Radiation to the heart during left breast cancer radiotherapy has proven to lead to late cardiac complications. It has been shown that treatment at deep inspiration breath hold (DIBH) reduces radiation to the heart in these treatments. We have implemented a workflow for DIBH treatment of left breast patients using surface imaging and visual feedback for patient simulation, setup, and treatment delivery. The dosimetric impacts to the heart and to the left anterior descending coronary artery (LAD) have been evaluated. Prior to treatment, patients are set up at DIBH based on surface information in order to reproduce the setup of the simulation day, without guaranteeing that the same lung inspiration level will be achieved every day. To study the effect of lung inspiration level variability, we have recreated several inspiration levels using deformable registration between computed tomography (CT) scans at DIBH and free breathing (FB), and compared doses at those inspiration levels. The successful implementation of left breast cancer treatment at DIBH using surface imaging for set up and monitoring purposes is shown in this thesis. Four patients have undergone treatment so far. Treatment takes 16.7 minutes on average for the first four patients, and has continually decreased in time as all have become more familiar with the procedure. Patient compliance to perform a DIBH was excellent and thus far every patient has been able to produce a reproducible and stable DIBH using the visual feedback. An analysis of the CT scans shows that on average the heart was displaced 0.8 cm from the chest wall and 1.5 cm inferiorly in the abdomen from FB to DIBH. A demons algorithm running on the Graphics Processing Unit (GPU) was used to obtain the deformable registration field from the FB and DIBH scans. The vector field was interpolated and extrapolated to recreate scans at 75%, 90%, and 110% inspiration levels, considering FB as 0% and DIBH as 100%. For these scans, the mean dose to the heart was decreased on average by 52%, 59%, 59% and 63% respectively compared to FB, while the volume of the heart receiving 50% of the prescribed dose decreased by 82%, 86%, 88% and 90% for the 75, 90, 100, and 110 percent inspiration levels. On average the mean dose to the LAD was decreased by 48%, 61%, 63%, and 69% for the 75, 90, 100, 110 percent inspiration levels compared to FB. The volume of the LAD receiving 50% of the perspired dose was decreased by 55%, 72%, 74%, and 81% for the 75, 90, 100, 110 percent inspiration levels compared to FB. The successful implementation of a DIBH has already provided a reduction in dose received to the cardiac structures by several patients. It is clear that all will benefit from a DIBH treatment, but the exact extent is patient dependent. Surface imaging has shown to be an effective tool for setup and delivery. Its accuracy for set up and delivery have proven to be superior to previous tattoos and SSD alignment as well as to have exposed previously unknown setup errors in six degrees of freedom.