Objective:

To quantify inter-observer discrepancies in postoperative breast cancer CTV contouring among regional oncologists and to qualitatively analyze the reasons for these contouring discrepancies to promote standardization and quality assurance.

Approach:

• Study Design: 22 frontline radiation oncologists independently contoured CTVs based on CT datasets of two post-surgical breast cancer cases (BCS and MRM).
• Evaluation Metrics: Inter-observer discrepancies were evaluated using Dice similarity coefficients (DSC) and Hausdorff distances (HD) against a consensus reference standard.

Key Findings:

• High overlap for large volumes (CTVp_breast and CTVp_thoracic wall) with DSC means of 0.80 ± 0.07 and 0.63 ± 0.08.
• Elevated mean 95% Hausdorff distances (HD95) for large volumes at 42.84 ± 50.62 mm and 45.73 ± 60.44 mm.
• Intermediate overlap for post-BCS focal volumes (CTV_tb and CTV_boost) with DSC mean of 0.56.
• Moderate consistency for supraclavicular volume (CTVn_L4 DSC mean: 0.57 ± 0.15) but poor for axillary Level III (CTVn_L3 DSC mean: 0.38 ± 0.26) and internal mammary nodes (CTVn_IMN DSC mean: 0.42 ± 0.14).

Interpretation:

Delineation consistency is acceptable for large volumes but poor for regional nodal stations, indicating a need for standardized anatomical training.

Limitations:

• Study reflects only a limited number of oncologists from regional hospitals, which may not represent broader practices.
• Manual contouring remains subjective and may vary significantly among practitioners.

Conclusion:

Standardized anatomical training is crucial to improve radiotherapy uniformity.