Background
ASTRO evidence-based guideline on dose-fractionation for whole breast irradiation (WBI) with or without a tumor bed boost. Developed by a task force addressing 5 key questions: dose-fractionation for WBI, indications and dosing for boost, and treatment planning techniques. Based on systematic literature review with ASTRO-approved evidence grading.
Interventions and follow up
Arm A: N/A — evidence-based guideline
Arm B: N/A
Primary endpoint: Synthesis of evidence on WBI fractionation and boost; guideline recommendatio
mFollow up: N/A
Arm B: N/A
Primary endpoint: Synthesis of evidence on WBI fractionation and boost; guideline recommendatio
mFollow up: N/A
Results
Preferred WBI fractionation: Hypofractionated WBI: 40 Gy/15 fx (2.67 Gy/fx) or 42.5 Gy/16 fx (2.65 Gy/fx) — preferred over conventional 50 Gy/25 fx for most patients
Conventional fractionation (50 Gy/25 fx): Acceptable when hypofractionation is not appropriate (e.g., simultaneous nodal RT where high-dose-per-fraction concerns exist)
Boost indications: Recommended for young age, close/positive margins, high-grade tumors; omission considered in favorable low-risk populations
Boost dosing: 10–16 Gy in 2 Gy/fx standard; hypofractionated boost schedules acceptable
Conventional fractionation (50 Gy/25 fx): Acceptable when hypofractionation is not appropriate (e.g., simultaneous nodal RT where high-dose-per-fraction concerns exist)
Boost indications: Recommended for young age, close/positive margins, high-grade tumors; omission considered in favorable low-risk populations
Boost dosing: 10–16 Gy in 2 Gy/fx standard; hypofractionated boost schedules acceptable
Adverse events
Main adverse events: Hypofractionation is associated with equivalent or superior long-term cosmesis and similar toxicity compared to conventional fractionation (per START, Ontario trial data). Tumor bed boost increases fibrosis rates; individualized decision-making recommended.
Conclusions
For women with invasive breast cancer undergoing WBI (with or without low axillary coverage), hypofractionated WBI is the preferred dose-fractionation scheme. The guideline formalizes a shift away from conventional fractionation as the default standard, based on robust RCT data showing equivalent efficacy and non-inferior cosmesis.
Key Limitations
Key Limitations: The guideline is based primarily on trials that did not include simultaneous nodal RT with hypofractionation — evidence for hypofractionated locoregional RT is more limited (supported by subsequent FAST-Forward, IMPORT HIGH data). Boost recommendations are based on extrapolation from invasive cancer trials; evidence for boost in DCIS is less robust. Not all patients are suitable for hypofractionation (e.g., significant breast asymmetry, institutional constraints).
Clinical Context
This ASTRO guideline catalyzed widespread adoption of 15- or 16-fraction WBI schedules in the United States, where conventional fractionation had remained prevalent despite European RCT data (START A, START B, Ontario) showing equivalence of hypofractionation since the mid-2000s. Subsequent FAST-Forward trial (40 Gy/15 fx vs 27 Gy/5 fx) supports even more abbreviated schedules as non-inferior.
References