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Purpose: To map the location of metastatic supraclavicular (SCV) lymph nodes (LNMs) in breast cancer patients with SCV node involvement and determine whether and where the radiation therapy clinical target volume (CTV) of this region could be modified in high-risk subsets.Methods and Materials: Fifty-five patients with metastatic SCV LNMs were eligible for geographic mapping and atlas coverage analysis.All LNMs and their epicenters were registered proportionally by referencing the surrounding landmarks onto simulation computed tomography images of a standard patient.CTVs based on selected SCV atlases, including the one by the Radiation Therapy Oncology Group (RTOG) were contoured.A modified SCV CTV was tried and shown to have better involved-node coverage and thus theoretically improved prophylaxis in this setting.Results: A total of 50 (91%) and 45 (81.8%) patients had LNMs in the medial and lateral SCV subregions, respectively.Also, 36 patients (65.5%) had LNMs located at the junction of the jugular-subclavian veins.All nodes were covered in only 25.5% to 41.8% of patients by different atlases.The RTOG atlas covered all nodes in 25.5% of patients.Stratified by the nodes in all the patients as a whole, 49.2% to 81.3% were covered, and the RTOG atlas covered 62.6%.The lateral and posterior borders were the most overlooked locations.Modification by extending the borders to natural anatomic barriers allowed the new CTV to cover all the nodes in 81.8% of patients and encompass 96.1% of all the nodes.Conclusions: According to the distribution of SCV LNMs, the extent of existing atlases might not be adequate for potential metastatic sites in certain groups of patients.The extension of the lateral and posterior CTV borders in high-risk or recurrent patients might be a reasonable approach for increasing coverage.However, additional data in more homogeneous populations with localized disease are needed before routine application.