Female patients treated with radiation for left-sided breast malignancy will undergo alternate fractions of ABC-assisted and VisionRT-assisted DIBH. Residual motion during breath-hold will be quantitatively assessed using MV fluoroscopy from the treatment beam itself (i.e., no additional radiation dose). The dosimetric impact of residual motion on organs at risk (heart and lung) will be assessed by applying rigid and/or deformable displacements to the planning CT images, computing the 3D dose map and comparing with the original planned dose map.