KEY POINTS
- The study analyzed 221 patients and 224 treated breasts enrolled in the prospective UNIQUE trial after breast-conserving surgery. In total, 1,105 pretreatment CBCT scans were reviewed fraction by fraction.
- Patients received whole-breast irradiation of 26 Gy in 5 fractions, with or without a simultaneous integrated tumor-bed boost to 30 Gy in 5 fractions. Treatment was delivered with IMRT or VMAT, either daily or on alternate days.
- CBCT-defined breast edema occurred in 83 of 224 breasts (37.1%). Among affected breasts, 96.4% developed edema immediately after the first fraction, and 50.6% reached maximal swelling at that point.
- The clinically relevant magnitude was smaller than the overall imaging-detected incidence: contour expansion exceeded 3 mm in 29.1%, 5 mm in 9.8%, and 8 mm in 2.7% of the full cohort. Maximum observed expansion was 11.2 mm.
- Most edema was clinically occult. Only 12 of 83 affected breasts (14.5%) had swelling detectable on examination, all grade 1, and no grade ≥2 acute edema occurred.
- Younger age and greater baseline breast thickness independently predicted swelling ≥3 mm. Each additional year of age reduced risk by about 4.2%, while each additional centimeter of breast thickness increased risk by approximately 26%.
- No patient required adaptive replanning. Two patients with edema >8 mm received a one-day treatment interval and symptomatic treatment; the remainder completed all five fractions without interruption. The study did not recalculate delivered dose on CBCT, so the actual dosimetric impact remains unproven.
CLINICAL TAKEAWAY
Breast shape can change after just one 5.2-Gy fraction, often without obvious clinical swelling. For patients with thicker breasts or other risk factors, early imaging review may be worthwhile, but this study does not yet show that CBCT-detected edema causes clinically meaningful underdosage or requires routine adaptive replanning.