TTF-1 may not be a reliable marker for differentiating metastasis from brain tumors

TTF-1 is widely used as an immunohistochemical marker of lung and thyroid tumors. However, TTF-1 expression has been described in tumors from other sites. The presence of TTF-1 expression in primary brain tumors is largely unclear and has not been clearly specified yet. We characterized expression of two TTF-1 clones in primary brain tumors with relevance to tumor types and grades. We studied immunohistochemistry with tissue micro-array, using both clones (8G7G3/1 and SPT24) in 45 primary brain tumors of different types and grades. Our cases consisted of 1 grade I, 7 grade II, 4 grade III, 20 grade IV astrocytic tumors; 9 meningiomas, 2 oligodendrogliomas, 1 schwannoma and 1 medulloblastoma. We have found TTF-1 nuclear staining using the SPT24 clone in 4 cases (3 cases were grade IV and 1 was grade III). Focal and weak staining was seen in three cases and moderate-strong and diffuse staining was seen in one case. All the tumors were negative with clone 8G7G3/1. Clone SPT24 was more sensitive but less specific. TTF-1 can also be expressed in primary brain tumors, particularly grade III to IV tumors. TTF-1 expression was a rare finding in previous studies, however strong and diffuse staining was not observed until today. We think that TTF-1 nuclear expression in high-grade astrocytic tumors cannot rule out primaries even when diffuse and strong staining. Clinical and pathological parameters should be evaluated together.