Call JS ShowModalDialog Then Return a Datatable to Parent Form

I call a modal dialog with a gridview using this code:
    Public Const STR_MODAL_SELECT_LIST_CODE = "<script type='text/javascript'>  window.showModalDialog('SelectList.aspx', null, 'unadorned:yes ;resizable:0 ;dialogWidth:550px ;dialogHeight:500px ;status:no ;scroll:no ;status=no;'); </script>"

 ScriptManager.RegisterStartupScript(upNOI, upNOI.GetType(), STR_MODAL_DIALOG_NAME, STR_MODAL_SELECT_LIST_CODE, False)

When the user selects the row, I populate the row into a datatable. But I am trying to figure out  how to return the datatable to the calling screen. I am not sure how to do this.