Background: Interval breast cancer is defined as a cancer that is detected
within 12 months after a negative mammogram. The failure of mammography to
detect breast cancer depends on testing procedures, radiologist interpretat
ion, patient characteristics, and tumor properties. Although errors by radi
ologists explain some interval cancers, another explanation is that the tum
or is rapidly growing and was too small to be detected on the last mammogra
m. To determine whether markers of tumor growth rate are associated with ri
sk of an interval cancer, we conducted a population-based study with the us
e of data collected statewide by the New Mexico Mammography Project. Method
s: Among women who received a mammographic examination from 1991 throughout
1993, we ascertained records of all patients with breast cancer diagnosed
within 12 months of a negative screening mammographic examination (interval
cancers) and corresponding tumor samples, when available. We selected an a
ge- and ethnicity-matched comparison group of control patients with screen-
detected breast cancers diagnosed during the same period. In tumor samples,
p53, bcl-2, and Ki-67 were examined immunologically and the apoptotic inde
x was assessed histologically. We used logistic regression to determine whe
ther interval cancers were associated with selected demographic, radiologic
, and biologic characteristics. Results: It is more likely that mammography
did not detect tumors with a high proportion of proliferating cells (>20%)
than tumors with a low proportion of proliferating cells (<5%) (odds ratio
[OR] = 4.09; 95% confidence interval [CI] = 1.14-14.65). The OR for mammog
raphic failure was 2.96 (95% CI = 1.07-8.20) among cancers that expressed p
53 compared with cancers that did not. Interval cancers also had fewer apop
totic cells. Approximately 75% of interval cancers appear to have tumors wi
th 5% proliferating cells or more. Younger women had a higher proportion of
rapidly proliferating and aggressive cancers. Conclusion: Rapidly growing
and aggressive tumors account for a substantial proportion of mammographic
failure to detect breast cancer, especially among younger women, who have a
high proportion of aggressive cancers.