cancer vs malignant tumor

cancer and malignant tumor both are nouns.

Word	Noun	Adjective	Verb	Adverb
cancer	Yes	No	No	No
malignant tumor	Yes	No	No	No

As nouns, malignant tumor is a hypernym of cancer; that is, malignant tumor is a word with a broader meaning than cancer:

cancer: any malignant growth or tumor caused by abnormal and uncontrolled cell division; it may spread to other parts of the body through the lymphatic system or the blood stream
malignant tumor: a tumor that is malignant and tends to spread to other parts of the body

Other hypernyms of cancer include malignant neoplasm, metastatic tumor.

cancer (noun)	malignant tumor (noun)
any malignant growth or tumor caused by abnormal and uncontrolled cell division; it may spread to other parts of the body through the lymphatic system or the blood stream	a tumor that is malignant and tends to spread to other parts of the body