The UK Standardisation of Breast Radiotherapy (START) trials evaluated the safety and effectiveness of hypofractionated radiotherapy (lower total doses in fewer, larger fractions) for early breast cancer. The 10-year follow-up of two randomised controlled trials (START-A and START-B) confirmed that appropriately dosed hypofractionated radiotherapy is safe and effective for women with early breast cancer. In START-A, 2236 women were treated with either 50 Gy in 25 fractions or 41.6 Gy or 39 Gy in 13 fractions over 5 weeks. At 10 years, local-regional relapse rates did not differ significantly between the 41.6 Gy and 50 Gy groups (6.3% vs 7.4%) or between the 39 Gy and 50 Gy groups (8.8% vs 7.4%). The 39 Gy group had fewer normal tissue effects, such as breast induration, telangiectasia, and oedema, compared to the 50 Gy group. In START-B, 2215 women were treated with either 50 Gy in 25 fractions or 40 Gy in 15 fractions over 3 weeks. At 10 years, the 40 Gy group had a lower local-regional relapse rate (4.3% vs 5.5%) compared to the 50 Gy group. The 40 Gy group also had fewer normal tissue effects, such as breast shrinkage, telangiectasia, and oedema, compared to the 50 Gy group. The results support the continued use of 40 Gy in 15 fractions as the UK standard of care for women requiring adjuvant radiotherapy for invasive early breast cancer. The 10-year results confirm that hypofractionated radiotherapy is safe and effective, with no significant difference in survival or late normal tissue effects between the hypofractionated and standard regimens. The findings support the adoption of hypofractionated radiotherapy as a standard treatment for early breast cancer.The UK Standardisation of Breast Radiotherapy (START) trials evaluated the safety and effectiveness of hypofractionated radiotherapy (lower total doses in fewer, larger fractions) for early breast cancer. The 10-year follow-up of two randomised controlled trials (START-A and START-B) confirmed that appropriately dosed hypofractionated radiotherapy is safe and effective for women with early breast cancer. In START-A, 2236 women were treated with either 50 Gy in 25 fractions or 41.6 Gy or 39 Gy in 13 fractions over 5 weeks. At 10 years, local-regional relapse rates did not differ significantly between the 41.6 Gy and 50 Gy groups (6.3% vs 7.4%) or between the 39 Gy and 50 Gy groups (8.8% vs 7.4%). The 39 Gy group had fewer normal tissue effects, such as breast induration, telangiectasia, and oedema, compared to the 50 Gy group. In START-B, 2215 women were treated with either 50 Gy in 25 fractions or 40 Gy in 15 fractions over 3 weeks. At 10 years, the 40 Gy group had a lower local-regional relapse rate (4.3% vs 5.5%) compared to the 50 Gy group. The 40 Gy group also had fewer normal tissue effects, such as breast shrinkage, telangiectasia, and oedema, compared to the 50 Gy group. The results support the continued use of 40 Gy in 15 fractions as the UK standard of care for women requiring adjuvant radiotherapy for invasive early breast cancer. The 10-year results confirm that hypofractionated radiotherapy is safe and effective, with no significant difference in survival or late normal tissue effects between the hypofractionated and standard regimens. The findings support the adoption of hypofractionated radiotherapy as a standard treatment for early breast cancer.