Set to record unknown cancer therapy (9)
'cancer therapy' is the definition.
(treatment option for cancer)
'set to record unknown' is the wordplay.
'set' becomes 'radio' (radio is a kind of set).
'to record' becomes 'log' (log is a kind of record).
'unknown' becomes 'y' (algebra).
(Another definition for radiology that I've seen is "medical treatment".)