This article has generated a strong hypothesis, which we believe can now aid in directing future studies. In an unselected group of 1101 patients, we have identified the base of the heart as a dose-sensitive region, where excess dose results in poorer patient survival (HR 1.25, 95% CI 1.01–1.56, p = 0.04). For those patients where heart contours were available (386 patients), neither the mean dose, V5 or V30 showed significance in univariate or multivariate analysis.