Purpose: There is growing interest in HER2 and its downstream signaling pathway molecules as treatment targets in human malignancies, including prostatic carcinoma. We used a standard Food and Drug Administration approved HER2 immunohistochemical kit to study HER2 expression in prostate cancer. We compared the results with those reported for mammary carcinoma.
Materials and methods: For this study we selected 216 specimens obtained from patients who underwent radical prostatectomy for primary untreated prostatic carcinoma. Immunohistochemical staining was performed using the HercepTest kit (Dako Corp., Carpinteria, California) in strict fashion according to the technical and analytical protocols outlined in the kit.
Results: Of the tumors 33 (15%) were positive for HER2, including 31 (94%) that were only weakly positive (2+). Of the HER2 positive tumors 97% were Gleason grade 7 or higher. Of the 33 positive cases 22 (67%) showed only a focal positive reaction for HER2 in discrete tumor areas.
Conclusions: HER2 is expressed in a minority of untreated primary prostatic carcinomas. Unlike in mammary carcinoma, HER2 positivity in prostatic cancer is usually weak in intensity and focal in distribution. While the former casts doubt on the usefulness of HER2 as a potential treatment target for most primary untreated prostatic cancer, the latter phenomenon may lead to false-negative results if the test is performed in small biopsies. Furthermore, HER2 staining of prostatic carcinoma can be technically and analytically reproducible provided that there is strict adherence to the outlined methodologies and interpretation.