This study is to compare radiation toxicity of accelerated partial breast irradiation (ABPI) with whole breast irradiation (WBI) in low-risk breast cancer.

This study is a single-center phase II randomized clinical trial to enroll low-risk breast cancer treat with breast-conserving surgery. All eligible patients are randomized to receive either ABPI (40Gy/10fx/2wks) or WBI(43.5Gy/15fx/3wks). Intensity modulated radiation therapy (IMRT) technique is used. Grade 2 or more acute and late toxicities are prospectively evaluated and compared.

Inclusion Criteria:

• Life Expectation: > 5 years
• Enrollment date no more than 12 weeks after breast-conserving surgery or no more than 8 weeks after adjuvant chemotherapy
• Histologically confirmed diagnosis of invasive ductal carcinoma (grade 1-2), or mucinous carcinoma, or papillary carcinoma, or tubular carcinoma, or medullary carcinoma: primary tumor ≤ 3.0cm in maximum diameter and pN0; or histologically confirmed DCIS: primary tumor ≤ 2.5cm in maximum diameter, low-medium grade
• Unifocal tumour (confirmed by diagnostic MRI)
• No lymphovascular invasion
• ER positive (defined as medium-strongly nuclear staining in >1% of the cancer cells)
• Negative radial resection margins of >= 2 mm
• Surgical clips placed in the tumor bed
• Written informed consent.

Exclusion Criteria:

• Stage Ⅱ-Ⅲ
• Multifocal tumors
• Histologically confirmed diagnosis of invasive ductal carcinoma (grade 3), - - - invasive micropapillary carcinoma, carcinoma of lobular in situ, invasive - - - lobular carcinoma
• Paget's disease of the nipple
• Underwent oncoplastic surgery of ipsilateral breast
• Underwent neoadjuvant chemotherapy or hormonal therapy
• Previous or simultaneous contralateral breast cancer
• Undergone ipsilateral chest wall radiotherapy
• Active collagen vascular disease.