How to contain a TextBox w/i an Ellipse when resizing?

Question

Hello,
I am trying to lay out a TextBox w/i an Ellipse w/i a larger control. When the larger control is resized, the aspect of the Ellipse can change. How can I resize the TextBox to remain entirely w/i the Ellipse?

        <Grid>
        <Ellipse x:Name="tbEllipse" StrokeThickness="1" Stretch="Fill" Fill="White" Opacity="100" />
        <ScrollViewer x:Name="TextBoxSV"  Height="Auto" Width="Auto"
            HorizontalAlignment="Stretch" VerticalContentAlignment="Stretch" HorizontalContentAlignment="Stretch" Margin="2" >
            <TextBox x:Name="TextBox" HorizontalAlignment="Stretch" VerticalAlignment="Stretch" TextWrapping="Wrap" Margin="1" />
        </ScrollViewer>
    </Grid>

Thanks for any advice...