Background Recent evidence supports hippocampal avoidance with whole brain radiotherapy (HA-WBRT) as the recommended treatment option in patients with good prognosis and multiple brain metastases as this results in better neurocognitive preservation compared to whole brain radiotherapy. However, there is often poor tumour control with this technique due to the low doses given. Stereotactic Radiosurgery (SRS), a form of focused radiotherapy which is given to patients who have a limited number of brain metastases, delivers a higher radiation dose to the metastases resulting in better target lesion control. With improvements in radiation technology, advanced dose-painting techniques now allow a simultaneous integrated boost (SIB) dose to lesions whilst minimising doses to the hippocampus to potentially improve brain tumour control and preserve cognitive outcomes. This technique is abbreviated to HA-SIB-WBRT or HA-WBRT+SIB. Methods We hypothesise that the SIB in HA-SIB-WBRT (experimental arm) will result in better tumour control compared to HA-WBRT (control arm). This may also lead to better intracranial disease control as well as functional and survival outcomes. We aim to conduct a prospective randomised phase II trial in patients who have good performance status, multiple brain metastases (4–25 lesions) and a reasonable life expectancy (> 6 months). These patients will be stratified according to the number of brain metastases and randomised between the 2 arms. We aim for a recruitment of 100 patients from a single centre over a period of 2 years. Our primary endpoint is target lesion control. These patients will be followed up over the following year and data on imaging, toxicity, quality of life, activities of daily living and cognitive measurements will be collected at set time points. The results will then be compared across the 2 arms and analysed. Discussion Patients with brain metastases are living longer. Maintaining functional independence and intracranial disease control is thus increasingly important. Improving radiotherapy treatment techniques could provide better control and survival outcomes whilst maintaining quality of life, cognition and functional capacity. This trial will assess the benefits and possible toxicities of giving a SIB to HA-WBRT. Trial registration Clinicaltrials.gov identifier: NCT04452084. Date of registration 30th June 2020.