This trial is designed to assess the efficacy of a novel carbon-fibre adjustable reusable accessory (CARA) for breast support to reduce skin toxicity and unwanted normal tissue dose in comparison to the current clinical standard for supine breast support during whole breast RT.

This study is designed to assess the effectiveness of utilizing the CARA positioning device for reducing the incidence and severity of acute skin reactions in the infra-mammary fold in patients undergoing WBRT. Secondary outcomes include patient reported outcomes and dose to normal body tissue and organs at risk. This is a randomized controlled trial. Participants will be randomly assigned to the following two arms:

Arm 1 patients will receive CARA breast support. The known benefits to using CARA for breast positioning are reduction in IMF skin folds during treatment, reduction in breast separation, and reduction in V50% body, V105% body and lung V20 Gy in treatment planning. No known risks to using CARA have been identified.

Arm 2 patients will not receive CARA breast support. Patients in arm 2 may be treated with no breast support, a small foam wedge, a thermoplastic shell or alternate supine breast support method according to the current standard of care at the treating centre. These methods have entered RT clinical practice over decades of practice without published evidence of impact on rates of MD. Published rates of MD for the control arm thus pertain to a cross section of these methods. The control arm of this study will look at all of these methods combined. There may be centre specific preference for the control method and stratification by centre will be done.

Inclusion Criteria:

• Age 18 or older.
• Able to provide informed consent
• ECOG performance status 0 - 2
• Stage 0-3 invasive breast cancer or DCIS scheduled to received adjuvant radiotherapy to the whole breast
• Any infra-mammary skin fold of >= 0.5 cm in the supine treatment position and/or palpable lateral breast tissue falling posterior to the mid-axilla line while in supine or treatment position.

Exclusion Criteria:

• Inability to give informed consent or comply with experimental arm of trial
• Previous RT to either breast or to the chest
• Planned boost to infra mammary area
• Use of Mepitel while on treatment
• Failure to heal surgical wound or significant post-operative wound infection
• Presence of significant connective tissue disease (e.g. systemic sclerosis, SLE)
• Known radiation hypersensitivity phenotype (e.g. ataxia telangiectasia etc.)
• Breast reconstruction
• Planned partial breast irradiation, unless the treated area includes the infra-mammary fold

Skin assessments carried out by blinded observers