Mine was diagnosed when they removed the entire tumour, I think thats the only way to diagnose, before that, they thought it was a nerve sheath tumour, they ruled that out and then thought it was lymphoma, right up until my results came back.
Have you spoken to Jim johnstone? he can put you in touch with any experts in your area or nearest, they can diagnose you if your current place cannot.
Do you use FB? this is pretty active
International Castlemans Disease Organization
and the web page which Jim runs, he is very active on FB