Geometrical and dosimetrical evaluation of different interpretations of a european consensus delineation guideline for the internal mammary lymph node chain in breast cancer patients

Background and Purpose: This study aimed at investigating the dosimetric impact on organs at risk, when the left- sided internal mammary lymph nodes (IMN) were delineated with two interpretations of the same guideline. Materials and Methods: The cohort consisted of 95 left-sided breast cancer patients with indication for irradiation of the CTVn_IMN treated at the Netherlands Cancer Institute (NKI). The NKI interpretation of the ESTRO guidelines was in the clinical structure sets (CTVn_IMN_NKI). A deep learning model was used as second interpretation of the guideline, based on a Danish consensus interpretation (CTVn_IMN_DK). The geometrical similarity was evaluated with the Dice Similarity Coefficient (DSC), volume, width, distance to sternal bone (SB) and maximum distance between the interpretations in the medial direction. Treatment plans were generated for both CTVn_IMNs. Mean heart dose (MHD) was correlated with the geometrical metrics. Results: 62 patients were eligible for analysis. The geometric comparison showed a median volume of 9.59 ml/ 7.19 ml for the CTVN_IMN_NKI/CTVn_IMN_DK along with a median DSC of 0.63. The width and distance from SB were significantly different, with a median width of 18.2 mm/14.7 mm and distance to SB of 3.4 mm/5.1 mm for CTVn_IMN_NKI/CTVn_IMN_DK. The MHD was significantly higher with the CTVn_IMN_NKI. The strongest correlation was found between MHD and maximum medial difference between the CTVn_IMN in slices where the heart was present. Conclusions: Differences in interpretations of the CTVn_IMN delineation guidelines were found, resulting in significant differences in MHD. For the individual patients, the dosimetric differences may impact treatment decisions, underscoring the need for strong consensus across borders.