Correlation of Chest Wall Separation and Central Lung Distance With Ipsilateral Lung Dose in Post-operative Breast Cancer Patients Treated With Intensity-Modulated Radiotherapy (IMRT): A Retrospective Study.

Objective Post-operative chest wall radiotherapy with or without regional nodal irradiation is a cornerstone of breast cancer management. With increasing adoption of hypofractionated intensity-modulated radiotherapy (IMRT), minimizing incidental lung irradiation remains important despite improvements in dose conformity. Conventional geometric surrogates such as central lung distance (CLD) and chest wall separation (CWS), widely used in two-dimensional (2D) and three-dimensional (3D) conformal radiotherapy, have been less extensively evaluated in the IMRT era. This study assessed the relationship between these geometric parameters and ipsilateral lung dose in post-operative breast cancer patients treated with hypofractionated IMRT. Methods Thirty-six post-operative breast cancer patients treated at the Department of Radiotherapy, All India Institute of Medical Sciences (AIIMS), Gorakhpur, India, were retrospectively analysed. Twenty-eight patients (77.8%) had right-sided disease and eight (22.2%) had left-sided disease. All patients received hypofractionated IMRT to the chest wall with or without supraclavicular fossa irradiation to a dose of 40 Gy in 15 fractions. CWS was defined as the tangential distance between the anterior and posterior extents of the contoured chest wall on axial CT images, averaged across three central slices. CLD was measured as the perpendicular distance from the posterior chest wall into the ipsilateral lung. Dosimetric parameters analysed included ipsilateral lung V5, V10, V20, and mean lung dose (MLD). Pearson correlation analysis was used to evaluate associations between geometric and dosimetric parameters. Patients were additionally stratified into two groups based on CWS (<16 cm and ≥16 cm), and intergroup comparisons were performed using a two-tailed independent samples t-test. Results CLD demonstrated significant positive correlations with all evaluated ipsilateral lung dose parameters, with increasing correlation strength from low-dose to higher-dose metrics (percentage lung volume receiving ≥5 Gy (V5): r = 0.445, p = 0.006; percentage lung volume receiving ≥10 Gy (V10): r = 0.500, p = 0.002; percentage lung volume receiving ≥20 Gy (V20): r = 0.579, p < 0.001; MLD: r = 0.592, p < 0.001). CWS showed weaker inverse correlations with lung dose parameters, reaching statistical significance only for lung V20 (r = -0.418, p = 0.011). On subgroup analysis, patients with CWS <16 cm demonstrated significantly higher lung V20 values compared with those with CWS ≥16 cm (29.01 ± 3.13% vs 26.83 ± 0.92%; t = 2.964, p = 0.005). MLD was also higher in the CWS <16 cm group (14.92 ± 0.94 Gy vs 14.13 ± 1.34 Gy), although this difference did not reach statistical significance (t = 1.989, p = 0.055). Conclusion CLD is a significant geometric predictor of ipsilateral lung dose in post-operative breast cancer patients treated with hypofractionated IMRT. Significant associations were observed between CLD and all evaluated lung dose-volume parameters, particularly lung V20 and MLD. CWS demonstrated a weaker but significant inverse association with lung V20. These findings highlight the continued relevance of simple anatomical surrogates in contemporary breast radiotherapy planning and suggest that CLD may serve as a practical indicator of increased lung dose exposure during treatment planning.