mastectomy vs excision

mastectomy and excision both are nouns.

Word	Noun	Adjective	Verb	Adverb
mastectomy	Yes	No	No	No
excision	Yes	No	No	No

As nouns, excision is a hypernym of mastectomy; that is, excision is a word with a broader meaning than mastectomy:

Other hypernyms of mastectomy include ablation, cutting out, extirpation.

mastectomy (noun)	excision (noun)
surgical removal of a breast to remove a malignant tumor	surgical removal of a body part or tissue
	the act of pulling up or out; uprooting; cutting off from existence
	the act of banishing a member of a church from the communion of believers and the privileges of the church; cutting a person off from a religious society
	the omission that is made when an editorial change shortens a written passage