S253
Clinical - Breast
ESTRO 2026
Purpose/Objective: The FAST-Forward trial (FF) (ISRCTN19906132) introduces an ultra-hypofractionated (UHF) one-week schedule for adjuvant radiotherapy of the breast. Prior to a possible Danish implementation, we assessed the compatibility of FF dose constraints with current Danish Breast Cancer Group (DBCG) guidelines [1] for whole breast irradiation (WBI) as well as local planning practice for partial breast irradiation (PBI). Moreover, setup and margin strategies were revisited. Material/Methods: Dose plans of patients receiving PBI at our institution, January 2023 – October 2025, were extracted from the treatment planning system to assess target coverage and organ-at-risk (OAR) doses. The patients were treated with 40 Gy in 15 fractions according to DBCG guidelines using a 3D conformal field-in-field technique with tangential or angled fields, an isotropic 6 mm CTV–PTV margin and open MLCs to free air in regions near the skin. Left-sided targets were treated in deep inspiration breath-hold (DIBH). The PTV was cropped 3 mm from the skin and used for normalization. The validity of the CTV–PTV margin was tested for our CBCT-based setup strategy using standard van Herk formalism [2] on a subset of 28 patients (16 left-, 12 right-sided). Results: A total of 311 patients (157 left-, 154 right-sided) were included. Delineation practice limits heart statistics to 52 patients for right-sided treatments.In Figure 1, dose readouts at selected relative volumes of the heart and the ipsilateral lung are presented as boxplot distributions for left- and right-sided treatments, respectively.In Figure 2, target coverage and OAR doses are given as the 80th and 95th percentiles for cohort readout values and compared with corresponding constraints from current DBCG and FF standard arm guidelines. As cohort values are more conservative than either guideline, we suggest new, tighter PBI constraints to supplement existing DBCG hard constraints. Scaled by total dose, the new constraints can also be applied in a national implementation of the UHF scheme.The setup-error analysis demonstrated good compliance with the applied clinical PTV margin for both left- (gated) and right-sided (non-gated) targets. To evaluate the influence on margins of reducing the number of fractions to five, we applied the methodology from [3], which resulted in negligible corrections (< 1 mm).
Conclusion: Doses from modern PBI planning are more restrictive than the standards set by DBCG and FF. We propose new, tighter PBI constraints, that when scaled by total dose, can be adopted in the UHF regime with unchanged setup and margin procedures. References: [1] Danish Breast Cancer Group locoregional radiotherapy guide ver. 4.0, see DBCG.dk. [2] van Herk M et al. The probability of correct target dosage: dose– population histograms for deriving treatment margins in radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 2000; 47:1121–35. [3] Gordon J J & Siebers J V. Convolution method and CTV-to-PTV margins for finite fractions and small systematic errors. Phys. Med. Biol. 2007; 52:1967–90. Keywords: Partial breast, dose limits, hypofractionation
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