Survival analysis and immunohistochemical study of HER-2 and AR (androgen receptor) expression in salivary duct carcinoma

Background: Because of its low incidence, the clinical characteristics of the salivary duct carcinoma (SDC) based on a statistical analysis with a large number of patients remain to be elucidated, and thus it has been impossible to standardize the optimal treatments of SDC including adjuvant systemic therapy. Aims: The present study aimed to determine the prognostic factors along with the clinical outcomes of patients with SDC and to evaluate the expression of several receptor molecules as treatment targets. Methods: We performed a statistical analysis and immunohistochemical examination of 16 patients with SDC who had undergone initial treatment in the Department of Otorhinolaryngology, Head and Neck Surgery, Keio University School of Medicine from 1996 to 2010. Results: The 3-year disease-free survival (DFS) and cause-specific survival (CSS) rates were 29.2% and 72.7%, respectively. At the time of the analysis, 6 patients are alive without the disease, 2 patients are alive with distant metastasis, whereas 7 patients had died of distant metastasis, and 1 patient had died of another cause (pulmonary embolism). We examined the prognostic value of the clinico-pathological factors such as age, sex, T classification, N classification, clinical stage, primary site, histological pre-existence of pleomorphic adenoma. A univariate analysis revealed that DFS was significantly correlated with age (p = 0.049), T classification (p = 0.018), and clinical stage (p = 0.029), whereas no factor was found to be correlated with CSS. A multivariate analysis demonstrated that age (≥61 vs. ≤60, risk ratio (RR) = 5.423, p = 0.042) and T classification (3, 4 vs. 1, 2, RR= 1.087, p = 0.020) were the independent prognostic factors for DFS. Positive expressions of HER2, ER (estrogen receptor), PR (progesterone receptor), AR (androgen receptor), and MIB-1 (index<20%) were found in 50%, 6%, 13%, 100%, and 69%, respectively. However, none of them showed significant correlation with survival. Conclusion: Frequent expressions of HER-2 and AR in SDC suggest that these receptors can be suitable molecular targets of systemic therapy for patients with SDC in which distant metastasis seems to be the largest obstacle to improving survival. In order to assess the efficacy of anti-HER-2 therapy and anti-androgen therapy for each receptor-positive SDC, a multi-institutional joint research system should be organized.