Accelerated Partial Breast Irradiation With IMRT in Early Breast Cancer

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.