Popup window refresh

Hi,
I have a child page which pop up from the parent page as follows:
(javascript)
function OpenPage()
{
     var RetVal=window.showModalDialog("childpage.aspx");
}
I have a radiobutton list and a agridview in that child page. I need to bind the Gridview on radiobutton_selected index change event.
But when I try to chnage the radio button, the child page opens again in the new tab..
how can I chnage radio button wihtout reopen the child page again in a new window???
 
please help me..
...