Background: Adjuvant radiation therapy for breast cancer improves survival but may expose thoracic organs to low-dose radiation, potentially increasing the risk of second primary lung cancer (SPLC). Prior studies have not adequately examined how SPLC risk varies across racial and ethnic groups or whether social factors influence incidence and survival outcomes. Objectives: To quantify racial and ethnic disparities in SPLC incidence and survival among breast cancer survivors treated with radiation and assess the potential protective role of marital status. Methods: We used SEER-17 registry data to identify patients diagnosed with a first malignant primary breast cancer from 2000 to 2022 who received radiotherapy. Patients who died within two months of diagnosis were excluded. Standardized incidence ratios (SIRs) were calculated to compare observed to expected SPLC cases across racial and ethnic groups, stratified by latency intervals (2–11, 12–59, 60–119, and ≥120 months) and marital status. Racial categories included White, Black, Asian or Pacific Islander (API), and American Indian or Alaska Native (AI/AN); ethnicity was categorized as Hispanic or non-Hispanic. Five-year overall survival after SPLC was estimated using Kaplan–Meier analysis, and mean age at death was calculated by subgroup. Results: Among 558,493 radiotherapy-treated breast cancer patients, 6,674 developed SPLC (1.19%). Risk varied significantly by race (p < 0.05). Black survivors had a 21% excess risk (SIR 1.21, 95% CI: [1.12–1.31]), peaking 5–10 years post-treatment (SIR 1.44, [1 .26-1.63]) and remaining elevated beyond 10 years (SIR 1.24, [1.05-1.45]). API patients had a 23% overall excess risk (SIR 1.23, [1.11-1.36]), while AI/AN patients had the highest overall risk (SIR 1.82, [1.24-2.57]), particularly in the 12–59 month window (SIR 2.29, [1.25-3.84]) and ≥120 months (SIR 2.38, [1.19-4.26]). White survivors had no sustained increased risk (SIR 0.96, [0.93-0.98]), aside from a short-term rise in the first post-treatment year. Hispanic patients had consistently lower SPLC risk (overall SIR 0.72, [0.65-0.79]), while non-Hispanics had a transient early increase but an overall neutral risk (SIR 1.01, [0.99-1.04]). Marital status was protective: married individuals had a 12% lower SPLC incidence (SIR 0.88, 95% CI 0.85–0.91). Five-year overall survival after SPLC was 28.0%, with significant variation by race and ethnicity (p = 0.002). API (32.2%) and AI/AN (32.5%) patients had the highest survival, followed by White (28.0%) and Black patients (25.6%). Hispanic patients had the best outcomes, with 37.4% five-year survival and a mean age at death of 65.5 years, compared to 27.0% and 55.0 years for non-Hispanics (p = 0.005). Married patients had higher five-year survival (31.8%) and older mean age at death (64.3 years) compared to unmarried patients (25.0%, 48.6 years; p < 0.001). Conclusion: Radiation-associated SPLC risk and prognosis vary significantly by race, ethnicity, and marital status. Black and API survivors face sustained elevated risk; AI/AN survivors experience high early and long-term risk, while White and Hispanic survivors show no increased risk overall. Hispanic and married patients experience better survival after SPLC. These findings underscore the need for race and context aware survivorship counseling and suggest consideration of personalized lung cancer screening thresholds for breast cancer survivors receiving radiotherapy.

Background: Adjuvant radiation therapy for breast cancer improves survival but may expose thoracic organs to low-dose radiation, increasing the risk of second primary lung cancer (SPLC). Racial and ethnic disparities and social factors influencing SPLC risk remain underexplored. Objectives: We quantified racial and ethnic differences in SPLC incidence and survival among radiotherapy-treated breast cancer survivors and assessed the potential protective role of marital status. Methods: Using SEER-17 (2000–2022), we identified patients with first primary breast cancer receiving radiotherapy, excluding those who died within two months. Standardized incidence ratios (SIRs) assessed observed versus expected SPLC cases by race and ethnicity and marital status, and five-year overall survival (OS) after SPLC and mean age at death were calculated. Racial categories included White, Black, Asian or Pacific Islander (API), and American Indian or Alaska Native (AI/AN); ethnicity was categorized as Hispanic or non-Hispanic. Results: Among 558,493 patients, 6674 developed SPLC (1.19%). Risk varied significantly by race (p < 0.05). AI/AN patients had the highest overall risk (SIR 1.82), particularly 12–59 months and ≥120 months post-treatment. Black (SIR 1.21) and API (SIR 1.23) survivors had sustained elevated risk, while White survivors showed no overall increase (SIR 0.96) and Hispanic survivors had lower risk (SIR 0.72). Married individuals had 12% lower SPLC incidence (SIR 0.88). Five-year overall survival after SPLC was 28.0%, with significant variation by race and ethnicity (p = 0.002). API (32.2%) and AI/AN (32.5%) patients had the highest survival, followed by White (28.0%) and Black patients (25.6%). Married patients had higher five-year survival (31.8% vs. 25.0%) and older mean age at death (64.3 vs. 48.6 years) compared to unmarried patients. Conclusions: SPLC risk and prognosis after breast radiotherapy differ by race, ethnicity, and marital status. These findings highlight the importance of context-aware survivorship counseling and support the consideration of personalized lung cancer screening for breast cancer survivors.