Introduction & Objectives: Determining the origin of metastatic tumours, particularly poorly differentiated ones, can be a diagnostic challenge. TTF-1 immunohistochemical stain is a sensitive marker for lung adenocarcinomas. However, positive labelling in adenocarcinomas of other origin, such as the prostate, has been reported. In this study, we sought to determine the incidence of TTF-1 expression in metastatic prostate cancer and determine whether TTF-1 expression may be associated with more aggressive behaviour.
Materials & Methods: We reviewed the clinicopathologic parameters of 98 cases of metastatic prostate cancer from 2015-2019 and determined the incidence of TTF-1 expression.
Results: Of the 98 cases of confirmed metastatic prostate cancer, 22% were positive for TTF-1 expression. The average age at time of metastasis was 71.9 years (range 51-89). The sites of involvement consisted of non-regional lymph nodes (28%), bone (26%), lung (20%), liver (20%) and other sites (6%). Among the TTF-1 positive cases, 45% presented with de novo metastasis with no known prior history of prostate cancer. Two of the positive cases were ductal type (one with bone metastasis and one with metastasis to lung) and one case showed neuroendocrine differentiation. The remainder of the cases were conventional acinar type. There was no significant difference in TTF-1 expression between cases with prior treatment (hormone therapy or radiotherapy) and non-treated cases (20% vs 27% respectively). There was no significant difference in frequency of lung, lymph node or bone metastasis. However, there was a statistically significant difference in liver metastasis, which was more frequent in TTF-1 positive cases compared to TTF-1 negative cases (45% vs 13%, p=0.001).
Conclusions: We found a significant incidence of TTF-1 expression in metastatic prostate cancer. This can pose a potential pitfall in determining the origin of metastatic tumours arising from sites other than the lung.
