Set to record unknown cancer therapy (9)

I believe the answer is:

radiology

'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).
'radio'+'log'+'y'='RADIOLOGY'

(Another definition for radiology that I've seen is " medical treatment".)

I've seen this clue in The Guardian.

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