K.A De Winter

1011 Background: APBI is now being used clinically and is being evaluated in 6 clinical trials in which several forms of fractionated external beam for APBI are being used. Recent publications and presentations from small single-institution series have reported significant short-term toxicities related to 3-D CEBT, a type of APBI under evaluation in NSABP B−39/RTOG 0413. Given those results, we believe it necessary to report the current levels of toxicity in the 3−D conformal portion of the APBI arm of our trial. Methods: NSABP B−39/RTOG 0413, a multi-institutional randomized phase III trial, compares whole-breast irradiation with APBI. Included is an electronic CT- data submission process that serves as the basis of a rigorous QA program used in all 3 types of APBI, assuring dosimetric compliance. Closely monitored data on patients randomized to the APBI arm and undergoing 3−D CEBT were reviewed for overall toxicity, specifically for toxicity related to fibrosis-cosmesis and fibrosis-deep connective tissue, based on CTCAEv3.0 and NSABP/RTOG CTCAE criteria. Results: 3,862 patients are enrolled (89.8% of target accrual). Toxicity data are available for 1386 patients randomized to APBI who have received 3−D CEBT, with 974 of the latter group in their 3rd year of follow-up. With a mean time on study of 41.0 months, no significant toxicity-related issues have been raised. The rates of fibrosis-cosmesis and fibrosis-deep connective tissue toxicities are: Grade 2 ≤12%, Grade 3 ≤3% and Grade 4/5 =0% for the 3-D CEBT used in the trial. Conclusions: Contrary to findings in recent published reports the 3-D conformal APBI toxicity rates in our trial are acceptably low. Our trial surpasses others reported in patient number and length of follow-up, emphasizing the importance of large phase III randomized trials with rigorous QA to determine treatment outcomes and avoid the bias that can arise from small single-institution series. The continuation of accrual to this trial is critically important so that APBI efficacy, long-term toxicity, and QOL outcomes can be accurately assessed. Supported by NCI-U10-CA-12027, U10-CA−37377, U10-CA−69974, U10-CA−69651, and U10-CA−21661.