search Parent and top level Parent

Hello,

I have some customers who have a parent and a top level parent
in the script when I get the parent, I get the top level parent, how can I get the parent

var cust = recInvoice.getLineItemValue('item', 'custcol_client_doctor_id', cntLne);               
                        nlapiLogExecution('debug', 'client doc', cust);// get the customer
                        var pardoc = nlapiLookupField('customer', cust, 'parent');
                        nlapiLogExecution('debug', 'parent doc', pardoc);

pardoc gives me the top level parent

Thanks for your help