INTRODUCTION:When irradiating the left breast, a small portion of the heart and left anterior descending coronary artery (LAD) are often included in the treatment field. Deep inspiration breath-hold (DIBH) techniques reduce dose to coronary structures, but are resource intensive and may not be tolerated by all patients. The aim of this study was to evaluate a simple multi-leaf collimator (MLC) modification technique with respect to target coverage and organ-at-risk sparing. METHODS:Forty nine patients with left-sided breast cancer, planned with a simultaneous integrated boost technique were retrospectively replanned with additional shielding of the LAD. Dose to the target volumes (whole breast and boost) and organs at risk (heart, ipsilateral lung and LAD) were assessed on both plans. RESULTS:Significant dose reductions were observed for all organs at risk when LAD shielding was introduced, with a reduction in mean LAD dose of 7.0 Gy, mean LAD planning risk volume (PRV) dose of 5.9 Gy, maximum LAD dose of 12 Gy and mean heart dose of 0.73 Gy. Target volume coverage was clinically acceptable for 96% of patients, using the left anterior descending coronary artery shielded plan (LADSP). No difference was observed between the standard plan (SP) and LADSP in nine patients (18%). CONCLUSIONS:For selected patients, the implementation of a simple MLC shielding technique can reduce the dose to cardiac structures, whilst maintaining breast and boost volume dosimetry. This technique is simple to implement and may be used as an alternative to DIBH for those patients who are unable to fulfill the selection criteria, or departments who are not resourced to perform DIBH.