"Yes, you do have cancer . . . "

"These are fateful but not necessarily fatal words," writes a man who has treated hundreds of cancer patients, Dr. John F. Potter, surgeon and director of Georgetown University's Lombardi Cancer Center.

"Almost half of all people who are so addressed do survive" -- at least five years, so the statistics show -- he writes in his new book, "How to Improve Your Odds Against Cancer," to be published by Frederick Fell next fall.

"Nevertheless," he continues, "these words come with stunning surprise. When the initial shock has worn off, the next reaction is one of disbelief -- that this cannot be happening to you.

"This phase will often progress to a 'Why me?' feeling. You may be consoled by one fact, however. You are at this moment at an absolute psychological low. From this point, almost invariably, your emotional status will improve. With the establishment of the diagnosis, uncertainty is eliminated, and your way is clear for more positive psychological reactions . . .

"Be assured that you can handle the situation. Hundreds of thousands of others have done so successfully and so can you, and you will. Have confidence in yourself, even in the face of the greatest psychological stress of your life. Take it a day at a time."

He has some other practical suggestions:"The more you know," the more you can learn about your disease, the better the decision you will make.Do your share to develop "a frank and open relationship" with your doctor. "He should be telling you what he knows about your condition, and you should be asking him" all the questions in your mind."There is enough time for you to obtain a second opinion about your condition . . . Act without delay but don't be panicked or pushed into almost immediate treatment.

"For example, there is good reason to be wary of the advice that a definitive surgical procedure must be performed at the same time or within a day of the return of a positive biopsy."Avoid the danger, however, of "deferring your decision for a prolonged period while you obtain repeated consultations" or "shop for the opinion you want to hear." Proceed promptly but without undue haste.