Mammography screening has significantly reduced the number of breast cancers cases involving large tumors or disease that has spread to the lymph nodes, according to a Swedish study.

Dr. Robert A. Smith of the American Cancer Society in Atlanta and his associates examined data from 20 to 40 years of observation for 1.1 million Swedish women. Over this period there were 23,092 breast cancer cases.

For women between 40 and 49 years of age, mammography screening was associated with significant reductions in the rate of node-positive cancers, tumors larger than 2 cm, and cancers of stage II or higher, the investigators report in the medical journal Cancer.

Significant reductions in all of these three parameters were also seen among women in the 50-to-69 year-old group.

The combined results for all ages (40 to 69 years) showed that mammograms were associated with a 17 percent to 37 percent reduction in advanced tumors.

"Reductions in tumors larger than 2 cm or with (positive) lymph nodes can be used as an early indicator of the effectiveness of screening," Smith and his colleagues conclude.