A U.S. District judge yesterday ordered the federal government to pay $300,000 in damages to a 56-year-old Hyattsville woman who underwent breast surgery and chemotherapy for cancer a year after she had been told by a Walter Reed Army Hospital staff doctor that he could not detect any breast tumors.

The woman said she had told the doctor before he examined her that she had detected a lump in her right breast, and that there was some history of breast cancer in her family.

Judge Gerhard A. Gesell, in a seven-page opinion, wrote that the doctor who examined the woman, Yoshie Matson, in January 1979, should have ordered periodic follow-up examinations of Matson even though he did not find any breast tumors after an initial, detailed examination. In January 1980, when the tumor became evident, Matson returned to Walter Reed and underwent surgery for cancer that had spread to six lymph nodes.

Gesell found that if Matson had been given proper follow-up examinations, it was probable that the tumor would have been discovered and removed before the cancer spread to six of her lymph nodes, which increases the risk that the cancer will recur.

Gesell wrote that the standard of medical care at the time required follow-up examinations in cases such as Matson's in which the woman is past menopause, has a history of lumps in her breasts as well as a relative who has suffered breast cancer, and is a person who specifically has sought a physician's advice because she detected a lump in her breast.

The delay in properly diagnosing Matson's case shortened her life expectancy and caused her "unnecessary shock" when the extent of her illness was determined and surgery ordered, Gesell wrote. Chemotherapy also had to be more extensive because of the delay, causing "additional pain and discomfort," Gesell wrote. Matson's husband, James A. Matson, also was awarded $50,000 for loss of his wife's companionship because of her reduced life expectancy and the increased severity of her illness.

Gesell's decision, reached after a nonjury trial, can be appealed by the government. The government contended that even if follow-up examinations were ordered, the cancer would have spread to Matson's lymph nodes before the tumor was detected.

Matson's lawyer, Martin H. Freeman, said yesterday that his client, who is employed by Sears Roebuck and Co., has had to reduce her work time because of her illness and still is undergoing chemotherapy.