Sensitivity and Specificity of Pre-Operative Staging of the Axilla When Using OSNA in Breast Cancer Patients

Introduction: All patients with a new diagnosis of breast cancer undergo pre-operative axillary ultrasound (US) and biopsy to identify axillary metastases. The efficacy of this approach at identifying metastatic disease is particularly important if an intra-operative technique of assessment is used. This paper is the first to report the sensitivity and specificity of pre-operative axillary ultrasound within the context of the routine use of OSNA (one step nucleic acid amplification) (Sysmex, Japan), a molecular assay technique for the intra-operative analysis of axillary lymph nodes. This knowledge will help counsel patients prior to the procedure and inform economic analysis of the implications of this approach. Method: Consecutive patients diagnosed with invasive breast cancer between July 2009 and February 2012 at a single centre were identified by interrogation of MDT (multi disciplinary team) and surgical databases and a prospective database of all patients undergoing OSNA analysis. A standardised data collection sheet was used to record tumour characteristics and the nodal stage for each patient. These results were used to document the efficacy of preoperative US and biopsy of the axilla in comparison to intraoperative OSNA analysis. The data were then compared between two groups of patients, those whose nodes had been assessed using OSNA (the OSNA group) and those nodes had been tested using standard histopathological examination (the non-OSNA group). Results: A total of 377 eligible patients were identified. Within the OSNA group (n = 240) the sensitivity of pre-operative axillary US was 29% which rose to 41% when micro-metastases were excluded. A specificity and positive predictive value of 100% was recorded. The negative predictive value was 74%. There were no significant differences to the efficacy when the nodes were assessed with standard histopathology. The detection rate of micro-metastases was significantly higher in the OSNA group (P = 0.001), but there was no statistically significant difference in those with macro-metastases (P = 0.457). Conclusions: There are no significant differences in the sensitivity and specificity achieved by pre-operative axillary US when compared to either post operative histopathology or OSNA molecular analysis as the gold standard. OSNA, however, does increase the detection of micro-metastases.