What the Dog Saw [86]
3.
Dershaw picked up a new X-ray and put it on the light box. It belonged to a forty-eight-year-old woman. Mammograms indicate density in the breast: the denser the tissue is, the more the X-rays are absorbed, creating the variations in black and white that make up the picture. Fat hardly absorbs the beam at all, so it shows up as black. Breast tissue, particularly the thick breast tissue of younger women, shows up on an X-ray as shades of light gray or white. This woman’s breasts consisted of fat at the back of the breast and more dense, glandular tissue toward the front, so the X-ray was entirely black, with what looked like a large, white, dense cloud behind the nipple. Clearly visible, in the black, fatty portion of the left breast, was a white spot. “Now, that looks like a cancer, that little smudgy, irregular, infiltrative thing,” Dershaw said. “It’s about five millimeters across.” He looked at the X-ray for a moment. This was mammography at its best: a clear picture of a problem that needed to be fixed. Then he took a pen and pointed to the thick cloud just to the right of the tumor. The cloud and the tumor were exactly the same color. “That cancer only shows up because it’s in the fatty part of the breast,” he said. “If you take that cancer and put it in the dense part of the breast, you’d never see it, because the whiteness of the mass is the same as the whiteness of normal tissue. If the tumor was over there, it could be four times as big and we still wouldn’t see it.”
What’s more, mammography is especially likely to miss the tumors that do the most harm. A team led by the research pathologist Peggy Porter analyzed 429 breast cancers that had been diagnosed over five years at the Group Health Cooperative of Puget Sound. Of those, 279 were picked up by mammography, and the bulk of them were detected very early, at what is called Stage One. (Cancer is classified into four stages, according to how far the tumor has spread from its original position.) Most of the tumors were small, less than two centimeters. Pathologists grade a tumor’s aggression according to such measures as the “mitotic count” — the rate at which the cells are dividing — and the screen-detected tumors were graded “low” in almost 70 percent of the cases. These were the kinds of cancers that could probably be treated successfully. “Most tumors develop very, very slowly, and those tend to lay down calcium deposits — and what mammograms are doing is picking up those calcifications,” Leslie Laufman, a hematologist-oncologist in Ohio, who served on a recent National Institutes of Health breast-cancer advisory panel, said. “Almost by definition, mammograms are picking up slow-growing tumors.”
A hundred and fifty cancers in Porter’s study, however, were missed by mammography. Some of these were tumors the mammogram couldn’t see — that were, for instance, hiding in the dense part of the breast. The majority, though, simply didn’t exist at the time of the mammogram. These cancers were found in women who had had regular mammograms, and who were legitimately told that they showed no sign of cancer on their last visit. In the interval between X-rays, however, either they or their doctor had manually discovered a lump in their breast, and these “interval” cancers were twice as likely to be in Stage Three and three times as likely to have high mitotic counts; 28 percent had spread to the lymph nodes, as opposed to 18 percent of the screen-detected cancers. These tumors were so aggressive that they had gone from undetectable to detectable in the interval between two mammograms.
The problem of interval tumors explains why the overwhelming majority of breast-cancer experts insist that women in the critical fifty-to-sixty-nine age group get regular mammograms. In Porter’s study, the women were X-rayed at intervals as great as every three years, and that created a window large enough for interval cancers to emerge. Interval cancers also explain why many breast-cancer