Heart Dose Reduction for Radiation Treatments of Left-Sided Breast Cancer by Using 4D Simulation and 4D Verification

Abstract

Purpose/Objective: Incidental heart irradiation during tangential photon therapy in patients with left-sided breast cancer can lead to cardiac abnormalities. The radiation dose to the heart may be different at various phases of the respiratory cycles due to the changes in the heart position. In this study, respiratory gated 4D-CT scans were performed and dose to the heart was determined at different phases of the respiratory cycle. Respiratory gated treatment was used to minimize the heart dose. Treatments were verified with a 4D on-board-imaging (OBI) and cine mode Portal Vision system. Materials/Methods: A GE Lightspeed CT scanner and Varian RPM respiratory gating system were used for the 4D-CT scanning. The position of an infrared marker placed on the patient was monitored by an in-room infrared camera, thereby assessing the phase of the respiratory cycle. The respiratory cycle was divided into 10 equally sized groups based the respiratory phase from 0% (inspiration) to 100%. A cine CT scan was performed and the resultant images were re-sorted to reconstruct the corresponding 3D image sets for 10 different respiratory phases. The images were transferred to an Eclipse planning station. A tangential field treatment plan was created. The heart position was defined for each respiratory phase. The volume of the heart within the 50% isodose line and the mean heart dose were calculated based on the 3D images for each phase within the respiratory cycle. A respiratory gated treatment plan in which the beam is only turned on when the separation between the heart and left breast reaches maximum was generated to minimize the heart dose. The OBI system with a kV x-ray tube and a kV imaging panel on each side of the gantry was installed. The OBI system also interacts with the respiratory gating signals. The gated OBI and cine mode Portal Vision system were used for 4D verification. Results: With 4D simulation, the heart position with respect to the tangential breast fields was found to be different at each respiration phase. The separation between the heart and left breast is maximum in the inspiration phase. Deep breathing can significantly increase the separation. During the expiration phase, the heart moved towards left as shown in the axial images and superior shown in the sagittal images. The total volume of the heart within the 50% isodose line was calculated for each respiratory phase. It shows a minimum at the inspiration phase and a maximum at the expiration phase. The mean heart dose was also calculated for each respiratory phase and also shows a minimum at the inspiration phase and a maximum at the expiration phase. The pre-treatment OBI fluoroscopy and 4D respiratory gated OBI radiographic images confirmed the gated treatment anatomy in the treatment room. Cine mode Portal Vision images were acquired during the treatment, and showed that the gated treatment matched with the respiratory phase as selected during the 4D simulation. Conclusions: The cardiac doses received during left-sided breast/chest-wall radiation therapy are dependent on the phase of the respiratory cycle. Gating therapy to the respiratory cycle is an efficient way to reduce the risk of radiation-induced heart injury. The gated OBI and cine mode Portal Vision system can accurately verify the respiratory gated treatments.