Dose-modeling study to compare external beam techniques from protocol NSABP B-39/RTOG 0413 for patients with highly unfavorable cardiac anatomy

The aim of this study was to select patients with heart anatomy that is specifically unfavorable for tangential irradiation in whole-breast radiotherapy (WBRT), to be used as an experimental cohort to compare cardiac dosimetric and radiobiological parameters of three-dimensional conformal external beam accelerated partial breast irradiation (3D-CRT APBI) to WBRT with techniques as defined by the National Surgical Adjuvant Breast and Bowel Project (NSABP) B-39/Radiation Therapy Oncology Group (RTOG) 0413 clinical trial.A dosimetric modeling study that compared WBRT and 3D-CRT APBI was performed on CT planning data from 8 patients with left-sided breast cancer. Highly unfavorable cardiac anatomy was defined by the measured contact of the myocardium with the anterior chest wall in the axial and para-sagittal planes. Treatment plans of WBRT and 3D-CRT APBI were generated for each patient in accordance with NSABP B-39/RTOG 0413 protocol. Dose-volume relationships of the heart, including the V5min (minimum dose delivered to 5% of the cardiac volume), biological effective dose (BED) of the V5min, and normal tissue complication probability (NTCP) were analyzed and compared.Despite expected anatomic variation, significantly large differences were found favoring 3D-CRT APBI in cumulative dose-volume histograms (p0.01), dose to the entire heart (mean difference 3.85 Gy, p0.01), NTCP (median difference, 1.00 Gy; p0.01), V5min (mean difference, 24.53 Gy; p0.01), and proportional reduction in radiobiological effect on the V5min (85%, p0.01).Use of 3D-CRT APBI can demonstrate improved sparing of the heart in select patients with highly unfavorable cardiac anatomy for WBRT, and may result in reduced risk of cardiac morbidity and mortality.