How To Correctly Uppercase Greek Words In .NET

We have ASP.NET application which runs different clients around the world. In this application we have dictionary for each language. In dictionary we have words in lowercase and sometimes we uppercase it in code for typographic reasons.

var greek= new CultureInfo("el-GR");
string grrr = "� όλη";
string GRRR = grrr.ToUpper(greek); // "� ΌΛΗ"

The problem is: ...if you're using capital letters then they must appear like this: f.e. � ΟΛΗ and not like � ΌΛΗ, same for all other words written in capital lettersSo is it possible generically to uppercase Greek words correctly in .NET? Or should I wrote my own custom algorithm for Greek uppercase?