Abstract Background While breast radiotherapy typically includes regional nodal basins, the treatment of the internal mammary nodes (IMN) has been controversial due to concern for long-term cardiac toxicity. For high risk patients where IMN treatment is warranted, there is limited data with regards to the degree of heart sparing conferred by modern techniques. In this study, we sought to analyze the specific heart sparing metrics conferred by deep inspiration breath hold (DIBH) in the setting of IMN irradiation. Methods From 2012 to 2015, 168 consecutive patients were treated with adjuvant left-sided radiotherapy using DIBH. Retrospective review identified 49 patients who received nodal irradiation, either to a supraclavicular field (SCF) and IMN (16), or to the SCF alone (33). Cardiac mean dose and dose volumes were calculated from free breathing (FB) and DIBH treatment plans, and compared by Wilcoxon signed-rank and Mann–Whitney U tests. Results DIBH achieved significant reductions in mean heart dose (p < 0.001) in both the IMN treated group from 6.73 Gy to 2.79 Gy (− 56.4%) and the IMN untreated group from 4.77 Gy to 1.55 Gy (− 63.7%). There was a 7.3% difference in relative reduction that was not statistically significant (p = 0.216). Relative reductions in heart dose volume measures were all significantly lower for IMN-irradiated patients (p ≤ 0.012), with the greatest deficits at V5 that gradually diminish with increasing dose (V25). Conclusions The relative heart sparing benefits of the DIBH technique are retained even with IMN inclusion. However, the addition of IMN irradiation is associated with an intrinsically greater heart dose, which translates to an estimated 9.2% proportional increase in the risk of a subsequent major coronary event. In the setting of effective cardiac sparing techniques, clinicians should take these considerations into account to guide when IMN treatment is warranted.
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