brain surgery vs surgery

brain surgery and surgery both are nouns.

Word	Noun	Adjective	Verb	Adverb
brain surgery	Yes	No	No	No
surgery	Yes	No	No	No

As nouns, surgery is a hypernym of brain surgery; that is, surgery is a word with a broader meaning than brain surgery:

brain surgery: any surgical procedure involving the brain
surgery: a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body

Other hypernyms of brain surgery include operation, surgical operation, surgical procedure, surgical process.

brain surgery (noun)	surgery (noun)
any surgical procedure involving the brain	a medical procedure involving an incision with instruments; performed to repair damage or arrest disease in a living body
	a room in a hospital equipped for the performance of surgical operations
	a room where a doctor or dentist can be consulted
	the branch of medical science that treats disease or injury by operative procedures