Validation of the ESTRO target volume consensus guideline for early-stage breast cancer in the DBCG Skagen Trial 1

Background and purpose: Adjuvant loco-regional radiotherapy reduces recurrence risk and enhances survival in early-stage breast cancer. The ESTRO consensus guideline on target volume delineation for elective radiotherapy of early-stage breast cancer, published in 2015, sought to standardise contouring practices. In the DBCG Skagen Trial 1, all clinical target volume (CTV) delineations followed the ESTRO guideline, allowing pre-planned prospective validation. This study aimed to characterise the loco-regional recurrence (LRR) pattern in accordance with target delineation as per the ESTRO guideline. Materials and methods: DBCG Skagen Trial 1 enrolled 2, 963 patients (2, 908 in the intention-to-treat cohort). LRR, with or without concurrent distant recurrence, were identified from imaging and medical records, and categorised as inside the CTV, marginal miss, or distant recurrence relative to the ESTRO-defined CTVs. Results: At median follow-up of 5.2 years (IQR 4.1–6.7), 78 patients developed LRR: 41 isolated LRR and 37 with concurrent distant recurrence. Among the 78 patients with LRR, 74 (95%) had recurrences located within the ESTRO-defined CTVs and were planned to receive the full prescribed dose. All local recurrences were within the CTVp, and all but three patients had regional recurrences within the CTVn. Only three marginal misses were identified. Conclusion: LRR in the DBCG Skagen Trial 1 occurred almost exclusively within the ESTRO-defined CTVs, providing prospective validation of the guideline, without evidence suggesting a need for modification of target volumes.