RESUMEN
Identifying breast cancers with HER2 overexpression or amplification is critical as these usually imply the use of HER2-targeted therapies. DNA (amplification) and protein (overexpression) HER2 abnormalities usually occur simultaneously and both in situ hybridisation and immunohistochemistry may be accurate methods for the evaluation of these abnormalities. However, recent studies, including those conducted by the Association for Quality Assurance of the Spanish Society of Pathology, as well as the experience of a number of HER2 testing National Reference Centres have suggested the existence of serious reproducibility issues with both techniques. To address this issue, a joint committee from the Spanish Society of Pathology (SEAP) and the Spanish Society of Medical Oncology (SEOM) was established to review the HER2 testing guidelines. Consensus recommendations are based not only on the panellists' experience, but also on previous consensus guidelines from several countries, including the USA, the UK and Canada. These guidelines include the minimal requirements that pathology departments should fulfil in order to guarantee proper HER2 testing in breast cancer. Pathology laboratories not fulfilling these standards should make an effort to meet them and, until then, are highly encouraged to submit to reference laboratories breast cancer samples for which HER2 determination has clinical implications for the patients.