Clinical Predictors of Dosimetric Precedence for Deep Inspiratory Breath Hold Radiation Therapy for Breast Cancer

Background/Objectives: Radiation therapy for left-sided breast cancer exposes the heart and lungs to incidental radiation, increasing long-term risk of cardiopulmonary morbidity. This study evaluated the dosimetric impact of DIBH compared with free breathing (FB) and explored patient- and treatment-related predictors of benefit. Methods: We retrospectively analyzed 90 patients with left-sided breast cancer or chest wall irradiation planned under both FB and DIBH. Dosimetric parameters included mean heart dose (MHD), mean lung dose (MLD), heart V5, and lung V20. Univariable analyses assessed associations between dose reductions (Δ%) and clinical factors (age, BMI, fractionation, boost, nodal fields, and smoking). Multivariable regression identified independent predictors. Results: DIBH significantly reduced mean doses to the heart (FB 4.25 ± 1.47 Gy vs. DIBH 2.64 ± 1.28 Gy; Δ −1.60 Gy, p < 0.001, and Cohen’s d = 2.46) and lung (FB 14.00 ± 3.45 Gy vs. DIBH 11.64 ± 3.32 Gy; Δ −2.36 Gy, p < 0.001, and Cohen’s d = 2.73). Similarly, heart V5 (median Δ −10.5%, p < 0.001) and lung V20 (median Δ −5.5%, p < 0.001) were significantly improved. Higher BMI was independently associated with smaller relative reductions in MHD (−1.4% per unit, p < 0.001) and MLD (−0.31% per unit, p = 0.019). Hypofractionation was linked to greater MHD and MLD reductions (+8.9%, p = 0.007), (3.7%, p = 0.035), respectively. Supraclavicular field irradiation increased lung exposure, while internal mammary chain fields elevated both lung and heart doses. Age showed no significant influence. Conclusions: DIBH provides robust and consistent reductions in cardiac and pulmonary radiation doses during left-sided breast irradiation. The degree of sparing is modulated by BMI, fractionation, and nodal field inclusion.