A new American Cancer Society study finds a recently reported drop in breast cancer cases in 2003 was preceded by a downturn in incidence that began in the late 1990s. A new American Cancer Society study finds a recently reported drop in breast cancer cases in 2003 was preceded by a downturn in incidence that began in the late 1990s, pointing to a leveling off in screening mammography as playing a significant role in the recent dip in cases. The study, appearing in Breast Cancer Research, confirms a sharp decrease in incidence from 2002 to 2003 among women 50 to 69 years that likely reflects reduced use of hormone replacement therapy (HRT). But it also found a downturn in incidence rates in all age groups above 45 years between 1999 and 2003, pointing to a leveling off in screening mammography.

The recent downturn in breast cancer incidence reported by researchers at MD Anderson Cancer Center followed a nearly two-decade long period during which breast cancer incidence rates increased by almost 40 per cent (1980-1998). Most of that increase has been attributed to increased utilization of mammography.

To explore how the recent drop related to earlier trends, researchers led by Ahmedin Jemal DVM, PhD, examined breast cancer incidence rates by tumor size, stage, and estrogen/progestin receptor (ER/PR) status and trends for in situ breast cancer among women age 40 years and older from 1975 through 2003, using incidence data from the nine oldest Surveillance Epidemiology and End Results (SEER) cancer registries of the United States. They found incidence rates of invasive breast cancer in women decreased in every age group in women 45 years and above between 1999 and 2003. Among women under age 60 or 70 years and above, the decrease began in 1998 or 1999. In contrast, among women aged between 60 and 69 years, all of the decrease occurred from 2002 to 2003.

The researchers say several factors point to a plateau in screening mammography as being the cause of the drop starting in the late 1990s: the decreases were seen in multiple age groups at roughly the same time; it was greatest for small tumors (2 cm or less) and localized or in situ disease that are most commonly detected by mammography; and the decrease coincided with a plateau in mammography usage as measured by national surveys.

The researchers say the rapidity of the decrease in breast cancer incidence rates following the dramatic reduction in use of HRT after the Women's Health Initiative published its results in July 2002 "is not inconsistent with the relationship being caused by withdrawal of HRT…. However, the recent decrease in HRT use cannot account for the reduction in breast cancer incidence that occurred before 2002, or for the decreased incidence in women age 75 and older.”

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