Laurie Cuttino MD, Dorin Todor PhD, Douglas Arthur MD, Rohini George, Lynn Pacyna CMD Medical College of Virginia Campus Department of Radiation Oncology 3D Conformal External Beam Radiotherapy (3D-CRT) for Accelerated Partial Breast Irradiation (APBI): What is the Correct Prescription Dose?

3D-CRT for APBI: What is the Correct Prescription Dose? APBI is increasing in popularity across the country 3D-CRT is a promising, non-invasive treatment technique with homogeneous dose delivery Upcoming NSABP/RTOG Phase III trial: 3D-CRT, multi- catheter brachytherapy, and Mammosite treatment are all included under a single treatment arm –Outcome equivalence of these techniques is anticipated –Target definition and dose coverage within this trial will be equivalent –Fractionation schemes have been proposed to account for the differences in dose homogeneity between the techniques Is the radiobiologic effect of these proposed fractionation schemes equivalent?

3D-CRT for APBI: What is the Correct Prescription Dose? The fractionation schemes for APBI: –brachytherapy = 34 Gy in 10 fractions –3D-CRT = 38.5 Gy 10 fractions This study is a comparison of equivalent uniform biologically effective dose (EUBED) EUBED is a combination of equivalent uniform dose (EUD) and biologically effective dose (BED) EUBED allows comparison of a non-uniform dose distribution (brachytherapy) with a homogeneous one (3D-CRT), while also accounting for fraction size

Five patients 3D-CRT and brachytherapy plans generated for each case Target and coverage were equivalent Calculations based on the assumption that the dose delivered with brachytherapy dose is the standard of care for APBI EUBED was calculated for each brachytherapy dose matrix Using this value, the EUD for the 3D-CRT plans was then calculated for the same fractionation scheme From this, we obtained an ideal fraction size for 3D-CRT of 3.74  0.03 Gy, and an equivalent prescription dose of 37.4 Gy (only 2.8% less than the proposed dose) Conclusions: The dose prescription for 3D-CRT APBI is biologically equivalent to brachytherapy, and validates the continued use of this fractionation scheme in the upcoming Phase III protocol 3D-CRT for APBI: What is the Correct Prescription Dose?