WPF Checkbox align to top left with multiline content

I've got a WPF Treeview with HierachicalDatatemplates for the nodes. The Child nodes are containing checkboxes for selecting the children. So far so good.

The content of the childnode should be a checkbox with multiline content several Textboxes and according to the viewmodel some pics.

When I put the content inside the checkbox, the box is always displayed in the vertical center of the content. But I want the checkbox to be on the top left of the content.

Here is an example WPF code:

<Window x:Class="Spielwiese.MainWindow" xmlns="http://ift.tt/o66D3f" xmlns:x="http://ift.tt/mPTqtT" Title="MainWindow" Height="350" Width="525">
<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="*" />
        <ColumnDefinition Width="*" />
    </Grid.ColumnDefinitions>

    <!--THIS IS WHAT I LIKE TO USE-->
    <CheckBox Grid.Column="0" Margin="5">
        <StackPanel>
            <TextBlock Text="Some text" />
            <TextBlock Text="Some more text" />
            <TextBlock Text="a lot of more text" />
        </StackPanel>
    </CheckBox>       


    <!--THIS IS MY ACTUAL WORKAROUND-->
    <Grid Grid.Column="1">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="auto" />
            <ColumnDefinition Width="auto" />
        </Grid.ColumnDefinitions>
        <CheckBox Grid.Column="0" Margin="0,3,5,0" />
        <StackPanel Grid.Column="1">
            <TextBlock Text="Some text" />
            <TextBlock Text="Some more text" />
            <TextBlock Text="a lot of more text" />
        </StackPanel>
    </Grid>
</Grid>

I would prefer the content to be inside the checkbock because of the selection of the nodes. I already tried to set verticalcontentalignment for the checkbock but it changed nothing.

Maybe somone has already solution for this little problem.