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.


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 (eudmean) and maximum equivalent fraction size (eudmax) were calculated for each patient using 100 different values of the alpha/beta ratio. The eudmean ranged from 3.67 to 3.69 Gy, while the eudmax 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.


CONCLUSIONS
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.
Cuttino
Laurie W
LW
Department of Radiation Oncology, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, VA 23298-0058, USA.
Todor
Dorin
D
Pacyna
Lynn
L
Lin
Peck-Sun
PS
Arthur
Douglas W
DW
D001918:Brachytherapy; D001943:Breast Neoplasms / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D006801:Humans; D020266:Radiotherapy, Conformal