The man who explained to President Reagan that he has cancer, Dr. Steven Rosenberg, is known as a research-oriented physician who is "extremely honest" with his patients, providing optimism but not false hopes.

"Humane, very humane," said one of his colleagues. Said another, "He is an extraordinarily honest man -- with his research, his work and his patients."

"He's a very concerned physician," said Dr. Paul A. Sugarbaker, a surgeon at the National Cancer Institute, where Rosenberg is chief of surgery. "He is extremely honest with patients. He takes time to tell them what is really going on, and he tries to put it all in a hopeful and optimistic way -- to put the disease in perspective without giving false hope."

"He didn't tell Mr. Reagan simply 'You're going to be okay'; he said your chances of the cancer recurring are 50-50," Sugarbaker said.

"He's a straightforward guy," White House spokesman Larry Speakes said after Rosenberg faced reporters with aplomb yesterday afternoon. "I think the Reagans appreciated the way he dealt with them."

Rosenberg, two weeks shy of his 45th birthday, is also well-known in the medical community as one of the editors of "the bible" for cancer specialists around the country, a frequently updated work called "Cancer: Principles and Practice of Oncology." He was also an editor for a major resource book on AIDS, released in May.

A graduate of Johns Hopkins undergraduate and medical schools, he received a PhD in biophysics at Harvard University, where he worked as a research fellow in immunology. An intern and resident at Peter Bent Brigham Hospital in Boston, Rosenberg came to NCI in 1970 and was named the institute's chief of surgery in 1974.

But for all his credentials in the medical community, Rosenberg was credited primarily by the White House for being instrumental in explaining clearly the president's illness -- not only to the nation but to the president and Nancy Reagan.

During Saturday's surgery, it was Rosenberg who stepped out of the operating room every 30 minutes to inform the first lady of the president's condition.

One White House official who has been in frequent contact with the president at the hospital over the weekend said Rosenberg had been a "crackerjack" about explaining to the family what was happening.

Rosenberg used what appeared to be the same manner when he met the press corps yesterday afternoon -- beginning his talk by saying "the president has cancer" and then explaining, clearly and calmly, what can be said about his health now and in the future.