U.S. District Court Judge Albert V. Bryan Jr. dismissed yesterday a $10 million malpractice suit against the federal government filed by a Manassas woman who alleged that doctors at the National Institutes of Health were negligent in diagnosing her life-threatening lung cancer.

Bryan called the case brought by Virginia C. Waffen, 29, a "tragic" one. Nevertheless, he ruled that actions by NIH doctors had not affected her chances of survival and were not "so reckless or deliberate as to warrant punitive damages."

Waffen, a mother of two, was not available for comment. One of her attorneys said he will file a motion for Bryan to reconsider his decision.

The attorney, Daniel J. Glanz, said, "It looks to us like the judge didn't understand" part of the testimony. "We have not given up on this."

Assistant U.S. Attorney Nash Schott, who defended the government in federal court in Alexandria, said, "We're obviously gratified by the verdict. It vindicates the reputations not only of NIH, but also of the many fine physicians who work there."

Dr. John L. Decker, director of NIH's Clinical Center and one of the physicians accused of negligence in the lawsuit, said he too was gratified by the decision. He added: "I take no joy in Mrs. Waffen's difficult situation; we wish her well and stand ready to be of whatever assistance is possible to her and to her physician."

Last month another Manassas woman, Judith Burke, 36, won a $1 million award against the government in a federal court suit in which she claimed that doctors at the Naval Hospital in Bethesda misdiagnosed her life-threatening breast cancer.

Waffen alleged in her suit that an X-ray taken at NIH in March 1981 had shown a tumor in her lungs, but that NIH doctors had signed a discharge summary statement stating that her chest X-ray was "within normal limits" and had not informed her of the true X-ray readings until February 1982.

Waffen underwent surgery to remove the tumor in late 1981 at Georgetown University Hospital after a private doctor found the tumor. Her suit alleged that NIH's delay in informing her of the tumor decreased her chances of survival.

NIH doctors said in pretrial statements that the results of Waffen's March X-ray had not been put in her file, that the discharge summary statement was inaccurate and that Waffen had not been informed of the X-ray's true reading until February l982.

But government lawyers argued that the delay had not decreased her chances for survival.

The nub of Bryan's 16-page decision dealt with this question, about which he said there was "a conflict in evidence" from expert witnesses called by the two sides. The witnesses described two different medical concepts for evaluating and measuring the growth of cancer tumors.

Bryan said he found the medical opinions of the government's witness, Dr. Joseph Aisner, "to be supported by the better reasoning."

Aisner, professor of oncology at the University of Maryland and assistant director of clinical affairs at the Cancer Center of the university's hospital, described Waffen's tumor as a slow-growing one that had already reached a critical stage in March and concluded that the delay in treatment "had no impact on her probability of survival," Bryan's decision said.

Bryan said his decision "is not one that affords any sense of satisfaction . . . but viewed as objectively as such a tragic case as this can be, it cannot find that the missing X-ray would have made a difference."