Session Item

Breast cancer radiotherapy is related to an increasing long-term risk of cardiac morbidity including arrhythmias and conduction disorders. This study aimed to assess radiation dose distribution to conduction nodes in patients treated with hypofractionated 3D conformal left-sided breast cancer radiotherapy.

Data of 187 patients referred for adjuvant left-sided breast cancer radiotherapy were evaluated. All patients underwent 3D conformal hypofractionated radiotherapy with a prescription dose of 40 Gy delivered in 15 daily fractions +/- an additional boost of 13.35 Gy. Delineation of heart subvolumes was performed on a non-contrast enhanced planning CT scan using the Duanes’s cardiac atlas. The conduction nodes including the sinoatrial (SAN) and atrioventricular nodes (AVN) were contoured according to the recently published French atlas. Dose volume histograms were generated. The mean heart dose (MHD), the Dmean and Dmax to conduction nodes and cardiac substructures were calculated and analysed with regards to irradiated target volumes. Correlation between cardiac dosimetric parameters was assessed.

The mean age was 51.3 years. Radiotherapy was delivered to the chest wall in 61% of cases and to the remaining breast in 49% of patients. Regional lymph nodes irradiation was performed in 76% of patients and included the internal mammary chain (IMC) in 50% of cases. The mean MHD was 3.24 Gy ranging between 1.02 Gy and 4.83 Gy. The mean Dmean to SAN was 1.32 Gy ranging between 0.68 Gy and 1.84 Gy. The mean Dmax to SAN was 2.06 Gy (0.66-3.6 Gy). Higher exposure of the AVN was noticed with Dmean/Dmax of 2.15 Gy/3.8 Gy. Doses to conduction nodes were statistically higher in chest wall radiotherapy when compared to radiotherapy to the remaining breast (p<0.01). IMC irradiation was strongly correlated to SAN and AVN exposure with Pearson coefficients ranging between 0.7 and 0.86 (p<0.01) for both Dmax and Dmean. Although the MHD was statistically correlated to doses to conduction nodes, we found a poor predictive value of MHD for both Dmean and Dmax to SAN and VAN, with linear regression coefficient R² <0.3. The highest value of R² coefficient was found between Dmean to AVN and Dmean to the right ventricle (R²=0.76).

This study showed that 3D conformal breast cancer radiotherapy yielded low doses (<4Gy) to SAN and AVN. Nonetheless, reporting conduction nodes exposure in breast cancer radiotherapy might be useful to further investigate dose-volume relationship of late radiation induced rhythmic and conduction disorders and better define specific heart dose constraints.