Potential for Sensitive Cardiac Substructure Sparing Using MR-Guided Radiation Therapy
Aldridge K, Morris ED, Ghanem AI, Zhu S, Movsas B, Chetty IJ, and Glide-Hurst C. Potential for Sensitive Cardiac Substructure Sparing Using MR-Guided Radiation Therapy. Int J Radiat Oncol Biol Phys 2019; 105(1):E728.
Int J Radiat Oncol Biol Phys
Purpose/Objective(s): Mounting evidence suggests that cardiac substructures are highly radiosensitive. However, these structures are not typically considered in treatment planning because they are not readily visualized on treatment planning CTs (TPCTs). This work sought to evaluate the potential of integrating soft tissue contrast provided by MR-guided radiation therapy for highly effective cardiac substructure sparing. Materials/Methods: A retrospective evaluation was conducted of 10 patients (11 lesions) in the upper thoracic region treated at varied breathing states (4 end-exhalation, 6 end-inhalation, 1 free-breathing) on a 0.35T MR-linac. A hybrid MR/CT cardiac substructure segmentation atlas propagated 13 cardiac substructures to TPCTs. Contour modifications were performed by radiation oncologists using registered 0.35T MRIs to elucidate substructures. Clinical step and shoot IMRT treatment plans were reoptimized to reduce substructure doses while maintaining target coverage and sparing other organs at risk. Monte Carlo dose calculations were performed on the TPCTs. Equivalent dose conversion to 2 Gy fractions (EQD2, α/β = 2) was used for dosimetric assessment including mean heart dose (MHD) and left ventricular volume receiving 5Gy (LV-V5). Dose to 0.03 cc (D0.03cc, a surrogate for maximum dose) and mean doses were evaluated for the left anterior descending artery (LADA), coronary arteries, and ventricles. As metrics of plan complexity, total monitor units (MUs) and treatment time were evaluated between planning approaches. Plans were evaluated for beam arrangement modification to further improve sparing. Results: Cardiac sparing plans reduced the MHD (0.8 ± 0.7, range: 0.1 to 2.5 Gy). Reoptimized cardiac sparing plans reduced LADAmean and LADA0.03cc (4.8-51.1% and 0.1 to 14.4 Gy, respectively). LV0.03cc was reduced >1 Gy for 7 patients while large reductions (>7%) in LV-V5 were observed in 4 cases. Left atrial mean dose was equivalent/reduced in all sparing plans (mean reduction 1.1 ± 1.4 Gy). The left main coronary artery was better spared in all cases for mean dose and D0.03cc. A left lung cancer patient exhibited >10 Gy reduction in D0.03cc to the LV, heart, left atrium, and pulmonary veins with sparing. Negligible increase in estimated delivery time was found with reoptimized plans (0.1 ± 1.3 min) with 7/11 plans having <100 MU change. New beam arrangements for preferential cardiac sparing were possible for 4 patients, leading to further reductions of dose metrics. Conclusion: By introducing cardiac substructures into treatment planning in the setting of MR-guided RT, an opportunity is presented for more effective sparing. Similar delivery times and MUs were observed between planning strategies, suggesting limited increase in plan complexity with cardiac sparing. Validation in a larger cohort with appropriate margins offers potential to reduce radiation-related cardiac toxicities.