Optimizing posterior neck coverage in regional nodal irradiation for breast cancer.

Patterns-of-failure studies suggest the posterior neck (PN) may harbor microscopic disease in patients with locally advanced breast cancer. We compared target coverage, lung dosimetry, and toxicity in patients treated with either an anterior oblique field (AO) or opposed oblique fields (OO) to address the superior axillary and supraclavicular (SCV) nodal regions. We identified 109 patients treated at a single institution with regional nodal irradiation utilizing a mono-isocentric technique with a "third field" for superior axillary and SCV coverage. SCV and PN clinical target volume (CTV) coverage was determined based on the delivered treatment plan. CTV coverage, lung dosimetry, and toxicity differences between AO and OO treatment groups were evaluated using Wilcoxon rank-sum and Fisher's exact tests. The AO group had 29 patients, and the OO group had 80. Both groups had similar body mass indices and anterior-posterior separations at the match line. The median V90% of the PN was significantly improved in the OO group (100%) compared to 30% in the AO group (p < 0.001). The median V90% in the axillary levels I to III were similarly improved in the OO group compared to the OA group (p < 0.001). The maximum dose, heterogeneity and V90% of the SCV, as well as the lung dosimetry were similar in both groups. Rates of toxicities including ipsilateral upper extremity lymphedema and pneumonitis (all grade 1), and moist desquamation were low and similar across groups. Compared to a single anterior field, opposed oblique fields provide superior axillary and posterior neck coverage without increasing toxicity.