WPF - Blend - Expression arc control rounded corner

Question

hello,

Did you have any idea how I can round the start and the end or my arc (from expression shapes) ?

What I have try :

        <ed:Arc Name="arc_Progress" Margin="1" Fill="Green" StartAngle="-90" EndAngle="{Binding ElementName=root, Path=ArcAngle}" ArcThicknessUnit="Pixel" ArcThickness="35" RenderTransformOrigin="0.5,0.5" Stretch="None" Grid.ColumnSpan="3" Grid.RowSpan="4">  
            <ed:Arc.Resources>  
                <Style TargetType="{x:Type Border}">  
                    <Setter Property="CornerRadius" Value="10"/>  
                </Style>  
            </ed:Arc.Resources>  
        </ed:Arc>  

What I have :

What I try to do :

Thanks for your help.