A Pattern Of Care Analysis Of Nodal Irradiation In The Post Z0011 Era: Results Of A Large Prospective Study

Since the publication of the ACOSOG Z0011 trial in 2010, there is a paucity of data analyzing the impact of radiation therapy (RT) field design for Z0011 eligible patients treated with breast conserving therapy (BCT). The RT fields in Z0011 have been retrospectively analyzed in a post-hoc analysis of only 124 patients in the sentinel lymph node (LN) biopsy arm; herein, we prospectively assess axillary nodal coverage and use of nodal irradiation in patients who met Z0011 criteria. From 2010-2015, 283 consecutive patients met Z0011 eligibility criteria (cT1-2N0, <3 positive nodes, no gross extranodal extension [ENE]), who underwent lumpectomy, sentinel LN biopsy, and RT. Axillary LNs were prospectively delineated on all patients according to RTOG guidelines. Radiation therapy was delivered with 3D conformal technique, and dose-volume histograms were generated. Univariate and multivariate analyses (MVA) by logistic regression were performed to determine association between clinicopathological factors and receipt of dedicated nodal RT (≥ 3 fields). Clinicopathologic characteristics were similar to those in the Z0011 trial: median age 56 years, median tumor size 1.6 cm, median number of positive LNs 1, total LNs sampled 3, and 32.5% N1mic. Eight-five percent of patients received tangents alone and 15% received an additional nodal field (15% supraclavicular, 3% posterior axillary boost and 1% internal mammary node RT). Among patients treated with tangents alone, 67% were treated in the supine and 33% in the prone position. The median V95% for Level 1 LNs was 33% and for Level 2 LNs was 14%, although the range was wide (0-100%). On univariate analysis, tumor grade, young age (≤50 years), greater number of positive LNs (2 vs 1), ENE, macrometastases (vs micro), and increasing time interval since the publication of Z0011 were associated with nodal RT (P < .05). On MVA, younger age (OR = 4.7; 95% CI = 1.9-11.7; P = .001), high grade (OR = 3.9; 95% CI = 1.1-14.2; P = .036), two positive LNs (OR = 4.7; 95% CI = 1.6-13.8; P = .005), and ENE (OR = 5.0; 95% CI = 1.9-13.1; P = .001) remained predictive for receipt of a dedicated nodal field. In this large, prospective series of Z0011-like patients receiving tangential breast RT, the volume of level 1 and 2 axillary LNs receiving prescription dose was relatively low. The proportion of patients who received directed nodal RT with an additional field (15%) was comparable to the rate reported in the post-hoc analysis on Z0011 RT fields (16.9%). Patients with younger age, higher grade, ENE, and two positive LNs influenced the delivery of nodal RT, reflecting that physicians perceived these features as prognostic for nodal recurrence risk. Dosimetric analysis on the impact of these features on axillary nodal coverage will be reported at the time of presentation. Longer follow-up is required to assess clinical outcomes in this cohort.