Diagnostic Work-up in HER2+ eBC: A Focus on Pathology

Heterogeneity of HER2-positive Breast cancer can be further classified into 3 subtypes: Biological (cellular level), Intra-tumoral (local tumor level) and temporal (at different time points).

Hormone receptor co-expression is classified as Biological (cellular level) and is seen is at the cellular level and this kind of biological heterogeneity is mainly represented by the co-expression of hormone receptors from the same cells that overexpress HER2.

Intra-tumoral heterogeneity is seen at local tumor and is named “spatial” heterogeneity. It is represented by the non-uniform HER2 expression within a single tumor with areas of HER2 positive and ares of HER2 negative.

Temporal Heterogeneity is the change in the HER2 status at different time points along the course of the disease- the conversion of HER2 status. The conversion could be seen when comparing the HER2 status of primary vs metastatic tumor OR pre- vs post- neoadjuvant therapy.

HER2 heterogeneous cancers may represent a distinct subset of HER2-positive breast cancer, with lower rates of pCR, lower levels of HER2 protein expression and may require different treatment approaches.

In contrast, HER2-enriched subtypes are truly HER2-driven and may be targets for chemotherapy de-escalation by avoiding combining HER2 positive treatment with chemotherapy.

When there is different biological features in the metastases as to primary tumor, for example, HER2 positive in primary tumor but HER2 negative in liver metastases, pathologist always use an alternative assay (FISH) to ensure the apparent loss of HER2 is not due to any of the possible technical problem.

Pathologist will also retrieve the original block of the primary tumor and stain together the primary tumor and the metastasis at the same time, with the same environment in the lab (same temperature and same humidity) in order to minimize any possible defects of the staining due to the analytical phase of the assay.

If the change of HER2 from primary to the metastasis is confirmed, it is not suggested to deny targeted treatment just because one of the metastases apparently loss HER2 status. We have to be reassure that there is no heterogeneity in the metastasis before stopping the targeted therapy.

Therapy-induced changes is not usually observed in the absence of neoadjuvant treatment. 8-30% conversion rate by IHC following neoadjuvant treatment with anti-HER2 therapy.

In KATHERINE exploratory analyses, around 8% of the patients lost HER2 positivity at residual tumor. In these 70 patients who have lost HER2 positivity, there have been no IDFS events among patients randomized to TDM1(n=28) and 11 IDFS events in patients randomized to trastuzumab(n=42). The magnitude of the benefit of TDM1 over trastuzumab was maintained even for those patients where the residual tumor apparently had lost HER2 positivity. Therefore, it is important to continue HER2 targeted therapy as whether the HER 2 status in the systemic micro-metastatic disease is lost or retained, remains unknown.

In conclusion, HER2 targeted therapy should not be withheld in cases where the residual tumor is HER2 negative following neoadjuvant treatment.

The pathologists’ role is to ensure accurate assessment of pCR, which is based on the absence of disease in the breast and the lymph nodes.

Pathological assessment is the gold standard for determining a complete response. 30-50% of patients with clinical CR who have residual breast cancer in the surgical specimen, 20% of patients with clinically suspected residual disease who actually have a pCR.

Pathologists also need to evaluate the tumor margins and define it as positive or negative. The tumor bed should be sampled extensively so as not to miss any residual disease. Patients should be re-operated on if there are tumor cells present at the resection