BY JOHN C. HAYES

The cost-effectiveness of annual screening mammography exams for 40 to 49-year-old women can be heavily influenced by factors that are not easily controlled. These include the natural history of the disease and an inherent variability in the sensitivity of the procedure, according to new research.

Current models have found considerable differences in the costs of mammography screening, contributing to the controversy about screening women younger than 50.

A cost-effectiveness model developed by a Stanford researcher Susan Plevritis, Ph.D., and presented at the RSNA meeting on Monday differed from others by incorporating the natural history of cancer and the operating characteristics-sensitivity and specificity-of screening mammography. Whether to screen annually or every other year is not a simple question, Plevritis said later.

If a tumor is detected when it is 1 cm in size, the cost-effectiveness of annual screening falls above the figure of $50,000 per year of life saved that is used in comparisons of annual and biennial screening, she said. But if the tumor is not detected until it is 1.2 cm in size, the cost-effectiveness of annual screening increases.

The size at detection, in turn, can be influenced by the rate of tumor growth. Faster rates of growth argue for annual screening, but with very high rates of growth, even annual screening may not be cost-effective, she said.

Plevritis placed the marginal cost of annual screening relative to biennial screening at $77,000. The marginal cost of biennial screening was $30,000. The analysis did not take into account the detection of DCIS.

"It's not a black-and-white question. There should be an awareness that we don't really know enough to make a decision," Plevritis said. "We need to know more about the course of cancer and the operating characteristics of screening mammography."