Gy (LV-V5).
Results: Thirteen of 15 fractions benefitted from MRgART (4 due to a 10% increase in target coverage, 9 due to reducing OAR doses). LV-V5 was reduced by >7% in all fractions for Patient 4. LADA maximum dose was reduced >1.5 Gy for all Patient 3 fractions. MRgART was beneficial for 3/4 fractions for Patient 2 due to pulmonary artery dose reduction. Negligible increase in delivery time was found for 6/13 re-optimized plans (<100 MU increase between base and adapted plan).
Conclusion: Sensitive cardiac substructures can be spared by leveraging MR image guidance with further sparing possible via MRgART. With validation in a larger cohort, it is suggested that robust cardiac substructure sparing will mitigate radiation-related cardiac toxicities.
Funding Support, Disclosures, and Conflict of Interest: Carri Glide-Hurst discloses travel and honorarium from ViewRay, Inc., Modus Medical, and Philips Healthcare for speaking engagements and research agreements with Philips Healthcare. Dr. Glide-Hurst is the PI for National Cancer Institute (NCI) R01CA204189.
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