HER2 Dual In Situ Hybridization: Correlations and Cautions

Area of Science:

• Oncology
• Pathology
• Molecular Diagnostics

Background:

• Accurate HER2 testing is vital for precision therapy in breast cancer.
• HER2 testing commonly uses immunohistochemistry and in situ hybridization.
• Brightfield dual in situ hybridization (DISH) has increasingly replaced fluorescence in situ hybridization (FISH).

Purpose of the Study:

• To evaluate the real-world performance of HER2 DISH testing.
• To identify potential pitfalls and limitations in routine HER2 DISH procedures.

Main Methods:

• Retrospective review of 273 HER2 DISH results from outside laboratories.
• Repeat FISH testing on 55 discordant DISH cases.
• Prospective review of 42 consecutive DISH cases for technical and interpretative quality.

Main Results:

• 13% of repeated FISH tests were discordant with initial DISH results.
• 33% of prospective DISH cases showed technical or interpretative limitations.
• Common issues included weak/absent signals and artifacts, causing misinterpretations.
• 42% of DISH tests were performed without immunohistochemistry.
• 44% of DISH reports lacked essential data like average HER2 signals/cell.

Conclusions:

• HER2 DISH is an efficient alternative to FISH but requires strict attention to technical quality.
• Potential pitfalls can lead to false-negative or false-positive HER2 results.
• Cotesting HER2 DISH with immunohistochemistry can improve accuracy and minimize errors.