TH-C-WAB-07: Optimization of Heart Block in the Left-Sided Whole Breast Irradiation.
PURPOSE
Blocks have been used to protect heart from potential radiation damage in left breast treatments. Since cardiac motion pattern may not be fully captured on co nventional 3DCT or 4DCT simulation scans, this study was intended to investigate the optimization of the heart block design taking the cardiac motion into co nsideration.


METHODS
Whole breast treatment plans using two opposed tangential fields were designed based on 4DCT simulation images for 10 left breast patients. Using an OBI system equipped to a Varian Linac, beam-eye viewed fluorosco py images were acquired for each of the treatment beams after patient treatment setup, and the MLC heart blocks were overlaid onto the fluorosco py images with an in-house software package. A non-rigid image registration and tracking algorithm was utilized to track the cardiac motion on the fluorosco py images with minimal manual delineation, and the tracked cardiac motion information was used to optimize the heart block design to minimize the radiation damage to heart while avoiding the over-shielding that may lead to underdosing breast tissues.


RESULTS
Heart moved under the influences of both respiratory and cardiac motion. For 4 out of 10 patients, it was observed that heart moved beyond the heart block into treatment fields. The average size of the heart portion irradiated inside the treatment fields was 0.4 mm (0.4mm-0.4 mm), 3.6 mm (0.8 mm - 5.1 mm), 4.2 mm (0.4mm - 6.3mm), and 3.9 mm (0.2 mm-5.7mm) for the four patients, respectively. The optimization process changed the positions of 1, 8, 10, and 5 MLC leaves (5 mm wide per leaf) respectively to achieve the optimal heart protection. Co nclusion: Simulation 4DCT based heart block design may not provide adequate heart protection for all the treatments. A fluorosco py-based method has been developed to adaptively optimize the heart MLC block to achieve optimal heart protection.