Individualized Clinical Target Volume for Irradiation of the Supraclavicular Region in Breast Cancer Based on Mapping of the Involved Ipsilateral Supraclavicular Lymph Nodes.

PURPOSE: To map supraclavicular fossa-involved lymph nodes (SCF-LNs) in patients with nonmetastatic breast cancer, evaluate the coverage of widely adopted atlases, and propose modified borders for individualized regional irradiation.

METHODS AND MATERIALS: M0 patients with biopsy-proven SCF-LNs who were SCF treatment-naïve were included. The SCF was spatially divided into subregions, with each node mapped on the original images. The geographic misses after the borders of multiple atlases were evaluated and factors affecting SCF-LNs' spread pattern were analyzed.

RESULTS

From 1998 to 2022, 209 patients with 1242 SCF-LNs were eligible. Patients had a median of 4 nodes. At least 537 nodes (43.2%) in 147 patients (70.3%) were lateral to the sternocleidomastoid muscle (SCM), and 403 nodes (32.4%) in 127 patients (60.8%) were dorsal to the anterior scalene muscle (ASM). In the 88 patients with ≤3 SCF-LNs, at least 66 nodes (39.1%) in 40 patients (45.5%) were lateral to the SCM, and 34 nodes (20.1%) in 29 patients (33.0%) were dorsal to the ASM. These nodes were not covered by the Radiation Therapy Oncology Group (RTOG) atlas and partly within the Radiotherapy Comparative Effectiveness atlas. One hundred four patients (49.8%) had 432 SCF-LNs (34.8%) beyond the upper border of the European Society for Radiotherapy and Oncology (ESTRO) atlas. In multivariate regression, nodal sizes were associated with wider spread in the primary group. Being triple-negative (TN) subtype was associated with less spread in the recurrent group. Situation-based clinical target volumes (CTVs) were theorized, in which for a sequential spread, the posterior border could be the posterior scalene muscle or even be more constringent; otherwise, it should touch the anterior trapezius surface.

CONCLUSIONS

SCF-LNs tend to spread laterally and dorsally beyond the RTOG borders, even in M0 stages with ≤3 SCF-LNs. The ESTRO upper border does not guarantee coverage with multiple SCF-LNs. Nodal burden and non-TN types are predictive of wider dissemination. A situation-based CTV is possibly feasible. Deciphering the SCF-LN spread route is needed.