To validate the ESTRO guideline on clinical target volume (CTV) delineation for breast cancer using a multi-centre dataset.

Patients with axial imaging of gross locoregional recurrence (LRR) were identified from 10 participating institutions. All patients received RT, albeit not to all regional node. The location of LRR was transferred to the corresponding area on representative axial computed tomography images and compared with ESTRO-CTV.

The locations of LRRs in 234 patients with 337 recurrence lesions were mapped. The ESTRO-CTV encompassed 97.6% of all LRRs, except in lymph node level 4 and the pectoralis muscle. Although 8.8% of level 4 failures occurred outside the ESTRO-CTV, cranial to the subclavian artery, all nodes were located within 6 mm cranially. Another 20% occurred posterolateral to anterior scalene muscles; however, 11/16 cases had simultaneous multiple lymph node recurrences, and 8/16 initially had N2-3 tumours. Local recurrence at the pectoralis muscle was prominent in patients undergoing mastectomy but not breast-conservation surgery (28% vs. 2.9%, P = .001).

Our mapping data demonstrated that the ESTRO-CTV, with some considerations, successfully encompassed most LRRs in patients undergoing contemporary management, thus validating ESTRO-CTV to be valuable for highly conformal radiation therapy techniques.