All the DIBH patients in this study were treated in the supine position. Another way to reduce dose to the heart is to treat patients in the prone position with free breathing. The dosimetric results of these two techniques are different. Also the contralateral breast dose is often evaluated in the prone treatments. A recent study (23) has shown that prone free breathing yields lower mean lung dose and V20 than supine DIBH. The mean heart dose and contralateral breast receiving >5 Gy were significantly lower in supine DIBH than prone free breathing. Although this study investigated only planned dose without considering patient motion, it still provides dosimetric information of prone free‐breathing and supine DIBH. For prone free‐breathing treatments, the interfraction motion (patient setup) is often greater than the intrafraction patient motion. Unfortunately, no study was found at this point to show the corresponding dosimetric impact.