Background
Phase III, open-label RCT. N=1221 patients (per protocol) with non-metastatic early breast cancer requiring locoregional radiation including regional nodal irradiation (RNI), treated with breast-conserving surgery or mastectomy. First large RCT to evaluate hypofractionated locoregional RT including nodal fields. Stratified by center, laterality, and receipt of systemic therapy.
Interventions and follow up
Arm A: Hypofractionated locoregional RT — 40 Gy in 15 fractions over 3 weeks (breast/chest wall + regional nodes) (N=614)
Arm B: Conventional fractionated locoregional RT — 50 Gy in 25 fractions over 5 weeks (breast/chest wall + regional nodes) (N=607)
Primary endpoint: Arm lymphedema (non-inferiority)
mFollow up: NR at primary analysis
Arm B: Conventional fractionated locoregional RT — 50 Gy in 25 fractions over 5 weeks (breast/chest wall + regional nodes) (N=607)
Primary endpoint: Arm lymphedema (non-inferiority)
mFollow up: NR at primary analysis
Results
Arm lymphedema (primary): 40 Gy/15f non-inferior to 50 Gy/25f — primary endpoint met
BCSS (exploratory): HR 0.53
OS (exploratory): HR 0.59 — survival analyses pre-specified as exploratory; trial not powered for these endpoints
BCSS (exploratory): HR 0.53
OS (exploratory): HR 0.59 — survival analyses pre-specified as exploratory; trial not powered for these endpoints
Adverse events
Acute skin toxicity: Grade ≥2 acute skin toxicity lower in the 40 Gy/15f arm
Lymphedema/late effects: lymphedema rates comparable between arms (non-inferiority confirmed); late toxicities (fibrosis, pneumonitis, cardiac) similar or favoring hypofractionation at available follow-up
Lymphedema/late effects: lymphedema rates comparable between arms (non-inferiority confirmed); late toxicities (fibrosis, pneumonitis, cardiac) similar or favoring hypofractionation at available follow-up
Conclusions
HypoG-01 demonstrates that 40 Gy/15f locoregional RT is non-inferior to 50 Gy/25f for arm lymphedema, supporting hypofractionation when regional nodal irradiation is required. Exploratory survival data directionally favored hypofractionation, though these results are hypothesis-generating only.
Key Limitations
Open-label design with lymphedema assessment subject to observer bias. Survival HRs (0.53, 0.59) are exploratory in an underpowered endpoint and likely reflect random variation rather than true superiority; they should not be cited as definitive efficacy data. Long-term follow-up for late cardiac and pulmonary effects of nodal RT is not yet available. Radiation field design was not fully standardized across centers. Applicability to patients receiving concurrent CDK4/6 inhibitors, PARP inhibitors, or ADCs (with ILD risk) is unknown.
Clinical Context
HypoG-01 supports extension of hypofractionation from breast/chest wall alone (established by FAST-Forward, START-B) to locoregional fields including nodal irradiation. The 3-week schedule offers meaningful patient convenience and resource benefits vs 5 weeks. Combined with FAST-Forward, 40 Gy/15f locoregional RT is expected to become a practice standard; ASTRO and ESTRO guideline updates are anticipated. ESMO-MCBS: not applicable (non-inferiority design).