The anticancer effects of radiation for patients with left‐sided breast cancer might be partly negated by radiation therapy (RT)‐associated cardiac toxicity. 1 , 2 , 3 Since the heart can be displaced medially, inferiorly, and posterior (i.e., away from the left breast) during deep inspiration in most patients, one approach to reduce incidental cardiac irradiation is to treat patients specifically during this portion of the respiratory cycle (e.g., using DIBH). This approach allows one to typically maintain coverage of the target tissues yet markedly reduce the degree of incidental cardiac irradiation. 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14