How to generate checkbox on grouping column in advanced datagrid : flex 3

This is my code .but it generate for all rows. I want checkbox for only where studyid displayed.

<mx:AdvancedDataGrid id="dgList" x="10" y="342" width="1001" height="175" sortExpertMode="true" borderColor="#992958" styleName="invoiceDataGrid">

                <mx:dataProvider>

                    <mx:GroupingCollection id="groupda" source="{CptArr}">

                        <mx:Grouping>

                            <mx:GroupingField name="patientname"></mx:GroupingField>

                            <mx:GroupingField name="studyId"></mx:GroupingField>

                        </mx:Grouping>

                    </mx:GroupingCollection>

                </mx:dataProvider>

                <mx:groupedColumns>

                    <mx:AdvancedDataGridColumn headerText="Patient Details" width="200"/>

                    <mx:AdvancedDataGridColumn dataField="cpt_code_val" headerText="CPT Code" width="100"/>

                    <mx:AdvancedDataGridColumn dataField="cpt_desc" headerText="CPT Description"/>

                    <mx:AdvancedDataGridColumn dataField="modifier" headerText="Modifier" width="100"/>

                    <mx:AdvancedDataGridColumn headerText="Fee Schedule" dataField="fee_schdule" width="100"/>

                    <mx:AdvancedDataGridColumn headerText="select" id="dgselect">

                    <mx:itemRenderer>

                        <mx:Component>

                            <mx:CheckBox visible="{data.cptdesc}"

                        </mx:Component>

                    </mx:itemRenderer>

                    </mx:AdvancedDataGridColumn>

                </mx:groupedColumns>

            </mx:AdvancedDataGrid>

output:

---------

but i want like this:

Thanks in advance for your help.