//...
  private
    FCanvas: TControlCanvas;
//...
procedure TForm1.FormCreate(Sender: TObject);
begin
  FCanvas:=TControlCanvas.Create;
  FCanvas.Control := RadioGroup1;
end;

procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
  FCanvas.Free;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  FCanvas.Pen.Color := clRed;
  FCanvas.MoveTo(0,0);
  FCanvas.LineTo(RadioGroup1.Width,RadioGroup1.Height);
  FCanvas.MoveTo(RadioGroup1.Width,0);
  FCanvas.LineTo(0,RadioGroup1.Height);
end;

procedure TForm1.Button2Click(Sender: TObject);
begin
  //Kreutz entfernen
  RadioGroup1.Repaint;
end;