Disposing of Microsoft.Office.Interop.Word.Application

Do you not need to call Quit?

app.Quit();

Perhaps try setting doc = null and calling GC.Collect()

Edit, not really my own code I forget where I got it but this is what I use to dispose of Excel, and it does the job maybe you can glean something from this:

public void DisposeExcelInstance()
{
    app.DisplayAlerts = false;
    workBook.Close(null, null, null);
    app.Workbooks.Close();
    app.Quit();
    if (workSheet != null)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(workSheet);
    if (workBook != null)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(workBook);
    if (app != null)
        System.Runtime.InteropServices.Marshal.ReleaseComObject(app);
    workSheet = null;
    workBook = null;
    app = null;
    GC.Collect(); // force final cleanup!
}