10 patients will be treated alternating daily between prone and supine position for breast radiotherapy to compare set-up accuracy. Prone treatment will be given on the odds days and supine treatment on the even days.

Inclusion Criteria:

• female patients
• multidisciplinary decision of adjuvant radiotherapy after lumpectomy for breast cancer
• minimum 18 years
• informed consent obtained, signed and dated before specific protocol procedures.

Exclusion Criteria:

• mastectomy
• need for axillary irradiation
• bilateral breast irradiation
• previous irradiation at the same time
• pregnant or breastfeeding
• mental condition rendering the patient unable to understand the nature, scope and possible consequences of the study.
• patient unlikely to comply with the protocol; i.e. uncooperative attitude, inability to return for follow-up visits, and unlikely to complete the study.