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* Context.--Accurate HER2 testing in breast cancer is crucial for appropriate precision therapy. HER2 testing is most commonly accomplished by a combination of immunohistochemistry and in situ hybridization techniques, as gene amplification is closely tied to protein overexpression. During the last 5+ years, brightfield dual in situ hybridization (DISH) has replaced fluorescence methods (fluorescence in situ hybridization [FISH]) in some laboratories. Objective.--To analyze routine HER2 DISH performance in the field. Design.--We reviewed our experience with HER2 DISH performed at outside laboratories and referred for patient care. Results.--Of 273 identified retrospective DISH results, 55 had repeated FISH testing at our institution; 7 (13%) were discordant. Additional cases had technical flaws hampering appropriate scoring. In 23 cases (42%), HER2 DISH was performed without immunohistochemistry. Slide review of a prospective cohort of 42 consecutive DISH cases revealed 14 (33%) with technical or interpretative limitations potentially jeopardizing results. Commonly identified problems include lack of or weak signals in most tumor cells, and silver precipitate or red signals outside of nuclei, resulting in false-negative or false-positive interpretations, respectively. Further, 44% (24 of 55) of DISH reports lacked complete data, specifically average HER2 signals/cell. Conclusions.--While HER2 DISH can be an efficient and effective alternative to FISH, we illustrate pitfalls and reinforce that careful attention to slide quality and technical parameters are critically important. HER2 DISH cotesting with immunohistochemistry could help minimize false-negative or false-positive HER2 results. doi: 10.5858/arpa.2019-0510-OA, The human epidermal growth factor receptor 2 (HER2) is encoded by the ERBB2 gene on chromosome 17. (1) Sentinel studies in the 1980s demonstrated that overexpression of this tyrosine kinase [...]

The article presents a case of a 69-year-old woman with a phyllodes tumor arising in the vulva. It examines whether phyllodes tumor is a site-specific tumor requiring the presence of ectopic breast tissue tumorigenesis. It also discusses the tissue distribution of phyllodes and other common differential diagnoses.