Purpose: Cardiac metastases lesions are invisible on noncontrast CT but visible on MRI scans. We present an MR-based treatment planning workflow with online plan adaptation to treat cardiac metastases with Elekta Unity MR-Linac.
Methods: Two cardiac metastases patients treated with Elekta Unity MR-Linac were enrolled in this study. 4DCT scans and a T2 MRI scan were acquired during simulation. The simulation MRI scan was imported into Monaco treatment planning system for physician to contour treatment target directly. Body and organ-at-risk (OAR) contours were created on average 4DCT scan and then rigidly transferred to simulation MRI scan together with mean electron density of each organ, which will be used by Monaco for synthetic CT creation for plan dose calculation. An 11-beam step-and-shoot IMRT plan was created on the simulation MRI scan as the reference plan. The dose was prescribed to cover at least 95% of the PTV with 30 Gy delivered in 3 fractions. During treatment, the reference plan was adapted to daily MRI scans to create adaptive plans online for treatment delivery.
Results: Intrafraction motion of the cardiac lesion due to respiration was less than 3 mm for both patients based on their 4DCT evaluation. In the reference plan, all dosimetric criteria from planning derivatives were met. The monitor units of the reference plans are 2267 and 2632, respectively. The IMRT QA measurements passed the gamma criteria of 3%/3mm with 100% and 95.8%, respectively. During treatment, inter-fraction anatomy changes were small based on the fusion of daily MRI to simulation MRI. Therefore, the adapt-to-position strategy was chosen to create adaptive plans for treatment. Average treatment time is 30 minutes per fraction.
Conclusion: An MR-based treatment workflow has been developed to treat cardiac metastases with MR-Linac. The treatment was successfully delivered with online plan adaptation.