Racial and Ethnic Disparities in Second Primary Lung Cancer After Breast Radiotherapy: A SEER Cohort Analysis (2000–2022)

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.