Dr. Cushman D. Haagensen, a leading cancer specialist who advocated radical mastectomies as the best hope for curing breast cancer, died Sunday at his home in Palisades, N.Y. He was 90 years old.

He died of pneumonia, his wife, Alice, said.

Dr. Haagensen had held surgery, research and teaching posts at Columbia University's College of Physicians and Surgeons since 1933 and performed operations until he was 75.

He was noted for a classic study, ''Diseases of the Breast,'' which was issued by the Saunders Publishing Company in completely different editions in 1956, 1971 and 1984. He was writing a nontechnical book on radical mastectomy when he died.

Concern Over Risks

In opposing breast reconstruction, Dr. Haagensen said he believed that cancer could be spread by another operation, that the cosmetic results he had seen were not esthetically successful and that if there was enough skin left to do an implant, the surgery had not been radical enough and the patient had less chance of survival.