The presence of ERα in breast cancer is an established prognostic marker determining the likelihood of response to anti‐hormonal therapy. PR is a marker of ERα functionality. ERα‐positive and PR‐positive tumours are more likely to respond to anti‐hormonal treatment than ERα‐positive and PR‐negative tumours; there is some evidence that ERα‐positive and PR‐negative tumours are more likely to respond to aromatase inhibitors than tamoxifen.9,10,11
In this study, we have explored the hypothesis that the dyes used in SLNB could interfere with hormonal receptor determination. A significant diminution in scoring was seen with cells treated with MB at 1:10 and 1:100 dilutions, with some samples rendered negative. This was not seen with PBV and IDC (table 1). These results suggest that an inherently weak ERα expresser could be rendered negative after exposure to MB with consequent withholding of potentially beneficial treatment. However, it is difficult to accurately relate in vitro findings to an in vivo situation. No similar effect was seen on the results of MNF116 immunolabelling, and the effects of these dyes on other markers would need case‐by‐case evaluation.
Hirsch et al6
measured MB tissue concentrations in breast tissue samples from a patient who had 0.1–0.2 ml of 1% MB injected, using spectrophotometry. This showed tissue concentration of 0.08, which is roughly equal to 1:10 dilution of the dye. The volume used for SLNB is larger, with 2 ml routinely injected for PBV and up to 5 ml for MB.1,4,12,13
Although the recommended site of injection for PBV in the UK is subdermal, around the areola, this is not the case with MB because of potential injection site complications, especially skin necrosis.14
The site of injection for MB is deeper in the breast tissue.13,15
Deep subareolar injections of MB have been previously described.12
Probably, with deeper peritumoral injections, the concentration of MB in the tumour could be in the region of 1:10 or even higher because of the larger volume used, whereas it should be lower with injections at sites more distant from the tumour. These concentrations are comparable to those that reduced ERα and PR status in our experimental system. The time durations of exposure to the chosen dyes were pragmatic: the shorter 4 h exposure to represent the duration after a straightforward excision transferred promptly to pathology, whereas the longer time was chosen to represent delayed fixation.
The concentration dynamics and the cell time exposures are likely to be highly complex. In cell line studies, direct and rapid exposure of the cells to the dye occurs. In the context of a complex tissue, even when the dye can be directly visualised pervading the tumour on gross analysis, the uniformity and certainty of cell exposure can only be hypothesised.
In summary, MB; which is one of the dyes used for SLNB worldwide, can reduce ERα and PR staining levels but not MNF116, in vitro. We suggest that if MB is used for SLNB, ERα should be checked on the core biopsy, if it was negative in the excised tumour.