Efforts have been made to match the molecular signatures tests with clinicopathological characteristics. The ASCO/CAP associations have emitted guidelines for interpretation of HR and HER2 expression by IHC with the intent to reduce the interobserver variability and to achieve a better correlation with the molecular classification. However, the capacity to discriminate between the luminal A and B subtypes by IHC is not ideal. Even with the standardization of the technique, there is a 30–40% discrepancy between IHC and multigene expression assays, with substantial implications in treatment decisions .
In our study, the observed range of Ki-67 expression was wide, with 2–70% vs 1–85% in the EndoPredict (Myriad Genetics, Inc., Salt Lake City, UT, USA) cohort and 2–50% vs 3–70% in theMammaPrint (Agendia, Inc., Irvine, CA, USA) cohort, low-risk and high-risk, respectively. However, the medians were slightly different for the two risk groups.
It is known that the Ki-67 assay has a moderate interobserver variability . The hot-spot vs. the whole-slide analysis of Ki67 has been an area of controversy, with the first being more practical by taking into account the more aggressive biology spot, acknowledging tumor heterogeneity. Thakur et al. evaluated the hot-spot vs. whole-slide Ki-67, identifying a strong correlation between the two methods (r = 0.938) . To reduce the interobserver variability, the International Ki-67 in Breast Cancer Working Group recommends if the stain is homogenous to count at least three randomly selected high-power (×40 objective) fields and if it is heterogenous, three fields in the tumor edge or hot spots, with some exceptions and scoring preferably 1000 cells with 500 at a minimum .
In the MammaPrint cohort, a low Ki-67 (< 20%) demonstrated a high sensibility (88%) and could modestly predict patients with low-risk of recurrence (PPV 0.61%, 0.46–0.75 CI 95%). Moreover, the Ki-67 and the molecular test, Mammaprint (Agendia, Inc., Irvine, CA, USA), had an overall concordance of 67.14% and a fair agreement (κ = 0.35 p = 0.001). Similar to our results and utilizing the same Ki-67 cutoff, Viale et al. reported a concordance of 71% (69–72 CI95%) between the molecular classification of Luminal IBC and Ki-67 in the EORTC 10041/BIG 3–04 MINDACT trial (κ = 0.35, 0.32–0.37 95% CI) . Additionally, another report demonstrated similar results (κ = 0.35) between Mammaprint and Ki-67 in 65 IBC patients, however, they utilize a different Ki67 cutoff (14%) . Similar to our study, Bösl et al. compared MammaPrint and EndoPredict with Ki-67, achieving a significant correlation with MammaPrint (p = 0.004) but not with EPclin score (p = 0.09) . Despite this encouraging concordance between Ki-67 and MammaPrint, in the EndoPredict cohort, the Ki-67 overall concordance was low and did not significantly correlate with the EPclin risk category (59.37%, κ = 0.168, p = 0.09). This mean that when patients were stratified by Ki-67, 30–40% in each cohort were assigned to other risk categories compared to molecular testing.
In clinical practice, the indication of adjuvant chemotherapy is based on the consideration of multiple variables such as patient age, tumor size, histological type and grade, PR status, and LVI. In our analysis, we did not observe a correlation between PR and Ki-67, EndoPredict, or MammaPrint. It is worth noting that only a small number of patients (11 and 6 patients in each cohort) had a PR expression of < 20%, highlighting the limited value of PR in the luminal classification of EIBC compared to Ki-67. Interestingly, a significant correlation between Ki-67 and NHG was observed in both cohorts (Chi-square EndoPredict χ2 = 4.68 p = 0.03 and MammaPrint χ2 = 6.32, p = 0.01), as has been previously reported [29–31].
The differences in the association between Ki-67 to MammaPrint and Ki-67 to EndoPredict could be due to the different patient selection criteria, clinicopathological differences between cohorts, and the acquisition of data from multiple centers, potentially introducing interobserver variability in the Ki-67.
Despite that molecular signature tests are an important tool to identify patients with low-risk of recurrence, the agreement between different tests is far from perfect. Pelaez et al. compared MammaPrint and EndoPredict, identifying an overall concordance of 72.5%, with a slight improvement using the EPclin score to an overall concordance of 75% , Similarly, Bösl et al. reported a concordance of 66% with more patients being placed in the low-risk category with MammaPrint .
Financially, the different molecular signatures tests have been evaluated with mixed results depending on the geographic location. A Canadian study found EndoPredict to be cost-effective with a ratio of $36,274 per quality-adjusted life-year (QALY), with a total gain of 379 QALYs/year . Meanwhile, in the United Kingdom (UK), EndoPredict was not identified as cost-effective with a threshold of £20,000/QALY. However, it was if the incremental cost-effectiveness ratio was £26,836/QALY . Moreover, a recent analysis in the UK showed that EndoPredict was cost-effective only if lymph node disease was present (1–3 positive nodes) with £30,000/QALY . On the other hand, MammaPrint in the USA was found to be cost-effective at a ratio of $10,000/QALY . Nevertheless, another study in the UK found that MammaPrint was not cost-effective compared to current clinical practice . Overall, in western European countries and the USA, molecular signature tests are cost-effective. The willingness to pay for QALYs in the healthcare systems of developing countries has yet to be studied. However, the cost of these tests might be onerous to healthcare systems in precarious situations.
Limitations to our study
Some limitations of our study include its retrospective nature and the potential for selection bias. Moreover, the groups tested with the different molecular signature tests were heterogeneous. However, this study represents a multicentric cohort of a large number of EIBC with molecular testing that allowed us the evaluation of the Ki-67 proliferation index to molecular signatures tests.