Whole Breast Irradiation vs Twice Daily vs. Once Daily Accelerated Partial Breast Irradiation

Inclusion Criteria: Tumor characteristics * Histologically confirmed invasive adenocarcinoma of the breast * Tumors must be ≤ 3 cm. Gross disease must be unifocal. Negative lymph nodes * Hormone receptor status: Estrogen receptor (ER) status known Progesterone status known if ER analysis is negative Marginal or borderline results are considered positive Surgery * Tumor removed with conservative breast surgery with adequate margin * Re-excision of surgical margins allowed * No prior breast implants * Prior axillary staging required for patients including 1 of the following: * Sentinel node biopsy alone (if sentinel node is negative) * Sentinel node biopsy followed by axillary dissection if sentinel node is positive) * Axillary dissection alone with ≥ 6 axillary nodes * Target lumpectomy cavity clearly delineated AND target lumpectomy/whole breast reference volume ≤ 30% based on postoperative pre-randomization CT scan * Final surgery (i.e., lumpectomy, re-excision of margins, or axillary staging procedure) within the past 42 days * No prior biologic therapy for this malignancy Chemotherapy * No prior chemotherapy for this malignancy * No concurrent chemotherapy during study radiotherapy Endocrine therapy * No prior hormonal therapy for this malignancy * Concurrent hormonal therapy allowed provided it is not administered during chemotherapy * No concurrent raloxifene, tamoxifen, or other selective estrogen receptor modulating drugs * No concurrent hormone replacement …