S212
Clinical - Breast
ESTRO 2026
and breast cancer respectively (table 1). These values were lower than the absolute and relative risk reduction observed in the IMPORT LOW trial.
https://doi.org/10.1016/j.ijrobp.2019.06.2509 Keywords: Breast cancer, proton therapy, arm symptoms
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Simulated risk of secondary malignancies after PBI vs. WBI in comparison to a clinical randomized trial Sophia Kiesl 1 , Mathias Duesberg 1 , Maximilian Aigner 1 , Luisa Allwohn 1 , Sophie Therese Behzadi 1 , Sophie Maier 1 , Rebecca Moser 1 , Jana Nano 1 , Stephanie Elisabeth Combs 1,2 , Kai Joachim Borm 1 1 Department of Radiation Oncology, TUM School of Medicine and Health, Munich, Germany. 2 Department of Radiation Sciences (DRS), Helmholtz Zentrum, Munich, Germany Purpose/Objective: A 10-year analysis of the IMPORT LOW trial recently demonstrated a (non-significant) tendency for secondary malignancies to occur more frequently following whole breast irradiation (WBI) than partial breast irradiation (PBI)[1]. To better contextualize the observation regarding secondary malignancies in randomized trials, we conducted a simulation study based on existing risk models comparing tangential external beam PBI and WBI. Material/Methods: For 22 patients with breast cancer, 3D conformal tangential treatment plans were generated for WBI and PBI using pre-existing planning CT scans. The mean lung and contralateral breast doses (Dmean) were evaluated in WBI and PBI plans arms with 40 Gy in 15 fractions according to the IMPORT LOW trial. Subsequently, the organ equivalent doses (OED) for relevant organs at risk were determined and the EAR (excess absolute risk = absolute additional cases of disease per 10,000 patient-years (PY)) were calculated in MatLab using the Schneider's model [2]. This voxel- based approach also takes fractionation into account. The calculations considered a mean patient age of 60 years and follow-up period of 10 years. The absolute risk difference between WBI and PBI was calculated as Δ EAR=EARWBI – EARPBI and the influence of tumor localization was assessed. Additionally, we calculated the risk ratio (RR(D)=1+ERR/Gy*Dmean; ERR=excess relative risk) for secondary malignancies based on existing normal tissue complication probability (NTCP) models for the lung and the contralateral breast assuming an ERR/Gylung = 0.11 and ERR/Gybreast = 1.0 [3]. Results: The estimated absolute risk reduction of PBI for secondary lung cancer after 10 years was 9.6/10.000PY and for breast cancer 0.6/10.000PY. The estimated relative risk of WBI vs. PBI was 1.23 and 1.22 for lung
The risk advantage of PBI was dependent on the tumor location: For lung cancer the greatest Δ EAR was found in medial tumors, for contralateral breast cancer in lateral tumors (figure 1).
Conclusion: In our simulation, PBI was associated with a lower absolute and relative risk of secondary lung and contralateral breast cancer compared to WBI. However, this effect is smaller than observed in the 10- year follow-up of the IMPORT LOW trial. Together with the findings from the DBCG trial, which reported no reduction in secondary cancer risk in the PBI arm, this calls for caution in overinterpreting the trend seen in IMPORT LOW. References: (1) Kirby, A. M. et al (2025). Partial-breast radiotherapy after breast conservation surgery for women with early breast cancer (UK IMPORT LOW): 10-year outcomes from a multicentre, open-label, randomised, controlled, phase 3, non-inferiority trial. The Lancet Oncology(2) Schneider, U. et al (2011). Site-specific dose-response relationships for cancer induction from the combined Japanese A-bomb and Hodgkin cohorts for doses relevant to radiotherapy. Theor Biol Med Model(3) Taylor, C. et al (2017). Estimating the Risks of Breast Cancer Radiotherapy: Evidence From Modern Radiation Doses to the Lungs and Heart and From
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