SU‐E‐J‐149: Heart Protection in Proton Therapy Using 4D Motion Analysis Based On Registration Between SENSE MRI and 4DCT

Purpose: Proton treatment on breast cancer requires optimal block on surrounding OARs including the heart to avoid radiation complications. The radiation dose to the heart is difficult to quantify since the heart moves under the influence of the respiratory motion and the cardiac motion; and as part of the cardiac motion the myocardium around the left ventricle rotates during the cardiac cycle. This study is to investigate the actual heart dose distribution by generating a 4D heart motion model through registration between SENSE MRI and planning CT. Methods: SENSE MRI has been utilized to acquire ECG gated 4D heart images for 4 patients. The slice thickness is 8mm, and the in plane spatial resolution is 1.4mm. 20 phases of MRI have been reconstructed to cover the whole cardiac cycle. The short axis (SA) images have been used to generate a 4D motion model through non‐rigid registration between phases. Anatomic land markers such as the papillary muscles and LV‐RV junctions have been used to constrain the registration to quantify the myocardial rotation. The reference MRI frame (end of systole) was registered to the treatment planning CT through model based meshless registration to translate the 3D dose distribution into the MRI image space and then the heart dose at different cardiac phases were derived based on registration results. Plan‐optimization was conducted to achieve optimal heart block. Results: The phase to phase registration demonstrated accuracy in the reconstructed motion. The average distance error at land markers is less than 1.5mm for two patients. The MRI‐to CT registration also has a satisfactory performance but still has the potential to be improved. A 4D dose reconstruction is possible given all the registration results. Conclusion: Based on the registration, 4D heart dose distribution can be generated to enable plan optimization for proton radiotherapy on breast cancer. © 2013, American Association of Physicists in Medicine. All rights reserved