Breast cancer in recent years has functioned as a pioneer tumour setting the stage for a new era of diagnostics and therapy in oncology. Estrogen (ER) and progesterone (PR) receptors followed by HER2 provided the first examples for targeted therapy and marked the beginning of the age of personalized medicine. Determination of ER and PR by immunohistochemistry (IHC) has almost completely replaced extract based methods because it can easily be integrated in standard histopathological procedures with no need of complicated fresh tissue logistics. Also, correlation to the number of tumour cells and their viability, admixture of normal, noninvasive and stromal cells is possible. But IHC suffers from lack of standardization and reproducibility appears to be suboptimal with up to 20% discordant results between local and central testing [1]. In a non-centralized test setting standardization can only be achieved by adherence of pathology institutes to guidelines and quality control [2]. The ASCO/CAP consented guidelines on ER and PR try to meet this requirement and parallel those on HER2 published 3 years earlier, in particular with regard to tissue fixation and test validation [3, 4].
ER positive has been defined as >1% labelled invasive tumour cells regardless of the staining intensity [3], because retrospective analysis of 14 studies on endocrine treatment provided evidence for this threshold. For PR the same threshold is valid and it was concurred that independent predictive information on endocrine responsiveness comes from PR testing. In most instances an ER-/PR+ status, however, may be due to inaccurate IHC. Proportion of positively labelled cells together with intensity should be reported, scores (Allred, Remmele) may be provided but are not mandatory. The guidelines leave it open what percentage of ER+ cells constitutes a strongly positive tumour which may be controlled by endocrine therapy only without need of chemotherapy [5]. No general recommendation is given on routine ER/PR testing of DCIS which may be useful in individual patients. Whereas breast recurrences are recommended for fresh ER/PR testing no statement on metastases can be found in the guidelines. Discrepant ER/PR status between primary and metastasis occurs in up to 20% [6]. Another critical issue in the guidelines is that in case of multiple tumours only one of the tumours (preferably the largest) is considered sufficient for ER/PR assessment [3].
Because of optimal fixation core biopsies are regarded as superior to resection specimens for testing as long as comparison with the resection specimens shows identity of tumour type and grade. Since discordance rates of 9–12% were reported between IHC and RNA expression signatures such as the recurrence score, it was concluded that a recommendation would be premature [3].
Pathologists should apply internal controls, e.g. normal duct, which demonstrate a spectrum of weakly to strongly stained cells and external on-slide positive and negative controls, which might include cell lines with known receptor content. In case that one of both controls is not sufficient, and no normal cells are represented in the tumour tissue or normal cells fail to demonstrate a heterogeneous labelling from weak to strong, the test can not be interpreted and has to be repeated. In addition external quality assurance methods to ensure ongoing accuracy in ER/PR testing are recommended, such as accreditation and at least bi-annual participation in proficiency testing programs with at least 10 challenges. Although it summarizes mostly what appears to be self-evident and at least the German proficiency testing program reveals ER/PR testing to be far less problematic than HER2 [7], the guideline has the merit to provide for the first time common standards on ER/PR testing in pathology.
References
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