Every year in the UK, around 37,000 people with breast cancer have radiotherapy. Radiotherapy uses carefully controlled radiation to destroy cancer cells. Most people now receive this treatment once a day for just 5 days, thanks to research done in the UK.

However, about 10,000 people still need a longer course of treatment, lasting up to 23 days. This is because they need an extra dose of radiotherapy called a boost. The boost is given to the area of the breast where the tumour was, and research has shown that it further lowers the chance of the cancer coming back. People who need a boost are often younger or have cancers that grow more quickly.

This research study is for people who need this boost treatment.

Having radiotherapy over many days can make life more difficult. It can slow down recovery, delay returning to work, and affect caring responsibilities at home. It also means more travel to hospital and can lead to more side‑effects. Longer treatment also uses more NHS staff time and increases carbon emissions from patient travel.

A recent study has already shown that the boost can be given at the same time as the main breast radiotherapy, reducing the treatment time to 15 days. Our new study now aims to find out whether the boost can be safely given during a 5‑day radiotherapy course.

To find this out, we will compare:

• One group receiving the standard boost during a 15‑day treatment schedule
• Two groups receiving different boost doses during a 5‑day treatment schedule

We plan to invite people from at least 40 radiotherapy centres across the UK. A computer will randomly place 4,828 participants into one of the three groups. This means:

• One‑third will receive the current standard treatment (boost over 15 days)
• Two‑thirds will receive the boost over 5 days

Throughout the study, we will collect information about:

• Side‑effects
• Changes to the treated breast
• Fatigue (extreme tiredness)
• Quality of life
• Treatment costs for both patients and the NHS

We will follow participants for five years.

The aim of this research is to show that giving the boost over 5 days is as effective as giving it over 15 days in stopping the cancer from coming back. We also want to show that side‑effects are the same or fewer, and that people recover more quickly with the shorter treatment