DEATH OF A FIRST LADY; No More Could Be Done, Mrs. Onassis Was Told

Jacqueline Kennedy Onassis returned to her apartment on Fifth Avenue on Wednesday, after doctors at New York Hospital-Cornell Medical Center had found that cancer had spread to her liver and had told her there was nothing more they could do for her.

Aggressive therapy, including antibiotic treatment of pneumonia that she developed early this week, was halted in keeping with the wishes Mrs. Onassis had expressed in a living will, a health worker familiar with her care said.

The type of cancer of the lymph system that Mrs. Onassis had, non-Hodgkins lymphoma, is often treatable for years, and she had been reported to be responding well to treatment. As recently as Sunday, Mrs. Onassis was seen walking in Central Park. But the health worker familiar with her her care said she had been unable to walk very far and had needed to be supported by her companion, Maurice Tempelsman.

Mrs. Onassis's condition was diagnosed in the winter. And in recent weeks, the health care worker said, she had been receiving radiation therapy to her brain, after the cancer was found to have spread there. She had also received chemotherapy directly into the brain. The cancer elsewhere in her body had initially responded to standard chemotherapy, including steroids, the health care worker said, but more recently had been found to have spread through her body. Immune System Weakened

The drug and radiation therapy weakened her immune system, leaving her vulnerable to infection. On Monday, she developed acute pneumonia, which was initially treated with antibiotics. On Wednesday, doctors found that the lymphoma had recurred in her liver, where it had been present earlier. They told her that the cancer was no longer treatable. The antibiotics were stopped when she left the hospital.

Mrs. Onassis signed a living will in February that had clearly expressed her wish not to receive aggressive medical treatment if she developed a grave illness and such measures would be futile.

"She had an aggressive cancer that was treated aggressively and that initially responded to therapy, but it came back in her brain and spread through her body," said the health care worker.

The health worker, who asked not to be identified, spoke out of concern that a mistaken impression of the care Mrs. Onassis had received at New York Hospital-Cornell Medical Center might have resulted from statements made earlier this week by Nancy Tuckerman, a spokeswoman for Mrs. Onassis. Ms. Tuckerman had been quoted as saying: "She's fine. She goes in for routine visits, routine treatment. That's what this is."

In an interview yesterday, Ms. Tuckerman acknowledged that she had understated the severity of Mrs. Onassis's condition on Monday and Tuesday. "We were trying to low-key this whole thing," she said, because "we really feel her medical situation is private and we did not feel we have to reveal everything to reporters."

"I was not trying to be deceptive," Ms. Tuckerman added. "I said what I thought was appropriate. We were trying to protect her and the children, because they could not make visits with ease." Treatment in December

Mrs. Onassis was healthy until early December, the health care worker said, when she was in Virginia and noticed a swelling in her right groin. A doctor there diagnosed a swollen lymph node, and suspecting an infection, prescribed antibiotics. The swelling diminished but did not completely disappear.

While on a boat in the Caribbean over the Christmas holidays, Mrs. Onassis developed a cough, swollen lymph nodes in her neck and pain in her abdomen.

After consulting by telephone with a doctor in New York City, she flew to Manhattan. The doctor found enlarged lymph nodes in her neck and in her armpit. A computerized type of X-ray, a CAT scan, showed that there were swollen lymph nodes in her chest and in an area deep in the abdomen, the retroperitoneal area.

A biopsy of one of the neck nodes showed that Mrs. Onassis had non-Hodgkins lymphoma. A pathologist noted that the cells were anaplastic -- that is, they were undeveloped, what doctors call "embryonic" or "primitive," indicating that the disease was highly malignant.

Mrs. Onassis was offered a chance to seek a second opinion elsewhere, but declined to do so. The pathology slides of her lymph nodes were sent to experts at a hospital in another city, and a doctor from that hospital came as a consultant to New York City. Misleading Early Success

In early January, Mrs. Onassis began receiving the first of four standard courses of chemotherapy for the lymphoma. The therapy included steroid drugs, and the initial treatment led to an apparent remission.

But in mid-March she developed weakness, became confused and had pains in her legs. A neurological examination indicated that the cerebellum portion of her brain had been affected.

Another type of scan, an M.R.I., showed that the lymphoma had disappeared from her neck, chest and abdomen but that it had spread to the membranes that cover the brain and spinal cord.

There was no sign of brain cancer as recently as November, when Mrs. Onassis had a CAT scan after falling off a horse in Virginia.

After the cancer was found in her brain, Mrs. Onassis received radiation therapy there and to her lower spinal cord for about a month, the health care worker said. The treatment relieved her weakness, but she continued to experience pain in her neck, for which she was given pain medications. During this time she received her care at home and as a hospital out-patient.

On April 14, Mrs. Onassis was admitted to New York Hospital-Cornell Medical Center after she developed a perforated ulcer in her stomach, a complication of the steroid therapy. Surgeons sewed up the hole in the stomach that day. Treatment Unsuccessful

But the cancer in her spinal cord and brain continued to worsen. Although a tube was placed in her brain to deliver an anti-cancer drug, a sophisticated but now standard form of therapy for this condition, the lymphoma in her brain did not respond to any therapy. During this period, she lost weight, her speech slowed, she was less alert and had difficulty walking.

On Monday, she developed shaking chills and became disoriented. Mr. Tempelsman and a nurse took her to New York Hospital, where she was admitted. Doctors diagnosed pneumonia and gave her antibiotics by injection.

She rallied initially, but on Tuesday her condition took a turn for the worse.

On Wednesday, a CAT scan showed that the lymphoma was in her liver in huge amounts. Another biopsy showed it was anaplastic non-Hodgkins lymphoma.

Doctors said there was nothing medicine could offer. After consultation with her family and in keeping with the intent of her living will, Mrs. Onassis asked to go home from the hospital.