Abstract: // Morad Elbouchtaoui1, 2, Iulia Tengher3, Catherine Miquel1, 2, Charlotte Brugière1, Amélie Benbara4, Laurent Zelek5, 7, Marianne Ziol 3, 5, 6, Fatiha Bouhidel1, 2, Anne Janin1, 2, Guilhem Bousquet1, 5, 7, * and Christophe Leboeuf1, * 1Université Paris Diderot, Inserm, UMR_S1165, Paris, France 2Pathology Department, Hôpital St Louis, APHP, Paris, France 3Pathology Department, Hôpital Jean Verdier, APHP, Bondy, France 4Obstetrics and Gynecology Department, Hôpital Jean Verdier, APHP, Bondy, France 5Université Paris 13, Villetaneuse, France 6Inserm UMR_S1162, Paris, France 7Oncology Department, Hôpital Avicenne, APHP, Bobigny, France *Co-senior authors Correspondence to: Guilhem Bousquet, email: [email protected] Christophe Leboeuf, email: [email protected] Keywords: HER2 overexpressing breast cancer; micromethods; cancer therapy; laser-microdissection; trastuzumab-based treatment Received: July 03, 2017 Accepted: April 03, 2018 Published: April 27, 2018 ABSTRACT An intraductal carcinoma, 55 mm across, was diagnosed on a total mastectomy in a 45-year-old woman. The 2 micro-invasive areas found were too small for reliable immunostainings for estrogen, progesterone, and HER2 receptors. In the sentinel lymph-node, a subcapsular tumor embole of about 50 cancer cells was identified on the extemporaneous cryo-cut section, but not on further sections after paraffin-embedding of the sample. Considering this tumor metastatic potential, we decided to assess HER2 status on the metastatic embole using pathological and molecular micro-methods. We laser-microdissected the tumor cells, extracted their DNA, and performed droplet-digital-PCR (ddPCR) for HER2 gene copy number variation. The HER2/RNaseP allele ratio was 5.2 in the laser-microdissected tumor cells, similar to the 5.3 ratio in the HER2-overexpressing breast cancer cell line BT-474. We thus optimized the adjuvant treatment of our patient and she received a trastuzumab-based adjuvant chemotherapy.