Molecular Evaluation of the SLNs
Use of molecular techniques to routinely detect lymph node metastases is a relatively rare practice in the United States (anecdotally far fewer than 0.1% of consult cases), although numerous commercial and noncommercial assays have been researched and developed in the past decade. The major selling point is that these assays can detect all metastases, including those missed on routine histopathology. Given detection of every last tumor cell is not our current goal, use of molecular assays for this purpose is unwarranted. However, a molecular assay to quantify and stratify lymph nodes in a way that parallels the conventional approach has gained traction in the United Kingdom and is worth mentioning, not least because it is controversial. One-step nucleic acid (OSNA) is reported to be used in 12% of UK hospitals, where it has replaced standard histopathologic evaluation of lymph nodes intraoperatively.
The OSNA method was developed by Tsujimoto and colleagues and reported in 2007. OSNA employs reverse transcription, loop-mediated isothermal amplification of CK19 messenger RNA (mRNA) to identify positive SLNs. Lymph nodes are sectioned longitudinally into four slices, with alternate slices used for mRNA polymerase chain reaction and routine pathology. In their original study of 325 lymph nodes from 101 patients, the authors claimed to be able to accurately classify a metastasis as negative, a micrometastasis, or a macrometastasis based on the CK19 titer. The CK19 mRNA copy number was correlated with size of the lymph node metastasis on histopathology to produce size-related ranges (eg, 250–5,000 copies/μL is a micrometastasis). There are many publications on this method, and they have all used the original discriminatory thresholds.
There is controversy over this method, and at least one analysis from Tiernan and colleagues has suggested the original threshold set for a macrometastasis is inaccurate. The authors also performed a meta-analysis of all studies of OSNA published until 2013. Although OSNA is highly sensitive and specific at detecting macrometastases (87% and 98%, respectively), up to 21% of patients identified as having macrometastases by OSNA would have disease reclassified as micrometastases on the corresponding histopathology. Given the current difference in treatment of the axilla around the micrometastasis/macrometastasis threshold, this potential inaccuracy is unacceptably high. To address this limitation, other authors have proposed a new cutoff that improves concordance between molecular and histopathologic methods, but it remains to be determined whether such strategies will affect utilization in the United States.
Another significant limitation of OSNA is that not all breast tumors express CK19 (negative in 11% of grade 3 tumors and 3% of grade 1 tumors), emphasizing the need to perform confirmatory CK19 IHC on the biopsied tumor prior to surgery.
Am J Clin Pathol. 2018;150(1):4-17. © 2018 American Society for Clinical Pathology