The use of one-step nucleic acid amplification (OSNA) and tumor-related factors in the axillary management of breast cancer: A predictive model

61 Background: Recent trends in surgical practice advocate selective use of axillary nodal clearance (ANC) following sentinel lymph node biopsy (SNB) in the treatment of breast cancer. We aimed to determine the effectiveness of one-step nucleic acid amplification (OSNA) using CK19 mRNA copy number and tumor-related factors in predicting non-sentinel axillary nodal involvement, in order to formulate local guidelines for ANC. Methods: Patients due to have SNB at our institution for invasive breast cancer as well as selected patients with high grade ductal carcinoma in situ with negative assessment of the axilla on pre-operative imaging were included. Alternate slices of each node were sent for assessment by either OSNA or Histopathology. Immediate ANC was performed if OSNA was positive. The CK19 mRNA copy number, the total tumor load (summation of m RNA copy number of positive nodes,TTL), the total nodal status at ANC and tumor characteristics including grade, tumor size and lymphovascular invasion (LVI) for each patient were determined. A model of risk probability was constructed using TTL and tumor related factors. Results: 664 nodes were examined from 425 patients who had SNB performed between 2011 and 2014. After excluding 8 patients who did not meet the study criteria, 648 nodes from 417 patients were included for analysis. The concordance between OSNA and histology was 91.4%; positive predictive value (PPV) and negative predictive value (NPV) was 77% and 97% respectively. Patients with TTL less than 1400 did not have additional non sentinel lymph node involvement. TTL (p