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

Cuttino LW, Todor D, Pacyna L, Lin PS, Arthur DW

Department of Radiation Oncology, Virginia Commonwealth University, Medical College of Virginia Campus, Richmond, VA 23298-0058, USA.

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 (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. 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.

Published 6 October 2006 in Am J Clin Oncol, 29(5): 474-8.
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