Though it takes a smaller toll than breast, colon or lung cancer, non-Hodgkin’s lymphoma is a leading cause of cancer death (Hodgkin’s disease is a mild and less common lymphoma). It will strike an estimated 45,000 Americans this year, and kill 21,000. Thanks to advances in treatment, 52 percent of all patients now live at least five years, up from 31 percent three decades ago. But as Mrs. Onassis’s experience suggests, curing or preventing the disease is still a distant hope.

Lymphoma draws its name from the lymphocytes, or white blood cells, it affects. These cells normally course through the bloodstream and the lymph system, a network of small glands concentrated in the neck, groin and armpits. Healthy lymphocytes proliferate only as needed to combat foreign invaders, such as viruses or bacteria. But cancerous ones multiply endlessly, crowding out healthy cells in the process. In its early stages, lymphoma may cause no symptoms other than a swollen gland like the one Mrs. Onassis noticed in her right groin last December. As the condition advances, it can also cause weight loss, night sweats and fevers. To diagnose the condition, and gauge its aggressiveness, doctors analyze tissue from an affected lymph node. Ironically, says Dr. John Laszlo of the American Cancer Society, fast-growing varieties like the one doctors diagnosed in Mrs. Onassis are often the most sensitive to therapy.

Aggressive lymphomas are normally treated with several different drugs simultaneously. When that approach fails, other strategies remain. Bone-marrow transplantation can help patients endure chemotherapy at doses that would normally kill them. And vaccine-style treatments sometimes quell drug-resistant tumors by tricking the immune system into attacking them. But as Mrs. Onassis’s experience suggests, a virulent lymphoma can still outpace any oncologist. Her cancer spread to her brain and spinal cord just two months after she started chemotherapy. Injecting drugs through a tube into her skull did no good, and the malignancy soon invaded her liver as well.

Unlike lung cancer, for which smoking is largely to blame, lymphoma has few clear causes. It’s common in people with HIV (the incidence rate has jumped 65 percent since the AIDS epidemic began), but the typical sufferer is HIV-negative and at least 50 years old. Studies suggest that lymphoma is more common in people with autoimmune diseases, such as lupus and rheumatoid arthritis, and in those exposed to certain chemicals. Some studies even suggest that prolonged use of dark hair dyes can promote the disease. For now, though, no one can say why the former First Lady was stricken, or how she might have been spared. Common Cancers

Non-Hodgkin’s lymphoma still ranks among the leading killers.

CANCER DEATHS PROJECTED FOR 1994

Lung 153,000

Colon and rectum 56,000

Breast 46,300

Prostate 38,000

Pancreas 25,900

Non-Hodgkin’s lymphoma 21,200

Ovary 13,600

Kidney 11,300

Skin (melanoma) 6,900

SOURCE: AMERICAN CANCER SOCIETY