Deformable image registration accuracy: impact of user-defined parameter selection on contour propagation for deep inspiration breath-hold and free breathing breast radiotherapy

Purpose Deformable Image Registration (DIR) is increasingly used in breast radiotherapy planning, but the impact of different parameter settings on clinical accuracy remains unclear. This study systematically evaluates how focus region selection and registration direction affect DIR performance, providing evidence-based recommendations for clinical implementation. Materials and methods We analyzed 73 patients with left-sided breast cancer who underwent both free breathing (FB) and deep inspiration breath-hold (DIBH) planning CT scans. Clinical target volume (CTV), heart, left anterior descending artery, and both lungs were retrospectively contoured by one radiation oncologist on both datasets. Deformable Image Registration was performed bidirectionally (FB↔DIBH) using four different approaches: no focus region, CTV-focused, heart-focused, and surgical clip-focused registration. Volume differences and Dice Similarity Index (DSI) were calculated to assess registration accuracy. Generalized estimating equations analyzed possible correlations. Results Focus region selection critically impacted DIR accuracy. CTV-focused registration achieved highest CTV overlap (DSI: 0.96 vs. 0.91 without focus, p < 0.005) but compromised other structures. Heart-focused registration optimized cardiac structure accuracy but reduced CTV precision. Registration direction showed minimal impact on volume differences but affected DSI values. Tidal volume significantly affected both CTV ( p < 0.005) and heart DSI ( p < 0.007), with opposite effects on each structure. Manual contour adjustment remained necessary in all cases regardless of parameter selection. Conclusion DIR performance in breast radiotherapy is highly dependent on user-defined parameters, with no single configuration optimizing all structures simultaneously. Focus region selection creates a trade-off between local accuracy and global registration quality. Clinical implementation requires structure-specific parameter optimization and mandatory manual review. These findings establish practical limitations and benefits of hybrid DIR algorithms for breast radiotherapy workflows and highlight the continued necessity of expert oversight.