Long‐term cardiac outcomes in breast cancer patients treated with helical tomotherapy: Evaluating the applicability of 3D‐based dose constraints for intensity modulated radiation therapy

Adjuvant breast radiotherapy has been associated with cardiac toxicity due to older 2D and 3D techniques, with a linear relationship between mean heart dose (MHD) and ischemic cardiac events. Cardiac dose distribution differs with modern techniques like intensity-modulated radiotherapy (IMRT), potentially affecting this relationship. This study evaluates long-term cardiac toxicity in breast cancer patients treated with tomotherapy to reassess 3D-derived dose constraints. Breast cancer patients treated with tomotherapy at Institut Curie from August 2010 to December 2015 were included. Patients had undergone breast-conserving surgery or mastectomy, with some receiving chemotherapy or trastuzumab. Tomotherapy was used for anatomically challenging cases. The primary endpoint was cardiac toxicity correlated with MHD; secondary endpoints were overall and disease-specific survival. Statistical analyses included logistic regression and Cox models. Among 179 patients, the median MHD was 7.04 Gy, with 95.6% having an MHD above 5 Gy. Sixty-six patients had cardiovascular risk factors, and 28.5% were obese. Over a median follow-up of 9.1 years, eight patients (4.5%) experienced cardiovascular events-all with pre-existing risks or obesity. No significant correlation was found between MHD and major coronary events (p = 0.607) or heart failure (p = 0.800). Cardiac mortality was absent, and 10-year overall and disease-specific survival were 88.0% and 94.3%, respectively. Cardiac events in patients treated with tomotherapy were rare and driven by pre-existing risk factors. The linear MHD-toxicity relationship observed in 3D radiotherapy may not apply to IMRT, potentially leading to overestimated risks. Long-term studies are needed to refine IMRT dose constraints.