A Northern Virginia woman who claims that doctors at the National Institutes of Health were negligent in diagnosing her lung cancer, is suing the federal government for $10 million in a malpractice suit in federal court in Alexandria.

In her suit, Virginia C. Waffen, 39, of Manassas said that when she was released from an NIH clinic in March 1981, Dr. John L. Decker, then chief of rheumatology at the Clinical Center, signed a discharge summary stating that her chest X-ray was "within normal limit."

Decker, now director of NIH's Clinical Center, discovered seven months later that the X-ray had showed a tumor in her lung, the suit alleges. Waffen, who underwent surgery that month after her regular physician detected the tumor, was not informed of the correct reading of her March 1981 NIH X-ray until February 1982, the suit says.

She said in an interview that the X-ray apparently was briefly lost.

Assistant U.S. Attorney Nash Schott, who is handling the case for NIH, and Decker declined comment. A spokesman for NIH also declined comment.

In a deposition taken in connection with the case, Decker said that when he signed the discharge summary he did not know Waffen's X-ray showed any abnormality.

Waffen, who has two teen-age sons and takes in foster children, said it was difficult for her to file the suit. "I always liked NIH . . . . I have a hard time saying anything against it even now . . . . Dr. Decker is one of the nicest doctors you'd ever want to meet," she said.

" . . . But if it happened to me who else could it happen to? . . . I don't know how my X-ray got lost. The doctors never saw it. It wasn't with my records."

Waffen's suit charges that, as a result of this incident, she was denied opportunity "for early treatment of a then existing lung cancer which increased the risk of danger and damage from that existing lung cancer, and substantially reduced the possibility of her survival."

The suit also charges that Decker and supervising staff "failed to adequately supervise and control medical care in their facility" and cites the lack of procedures "to assure that X-ray reports are properly placed in the charts of the patients."

Waffen said she does not know how much longer she may live. "It could be one year, it could be three years, it could be six months."

In a recent well-publicized incident, a 20-month-old boy who was a patient at NIH died after being given the wrong intravenous saline solution. An attorney for the boy's parents has said they are planning to sue NIH for $3.5 million.