Three-dimensional conformal external beam radiotherapy (3D-CRT) for accelerated partial breast irradiation (APBI) - What is the correct prescription dose?

Objective: This study is an evaluation of the biologic equivalence of the dose prescriptions for brachytherapy and 3-dimensional conformal external beam radiotherapy (3D-CRT) accelerated partial breast irradiation (APBI), using actual patient dose matrix data, and is based on the concept of equivalent uniform biologically effective dose (EUBED). This formalism allows a nonuniform dose distribution to be reduced to an equivalent uniform dose, while also accounting for fraction size. Materials and Methods: Five computed tomography scans were selected from a group of patients treated with multicatheter interstitial APBI. Dose matrices for the brachytherapy plans were computed and analyzed with in-house software. For each patient, the EUBED for the brachytherapy dose matrix was generated based on calculations performed at the voxel-level. These EUBED values were then used to calculate the biologically equivalent fraction size for 3D-CRT (eud). Results: The mean equivalent fraction size (eud(mean)) and maximum equivalent fraction size (eud(max)) were calculated for each patient using 100 different values of the alpha/beta ratio. The eud an ranged from 3.67 to 3.69 Gy, while the eud(max) ranged from 3.79 to 3.82 Gy. For all values of the alpha/beta ratio, the maximum fraction size calculated to deliver a biologically equivalent dose with 3D-CRT was 3.82 Gy, with an equivalent total prescription dose of 38.2 Gy. Conclusion: Utilizing a wide range of established radiobiological parameters, this study suggests that the maximum fraction size needed to deliver a biologically equivalent dose using 3D-CRT is 3.82 Gy supporting the continued use of 3.85Gy BID in the current national cooperative trial.