We read with interest the article by Tamimi and coworkers recently published in this journal 1. The authors compared the molecular subtypes of invasive carcinoma versus ductal carcinoma in situ and found significant differences, as expected 1. However, we have some concerns regarding the criteria used in the study.
First, the authors classified estrogen receptor (ER)-positive/human epidermal growth factor receptor (HER)2-positive tumors as luminal B (LUMB). Although this classification is in accordance with that used in the Carolina Breast Cancer Study 2, the LUMB tumors – as identified by gene expression profiling – were all negative for HER2 3. The LUMB tumors are defined as tumors that show low to moderate expression of luminal specific genes, including the ER cluster 34. Extrapolating these findings to routine practical use, one must use semiquantitative immunohistochemistry (Allred-score, Q-score, or an H-score like method) 5678 to define and distinguish luminal A (LUMA) and LUMB tumors. A large amount of information is lost when one labels a tumor as a mere ER-positive one, because a tumor in which 15% of cells exhibit weak ER staining is biologically different from one that demonstrates strong intensity staining in about 90% of cells. Although the vast majority of ER-positive tumors show strong immunoreactivity, approximately 20% of tumors exhibit variable ER expression. ER expression in breast carcinoma is a continuous variable, which has been demonstrated not only by immunohistochemistry and ligand binding assay, but also by quantitative RT-PCR assays 691011. Moreover, using data from the NSABP B-14 clinical trial, Baehner and coworkers 12 demonstrated that the greater benefit from tamoxifen is seen in patients with greater ER expression, as determined by RT-PCR.
Although it is difficult to define a cut-off, any ER-positive/HER2-negative tumor showing diffuse and strong ER expression in two-thirds of the tumor (an H-score of 200 or higher) could be considered to be a LUMA tumor and the remainder of ER-positive/HER2-negative tumors could be considered LUMB. Although not the most accurate, this arbitrary cut-off is simple and keeps the category of LUMA tumors as pure as possible using immunohistochemistry. The ER-positive/HER2-positive tumors could similarly be subdivided into LUMA-HER2 hybrid (LAHH) and LUMB-HER2 hybrid (LBHH), based on ER expression levels. The LBHH tumors probably correspond to the originally described luminal C tumors 3. LAHH tumors definitely exist but do not have a molecular correlate. We believe that this distinction is necessary before studies utilizing surrogate immunohistochemical markers are undertaken, because HER2-positive tumors should be separated from pure luminal tumors, which should be further categorized as LUMA and LUMB tumors.
Second, the authors considered HER2 2+ expression by immunohistochemistry to be a positive finding. Numerous studies have shown that only one-quarter of immunohistochemical score 2+ cases demonstrate HER2 gene amplification by fluorescence in situ hybridization 13. The authors did mention that 'the results of analyses in which HER2 positivity was defined as 3+ were very similar to those presented with a definition of 2+ and 3+'. However, the more important question is about the comparison of '2+ only' cases with '3+ only' cases.