Quality assurance of internal mammary node irradiation in the DBCG IMN2 study

Purpose/objective: The Danish Breast Cancer Group (DBCG) IMN2 study investigated the gain from internal mammary node irradiation (IMNI) in node-positive breast cancer patients. IMNI was indicated in right-sided patients, but not in left-sided. Target volume delineations were based on bony landmarks in contrast to the contemporary vessel-based ESTRO consensus guideline. Our objective was to compare IMNI doses in right-sided versus left-sided patients. Material/methods: Treatment plans and delineated structures including CTVn_IMN (IMN_old) from 2008 to 2014 were collected from the DBCG RT Nation study. During the study period, IMN_old was only delineated in right-sided patients. Right and left-sided CTVn_IMN structures were auto-segmented following the ESTRO guidelines (IMN_ESTRO). Due to cranial discordance between IMN_old and IMN_ESTRO, the IMN_ESTRO models were separated into IMN_ESTRO_cranial and IMN_ESTRO_intercostal space(IC)1-3, IC1-4, and IC4_only. Results: Treatment plans for 2837 patients were available (62.5 % of patients in the IMN2 study). In right-sided patients, the median IMN_old dose coverage (92.4 %) was higher than IMN_ESTRO (71.7 %), p < 0.001. Dose coverage in IMN_ESTRO_IC1-3 was comparable to IMN_old. Comparing IMN_ESTRO_IC1-3 in all patients by laterality, the median CTVn_V90% was 94.6 % (IQR 64.8–100.0) in right-sided patients and 20.4 % (IQR 0.9–55.8) in left-sided patients, p < 0.001. For right-sided patients, median CTV_V90% was 82.3 % in IMN_ESTRO_IC4_only. Median mean heart doses were lower in right-sided patients (1.2 Gy) than in left-sided (2.3 Gy), p < 0.001. Median mean lung doses were higher in right-sided patients (16.0 Gy) than in left-sided (12.7 Gy), p < 0.001. Conclusion: For IMN_ESTRO_IC1-3, we found a significantly higher IMN dose coverage in right-sided than in left-sided patients supporting treatment according to study guidelines in the DBCG IMN2 study.