Participants undergoing radiation after breast conserving surgery for an early breast cancer (either Ductal Carcinoma In Situ (DCIS), or Early Stage Invasive breast cancer), and are at increased risk of developing a skin reaction because of their large breast size.

After breast conserving surgery (also known as a 'lumpectomy'), women with either DCIS or early stage invasive breast cancer receive radiation to the breast to decrease the risk of cancer recurrence. Breast radiation is usually done with women lying on their back ("supine"). Some women develop temporary breakdown of the skin (moist desquamation). This skin reaction can be painful and has been linked to long term side effects such as chronic pain and decreased quality of life.

This study is being done because women with large breasts have higher rates of skin breakdown (called 'moist desquamation') and breast pain during and shortly after radiation therapy is complete. It is unclear if such skin reactions and pain would be improved by alternating treatment position - namely lying on your belly ("prone") during their radiation treatment.

The risk of moist desquamation in large breasted women remains unacceptably high and reactions tends to be severe and produce significant permanent and delayed side effects. Evidence suggests that the use of a prone breast IMRT technique has the potential to decrease the risk of moist desquamation in large breasted women to the levels that are now seen when average/smaller breasted women are treated with supine IMRT. As prone breast XRT is currently only offered at 6 of 15 of the Ontario Cancer Centres polled for the purposes of providing motivation for this study, a multicentre RCT is feasible to confirm and quantify the improvement provided by the prone technique and provide Level 1 evidence for it to be adopted world-wide.

Inclusion Criteria:

• confirmed histological diagnosis of breast carcinoma or ductal carcinoma in situ (DCIS);
• treated with BCT;
• no indication for treatment of regional LN;
• Women with a bra size of 40 inches or greater, or a pre-surgery cup size of D or greater

Exclusion Criteria:

• Regional Lymph Node XRT indicated;
• Bilateral breast cancer;
• unhealed wound (skin not closed and/or infection);
• previous XRT to the same breast;
• unable to lie prone;
• presence of active connective tissue disease;
• pregnancy;
• unacceptable heart exposure (as measured by > 10% of the heart receiving 50% of the prescribed dose, i.e. V25Gy > 10%);
• adequate coverage of postoperative tumour bed not technically possible