Electrons Only Internal Mammary Chain (Imc) Irradiation Technique Reduces Cardiac Toxicity Compared To Mixed Electron-Photons Technique In Left-Sided Breast Cancer

Treatment of locally advanced breast cancer includes adjuvant radiation therapy to the chest wall/breast and regional nodal irradiation. Internal mammary chain (IMC) irradiation remains debatable among radiation oncologists mainly due to concerns of increased toxicity to the heart where each additional Gray of radiation has been reported to increase the risk of major coronary events by 7.4%. We performed a comparative dosimetric study between two common techniques used for IMC irradiation, electrons only versus hybrid electrons-photons, with the aim of establishing the least toxic to the organs at risk. We included 20 patients with left-sided breast cancer who received IMC irradiation at our department over a two years period. For each patient, two different treatment plans were generated using a treatment planning system with electron Monte Carlo algorithm. Tangential and supraclavicular fields were similar in the two plans. In the first plan, IMC was treated with electrons only versus (vs.) with both photons and electrons (20% photons, 80% electrons) in the second plan. Prescription was 50 Gy in 25 fractions for all plans. Dose volume histogram data were obtained for each of the two plans for every patient. The means of each of the following variables were compared between the two plans: mean heart dose, heart V5Gy, Left anterior descending artery (LAD) maximum (max) and mean dose, V20Gy of the ipsilateral lung, mean lung dose, and skin max dose. The means of the above variables were compared using dependent samples t-test in statistical analysis software. In the two techniques of IMC irradiation, target coverage was adequate. The average of the mean doses to all organs at risk, except skin, was lower using electrons only vs. mixed electrons-photons respectively as follows: mean heart dose was 2.99 Gy ± 1.4 vs. 6.27 Gy ± 1.3 (p-value< 0.0001). Heart V5 Gy was 11.9% ± 7% vs. 53.8% ± 7% (p-value< 0.0001). Mean LAD dose was 11.96 Gy ± 8.2 vs. 14 Gy ± 7.8 (p-value = 0.003). Max LAD dose was 22.9 Gy ± 12.8 vs. 25 Gy ± 11.4 (p-value = 0.012). Mean lung dose was 12.67 Gy ± 1.9 vs. 13.46 Gy ± 1.8 (p-value< 0.0001). Lung V20 Gy was 24% ± 5% vs. 24.5% ± 5% (p-value=0.046). Max skin dose was the only variable that was higher using electrons only vs. electrons-photons: 58.8 Gy ± 1.2 vs. 57.3 Gy ± 1.9 respectively (p-value = 0.002). The difference was most pronounced for the heart, where the mean decreased by around 50% and V5Gy by around 75% in favor of the electrons only technique. With only a slight increase in the maximum skin dose, Electrons only IMC irradiation technique significantly reduced heart, LAD and lung doses compared to the mixed electrons-photons technique. Most importantly, our findings show a 3 Gy absolute decrease in the mean heart dose using electrons only, which potentially leads to around 22% reduction in the risk of major coronary events.