To measure cardiac tissue doses in left-sided breast cancer patients receiving supine tangential field radiotherapy with multileaf collimation (MLC) cardiac shielding of the heart and to assess the effect on target volume coverage.Sixty-seven consecutive patients who underwent adjuvant radiotherapy to the left breast (n = 48) or chest wall (n = 19) in 2009/2010 were analysed. The heart, left anterior descending coronary artery (LAD), whole breast and partial breast clinical target volumes (WBCTV and PBCTV) were outlined retrospectively (the latter only in patients who had undergone breast-conserving surgery [BCS]). The mean heart and LAD NTD mean and maximum LAD doses (LAD max ) were calculated for all patients (NTD mean is a biologically weighted mean dose normalised to 2 Gy fractions using a standard linear quadratic model). Coverage of WBCTV and PBCTV by the 95% isodose was assessed (BCS patients only).The mean heart NTD mean (standard deviation) was 0.8 (0.3) Gy, the mean LAD NTD mean 6.7 (4.3) Gy and the mean LAD max 40.3 (10.1) Gy. Coverage of the WBCTV by 95% isodose was <90% in one in three patients and PBCTV coverage <95% (range 78–94%) in one in 10 BCS patients.The use of MLC cardiac shielding reduces doses to cardiac tissues at the expense of target tissue coverage. Formal target volume delineation in combination with an assessment of the likelihood of local relapse is recommended in order to aid decisions regarding field and MLC placement.