oversize canvas cause radiabutton path render as oval

In MaterialDesignTheme.RaidoButton.xaml file, the canvas is large than viewbox, it will cause multi radio button look like oval, not circle.

three radio in uniform grid with 3 columns

                        <Viewbox Width="18" Height="18"  VerticalAlignment="{TemplateBinding VerticalContentAlignment}">
                            <Canvas Width="24" Height="24">
                                <Path x:Name="Graphic"
                                      Data="M12,20A8,8 0 0,1 4,12A8,8 0 0,1 12,4A8,8 0 0,1 20,12A8,8 0 0,1 12,20M12,2A10,10 0 0,0 2,12A10,10 0 0,0 12,22A10,10 0 0,0 22,12A10,10 0 0,0 12,2Z" 
                                      Fill="{DynamicResource MaterialDesignCheckBoxOff}" />
                                <Ellipse x:Name="InteractionEllipse" Fill="{TemplateBinding Foreground}" Width="0" Height="0" Canvas.Top="12" Canvas.Left="12" Opacity="0" RenderTransformOrigin="0.5,0.5" >
                                    <Ellipse.RenderTransform>
                                        <TransformGroup>
                                            <ScaleTransform/>
                                            <SkewTransform/>
                                            <RotateTransform/>
                                            <TranslateTransform/>
                                        </TransformGroup>
                                    </Ellipse.RenderTransform>
                                </Ellipse>
                            </Canvas>
                        </Viewbox>

I wonder if there a easy way to fix it, such as just changing some property like uselayoutround.
However, it's not working.
Another way is change all canvas size to fix size 18*18, but it will not work properly in large radio button.
So, I think that replacing canvas to grid with 2 ellipse maybe a solution(just do as MahApps.Metro).

ref:

                        <Grid x:Name="PART_CHECKBOX">
                            <Rectangle Margin="-6 0" Fill="{DynamicResource TransparentWhiteBrush}" />
                            <Ellipse x:Name="normal"
                                     Width="18"
                                     Height="18"
                                     Fill="{TemplateBinding Background}"
                                     Stroke="{TemplateBinding BorderBrush}"
                                     StrokeThickness="{TemplateBinding BorderThickness}" />
                            <Ellipse x:Name="Checked1"
                                     Width="10"
                                     Height="10"
                                     Fill="{DynamicResource HighlightBrush}"
                                     Opacity="0" />
                            <Ellipse x:Name="disabled"
                                     Width="18"
                                     Height="18"
                                     Fill="{DynamicResource SemiTransparentWhiteBrush}"
                                     Opacity="0"
                                     StrokeThickness="{TemplateBinding BorderThickness}" />
                        </Grid>

Regards.

从 MaterialDesignTheme.RaidoButton.xaml 开始，检查 Graphic 和 InteractionEllipse 路径周围的 Viewbox 和 Canvas 尺寸设置。重现包含三个单选按钮的 UniformGrid 示例，然后比较不同单选按钮尺寸下的渲染结果。完成标准是：在报告的布局中以及更大的控件中，单选路径保持圆形而不是椭圆形。