To contrast breast radiation exposure from chest radiotherapy in 2006-2021 with 1965-1997, and to compare breast cancer (BC) risk 25 years after treatment predicted by two models. Radiation dose distributions to the breast from 101 chest radiotherapies given 2006-2021 for Hodgkin lymphoma (HL) or other lymphoma in one German and two Dutch hospitals were compared with doses received by 505 Dutch HL patients treated 1965-1997 and sampled into a nested case-control study, weighted to represent a HL patient cohort. Dose-volume histograms, mean dose and doses to 10 breast segments were evaluated. Absolute BC risks 25 years after historic and recent treatments were estimated for low- and high-risk profiles (HL stage III vs I at age 24 in 2010, treatment-induced menopause at age 24 vs none, first live birth at age 20 vs nulliparous, BC family history no vs yes, no HL relapse, non-smoking at HL diagnosis, exposed to anthracyclines and cardiotoxic radiotherapy). Average mean breast dose decreased from 21.4 Gy for historic to 3.0 Gy for recent treatments. The breast volume receiving ≥20 Gy (V20) decreased from 48.7% to 2.2%, and the lowest dose to the highest exposed 20% of breast volume (D20) from 39.0 Gy to 2.8 Gy. Most metrics decreased during 2006-2021. Using quadrant-specific doses, median absolute BC risk 25 years after treatment decreased from 21.0% to 6.9% for historic vs. recent treatments for a high-risk patient, and from 3.1% to 0.9% for a low-risk patient, respectively. Using mean dose, median absolute BC risks decreased from 20.3% to 6.0% and from 3.1% to 0.9% for the high- and low-risk patient, respectively. Modern radiotherapy treatments expose the breast to lower radiation doses and irradiated volumes than those used before 2000. BC risk predictions using mean breast dose were slightly lower than predictions using quadrant-specific doses.
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