Your father has a condition called carcinoma of unknown primary site.
Usually patients present with enlarged lymph node
which on pathological examination show cancer cell
This usually occurs in the head and neck but can also occur elsewhere.
Patient needs a complete ENT examination, blind biopsies from several sites from the head, throat. CT MRI
and PET scan often help.
The pathologist can do an IHC and give an indication of the likely origin of the tumor
But inspite of all these, several patients continue to have metastases without obvious primary cancer.
The treatment of these will involve multiple modalities and often radiotherapy to the involved nodes may be necessary