Paging issue in datagrid radiobutton

Hi,I am working on Silver Ligth 4,MVVm Architecture,i am using DataGrid with Radiobutton,
i have paging issue in this DataGrid,i have more records in DataGrid,First i selected
First Record in DataGrid,then i scrolled the DataGrid in bottom,then i selected another Record,
again i have scrolled datagrid into bottom and now i selected another Record,now i scrolled to grid grid Record,
now i have observed that three Records are selected,i think it is wrong,Radio button should selected
one record at a Time ,here three records are selected at a time,can any one help me how to overcome
paging issue in Datagrid with Radiobutton,how to make it selecte one record at a time,
my code


<sdk:DataGrid x:Name="DtgDDetails" Grid.Row="0" Grid.Column="1" Grid.ColumnSpan="6"
AutoGenerateColumns="False" CanUserReorderColumns="True" CanUserSortColumns="True"
CanUserResizeColumns="True" HeadersVisibility="Column"
HorizontalAlignment="Stretch" HorizontalContentAlignment="Stretch"
VerticalAlignment="Stretch" VerticalScrollBarVisibility="Auto"
MinHeight="150" MaxHeight="150" Background="{x:Null}"
HorizontalScrollBarVisibility="Auto" IsReadOnly="True" TabIndex="21"
ItemsSource="{Binding PagedDetailsView,Mode=TwoWay,Source={StaticResource ReportVMKey}}"
SelectedItem="{Binding SelectedPatienteport,Mode=TwoWay,Source={StaticResource ReportVMKey}}"
>
<sdk:DataGrid.Columns>

<sdk:DataGridTemplateColumn x:Name="DthSelectPatientDetails" CanUserReorder="True" CanUserResize="True" Header="Select" Width="auto" MinWidth="70" >
<sdk:DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<RadioButton x:Name="RdoSelect" GroupName="rdo" TabIndex="22" HorizontalAlignment="Center"
Command="{Binding SelectedPatientReportCommand,Mode=TwoWay,Source={StaticResource ReportVMKey}}" IsChecked="{Binding IsSelected,Mode=TwoWay}" ToolTipService.ToolTip="Select Patient" />
</DataTemplate>
</sdk:DataGridTemplateColumn.CellTemplate>
</sdk:DataGridTemplateColumn>